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1. Elements and Compounds
Exercises
Subjective
1. Answer the following questions.
(a) What are subatomic particles? How can you say that an atom is
electrically neutral?
Ans:
An atom constitutes even smaller particles, protons and
neutrons which make up the nucleus, and electrons, which
revolve around the nucleus. These are called subatomic
particles.
Both protons and electrons are electrically charged particles.
In a naturally occurring atom, the number of electrons is equal
to the number of protons. The positive charge of protons is
balanced by the negative charge of electrons. As a result, an
atom as a whole is electrically neutral.
(b)Define
(i)Element: An element is a pure substance that cannot be split
into simpler substances by any physical or chemical means.
(ii) Compound: A chemical compound is a pure substance formed
by chemical combination of two or more elements in a definite
ratio.
(iii)Valency: Valency is the combining capacity of an atom.
(iv)Atom: An atom is the smallest particle of an element.
(c) What is the difference between atomic number and mass number?
The number of protons in the nucleus of an atom is known as the
atomic number of the element.
The total number of protons and neutrons within the nucleus is
known as the mass number of the element.
(d)What is an ion? Give a few examples of positive and negative ions.
If an electron is added to a neutral atom, it will become, negatively
charged. On the other hand, if an electron is removed from an atom,
it will become positively charged. The charged atoms so formed are
known as ions.
For example:
(sodium ion, which is a positive one)
Na – 1e Na+
(sodium atom) Cl + 1e Cl–
(chlorine atom)
(chloride ion which is a negative ions)
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(e) Write the chemical formula of
(i) calcium oxide
: CaO
(ii) iron sulphide
: FeS
(iii) sodium bicarbonate : NaHCO3
(f) How many valence electrons are present in the following elements?
(i) Hydrogen
: 1
(ii) Lithium
:1
(iii) Fluorine
: 7
(iv) Helium
:2
(v) Magnesium : 2
(vi) Carbon
:4
(vii) Aluminium : 3
(viii) Nitrogen : 5
(ix) Oxygen
: 6
(x) Chlorine
:7
2. Distinguish between:
(a) Element and Compound
Compound
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Element
1.
A compound contains atoms of An element is a pure chemical
different elements chemically substance made of same type
combined together in a fixed of atom.
ratio arranged in a defined
manner through chemical
bonds. An element is a pure
chemical substance made of
same type of atom.
2.
A compound can be separated An element cannot be split
into simpler substances by into simpler substances by
chemical reactions.
any physical or chemical
means.
3.
The list of compounds is There are about 117 elements
endless but can broadly be that have been observed. Can
classified as ionic and covalent. be classified as metal, nonmetal or metalloid.
4.
A compound is represented An element is represented
using a formula. An element using symbols.
is represented using symbols.
5.
Water (H2O), Sodium chloride Iron, copper, silver, gold etc.,
(NaCl), Sodium bicarbonate are examples of elements.
(NaHCO3) etc., are examples
of compounds. Iron, copper,
silver, gold etc., are examples
of elements.
TH—Chemistry For ICSE—7
(b)Atom and Molecule
Atom
Molecule
1.
An atom is the smallest particle A molecule is a chemical
of an element.
combination of several atoms.
2.
An atom cannot exist freely.
3.
An atom is the smallest part of A molecule is the smallest
an element that can take part in part of an element or a
a chemical reaction.
compound.
4.
All atoms of a particular The molecule of an element
element are identical and have constitutes of an atom or
same properties.
atoms of the same kind.
The molecule of a compound
constitutes of atoms of two or
more different kinds.
A molecule can exist freely .
(c) Electron and Proton
Protons are found in the nucleus but electrons are found in an
electron cloud around the nucleus, arranged in several orbitals.
Protons are positively charged while electrons are negatively charged.
Objective
3. Answer the following.
(a) Which noble gas element has two electrons in the outermost shell?
Ans: Helium
(b)What is the valency of O in H2O?
Ans: 2
(c) What is the valency of C atom in CO and CO2?
Ans: 4 in both
(d)What is the valency of Ca and O in CaO?
Ans: 2 in both
(e) In the formation of LiF, one electron from Li is transferred to F. How
many electrons are there in the outermost orbit of each atom of LiF?
Ans: 8
Ca atom has two valence electrons in its outermost shell, which it
donates to two Cl atoms; each Cl atom accepts one electron. What is
the valency of Ca and Cl?
Ans: Ca: 2, Cl: 1
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4. Fill in the blanks.
(a) An element is a pure substance that cannot be split into simpler
substances by any chemical means.
(b) The Latin name of gold is aurum.
(c) A compound is a pure substance that can be split by chemical means
into two or more elements.
(d) Elements are made of very minute particles called atoms.
(e) The smallest unit of an element is an atom and that of a compound
is molecule.
(f) The valency of sodium and potassium is one, they are known as
alkali metals.
(g) Electron is a subatomic particle which moves around the nucleus
of an atom.
(h) The chemical formula of ammonia is NH3.
(i) Elements with 3 or 5 electrons in their outermost orbit are nonmetals.
5. Write True or False for the following statements. Also rewrite the false
statements correctly.
(a) Elements can be split into simpler substances. False
Elements cannot be split into simpler substances by any chemical
means.
(b)Chemical compounds can be split into simpler substances. True
(c) Two atoms have same number of protons and different number of
neutrons. They are different elements. False
They are isotopes.
(d)Two atoms have different protons but same number of neutrons.
They are isotopes. False
They are different elements.
(e) Electrons and protons are present inside the nucleus. False
Neutrons and protons are present inside the nucleus.
(f) Protons are present outside the nucleus and electrons are present
inside the nucleus. False
Electrons are present outside the nucleus and Protons are present
inside the nucleus
(g)Protons and neutrons are present inside the nucleus. True
(h)Carbon contains 6 protons, 6 neutrons and 6 electrons. True
(i) Lithium contains 3 protons, 3 neutrons and 3 electrons.False
A lithium atom contains 3 protons, 4 neutrons and 3 electrons
(j) Protons are neutral particles. False
Protons are positively charged particles.
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TH—Chemistry For ICSE—7
Skill based
6. Identify the highlighted substances as elements or compounds.
compound
(a)Water is said to be a supporter of life.
(b)Gold particles found in a river bed are 24 carat gold. element
(c) Green plants absorb carbon dioxide in the sunlight, and produce
sugar and oxygen.
Carbon dioxide and sugar: compound
Oxygen:
element
(d) When iron and sulphur powder is heated, a black substance is
produced.elements
(e)Ozone produced in the upper atmosphere protects us from ultraviolet
radiation.element
(f) A pencil contains a black solid core of graphite inside a wooden
element
casing.
elements
(g) A diamond in a 16 carat gold ring. 7. 24 -carat gold is an element but 22-carat gold is a mixture. Explain how.
Ans: 24-carat is 100% gold. Hence it is an element.
22-carat gold is 91.7% gold. Hence is a mixture.
8. Find out.
(a) A substance is analyzed in a laboratory, and when viewed under an
electron microscope, it is revealed that it contains only one kind of
atom. Is the substance an element, compound or mixture?
Ans: Element
(b)Why do you think a chemical equation should be balanced?
Ans:
The Law of Conservation of Mass, states, “In an ordinary chemical
reaction, matter is neither created nor destroyed.” This means
that during a reaction any atoms are neither gained nor lost.
(c) The air we breathe is an element, compound or a mixture?
Ans: Mixture
(d)A magnesium ribbon is burnt in the air to form a white powdery
substance— magnesium oxide (MgO). Is this oxide an element,
compound or a mixture?
Ans: Compound
(e) Aqua regia is a liquid used by alchemists to separate silver from gold.
Is it an element, compound or mixture?
Ans:
Aqua regia is a mixture, formed by freshly mixing
concentrated nitric acid and hydrochloric acid, optimally in a
volume ratio of 1 : 3.
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(f) In H2, HCl and H2O—what do H and 2 denote?
Ans:
H denotes Hydrogen, and 2 denotes the number of atoms in a
hydrogen molecule or water molecule.
Multiple choice
9. Choose the correct answer.
(a)The formula of magnesium oxide is
(iii) MgO
(b) Which among the following is/are subatomic particle(s)?
(i) electron (ii) proton
(iii) neutron (iv) all of these
,
12
is
the
(c) In the symbol C12
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(ii) mass number
(d)Which of the following shows a variable valency?
(i) phosphorus (ii) iron
(iii) copper (iv) all of these
(e) The chemical formula K2O indicates that it consists of
(i) elements potassium and oxygen
(ii) two atoms of potassium and one atom of oxygen
(iii) both (i) and (ii)
(f) Which of the following shows valency 1?
(i) Na
(g) How many elements occur naturally?
(ii) 92
(h) Which of the following is correct if the valency of Ca is 2 and that
of Cl is 1?
(ii) CaCl2
(i) Compounds are substances that are made of
(iii) Two or more elements
(j) The chemical symbols of phosphorous, iodine, argon, iron, copper
and mercury are:
(iv) P, I, Ar, Fe, Cu, Hg
(k)The lightest gas is
(iv) Hydrogen
(l) Which element has the same number of valence electrons as 19
9 F?
(iii) 35
17 Cl
(m) Which among the following is monovalent?
(ii) Lithium
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2. Physical and Chemical Changes
Exercises
Subjective
1. Answer the following questions.
(a) Define a chemical change. Give four characteristics of a chemical
change.
Ans:
Those changes, in which new substances with entirely different
properties from those of the original substances are formed, are
called chemical changes.
Characteristics of a chemical change
• A chemical change produces entirely a new substance.
• In a chemical change the way in which atoms in molecules are linked
up gets altered.
• A chemical change is a permanent change. It cannot be reversed by
physical methods.
• Mass of original substance is different from that of the newly formed
one. However, the total mass of the reactant and product remains the
same.
(b) Define a physical change. Give four characteristics of a physical
change.
Ans:
Those changes in which the physical properties such as
texture, shape, size and state of the substance change but their
composition remains the same, are called physical changes.
Characteristics of a physical change
• The substance may change its state, shape, size, appearance but the
composition does not change.
• In a physical change, molecules rearrange themselves but there is no
change in their internal structures.
• Physical changes are temporary and can be generally reversed under
suitable conditions.
• In a physical change, mass of the substance remains constant.
(c) How does the law of conversation of mass apply to physical and
chemical changes?
Ans:
Law of conservation of mass states that matter can neither be
created nor destroyed during a chemical change.
Due to this number of atoms of each element or compound before
and after the reaction is same. So the total mass of matter before and
after the change also remains the same.
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(d)With the help of an example show that atoms rearrange themselves
to form a new substance.
Ans:
In a chemical change, the atoms in the reactants rearrange
themselves and bond together differently to form one or more
new products with different characteristics.
For example:
When barium chloride and sodium sulphate react, barium and sodium
radicals exchange their positions and the product barium sulphate
precipitates out of the solution.

2NaCl (aq) + BaSO4
BaCl2 (aq) + Na2SO4 (aq)
barium sodium
sodium barium
chloride sulphate chloride sulphate (e) How many types of chemical changes are there? Explain each with
an example.
Depending upon how a reaction is taking place, chemical reactions
are classified into different types:
They are (i) combination reaction, (ii) decomposition reaction,
(iii) displacement reaction and (iv) double decomposition reaction.
(i) In a combination reaction, two or more reactants combine to give a
product.
Magnesium combines with oxygen in air during burning to form
magnesium oxide. The reaction can be written as
2MgO (s)
2Mg (s) + O2 (g) 
magnesium oxygen magnesium oxide
(ii) In a decomposition reaction, one substance breaks down into two
or more simpler substances.
On heating, baking soda decomposes into sodium carbonate, water
vapour and carbon dioxide.
The reaction can be written as follows:
heat
Na2CO3 (s) + H2O (g) + CO2 (g)
2NaHCO3 (s)
(iii) In a displacement reaction, a more reactive element displaces a less
reactive element from a compound.
If you dip an iron nail into a copper sulphate solution, it becomes
reddish. The iron displaces copper out of its compound.
The chemical equation for the reaction involved is as follows.
FeSO4 (aq) + Cu (s)
Fe (s) + CuSO4 (aq)
Iron copper sulphate
iron sulphate
copper
(iv)In a double decomposition reaction, the positive and negative
radicals of two reactants displace each other forming two new
compounds.
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When an aqueous solution of silver nitrate is mixed with hydrogen
chloride, a white precipitate of silver chloride along with nitric acid
is formed. Nitric acid is also formed.
AgNO3 (aq) + HCl (aq)
 AgCl
+HNO3
silver nitrate hydrogen
silver
nitric acid
chloride chloride
(f) Explain electrolysis of water with diagram.
Refer to Decomposition of water, page 26 and Fig. 2.14.
(g) Differentiate between
(i) Physical and chemical changes
Refer to Table 2.1 Differences between physical and chemical changes.
(ii) Combination reaction and decomposition reaction
In a combination reaction, two or more reactants combine to give
a product. On the other hand in a decomposition reaction, one
substance breaks down into two or more simpler substances.
(iii) Displacement reaction and double decomposition reaction
In a displacement reaction, a more reactive element displaces a less
reactive element from a compound. In a double decomposition
reaction, the positive and negative radicals of two reactants displace
each other forming two new compounds.
2. Write balanced chemical equations for the following reactions taking
place and indicate the state of each substance by using an appropriate
symbol.
 Sodium Hydroxide + Hydrogen
(a)Sodium + Water
+H2
2Na (s)
+ 2H2O (l) 2NaOH
(b)Sodium Hydroxide + Hydrochloric Acid  Sodium Chloride + Water
2NaOH (s) + 2HCl (aq)  2NaCl (s)
+ 2H2O (l)
heat
Sodium Carbonate + Water + Carbon Dioxide
(c) Sodium Bicarbonate
heat
Na CO (s) + H O (g) + CO (g)
2NaHCO (s)
3
2
3
2
2
(d)Water + Carbon Dioxide  Carbonic Acid
 H2CO3 (aq)
H2O (l) + CO2 (g)
(e) Potassium Chloride + Silver Nitrate  Potassium Nitrate + Silver
Chloride
KCl (s) + AgNO3 (aq)  KNO3 (s) + AgCl (S)
(f) Calcium Carbonate heat Calcium Oxide + Water + Carbon Dioxide
CaCO3(s) heat CaO (s) + H2O (l) + CO2 (g)
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(g)Iron + Oxygen + Water  Iron Oxide
4Fe (s) + 3O2 (g)+ xH2O (l) 2Fe2O3.xH2O(s)
(rust)
+Oxygen  Aluminium Oxide
(h) Aluminium
4Al (s)
+ 3O2(g) 2Al2O3(s)
(i) Calcium Oxide + Water  Calcium Hydroxide
CaO (s)
+ H2O (l) Ca(OH)2
3. Balance the following chemical equations and classify each equation as
Combination reaction, Decomposition reaction, Displacement reaction
or Double decomposition reaction.
(a) Zn(s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) Displacement reaction
(b)3NaOH (aq) + H3PO4 (aq)  Na3PO4 (aq) + 3H2O (l)
Displacement reaction
(c) 2Al + 3H2SO4 (aq) Al2 (SO4)3 (aq) + 3H2 (g)
Displacement reaction
(d)CaCl2 (aq) + 2AgNO3 (aq) Ca(NO3)2 (aq) + AgCl (s)
Double decomposition reaction
(e) Hg (s) + S (s)  HgS (s) Combination reaction
heat
Decomposition reaction
(f) 2KClO3 (s)  2KCl (s) + 3O2 (g)
(g) 2AlCl3 (aq) + 3Ba(OH)2 (aq) 2Al (OH)3 + 3BaCl2 (aq)
Double decomposition reaction
(h)2CO + O2  2CO2Combination reaction
Displacement reaction
(i) SnO2 + 2H2  Sn + 2H2O
(j) 2H2O  2H2 + O2Decomposition reaction
Displacement reaction
(k) 2NaNO3  2NaNO2 + O2
Combination reaction
(l) N2 + 3H2  2NH3
(m)4P + 5O2  2P2O5Combination reaction
Displacement reaction
(n)Mg + 2HCl  MgCl2 + H2
Combination reaction
(o) H2 + Cl2  2HCl
Objective
4. Fill in the blanks.
(a) Those substances that react together in a chemical reaction are called
reactants.
(b)The substance that is formed in a chemical reaction is called product.
(c) A chemical equation is balanced by changing the number of atoms
of reactants and products.
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(d)A balanced chemical equation follows the law of conservation of mass.
(e) Chemical changes occur due to rearranging of atoms.
5. Identify each statement as true or false. Also write the false statements
correctly.
(a) In a physical change, no new substance is formed. True
(b) In a chemical change, a chemical reaction occurs. True
(c) When milk turns sour, it is a physical change because a change in
odour does not indicate a chemical change. False
Milk turning sour is a chemical change because a new substance
is formed.
(d)On burning magnesium ash is formed. This is a physical change
because magnesium looks different. False
On burning magnesium ash is formed. This is a chemical change
because magnesium looks different.
(e) On mixing citric acid and baking soda, carbon dioxide is produced
and the temperature decreases. This must be a chemical change. True
(f) With a physical change, energy is either given off or absorbed. False
With a chemical change, energy is either given off or absorbed.
(g) A chemical change can be easily reversed. False
A chemical change cannot be reversed.
6. Which of the following is a balanced equation?
(a) H2 + O2  H2O
(b)C + O2  CO2
(c) Mg + O2  2MgO
(d)Fe + 2S  FeS
Ans: (b) C + O2  CO2
Skill based
7. Give reasons
(a) Breaking of a glass tumbler is an irreversible but physical change.
Ans: Though breaking of a glass tumbler is an irreversible change,
yet no new product is formed in this reaction. Only the size or
shape of the object changes, but the substances of which they
are made remain the same. Hence it is a physical change.
(b)Iron can displace copper from copper sulphate solution but the
reverse is not possible.
Ans:
Iron can displace copper from copper sulphate solution but the
reverse is not possible because iron is more reactive than copper.
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(c) A chemical equation must be balanced.
Ans: Law of conservation of mass states that matter can neither be
created nor destroyed during a chemical change. Hence, the
number of atoms of each element or compound before and after
the reaction should be same. So a chemical equation must be
balanced.
(d)Sodium forms a positive ion and chlorine a negative ion.
Ans:
The sodium atom has one electron in the outermost shell. The
sodium atom can achieve stability easily by losing one electron.
When it does so, the total number of protons in the atom would
be greater than the total number of electrons. As a result, the
atom as a whole becomes positively charged, or becomes a
positive ion.
The chlorine atom has seven electrons in the outermost shell. In
contrast to sodium, the easier way for the chlorine atom to achieve
stability is to gain one electron rather than lose seven from its
outermost shell. In doing so, the additional electron makes the atom
negatively charged or a negative ion.
(e) In electrolysis of water, hydrogen is evolved at cathode, and oxygen
at anode.
Ans: Hydrogen being a positive ion is collected at the cathode which
is the negative side of the battery, and oxygen being a positive
ion, is collected at the anode which is the positive side of the
battery.
(f) Elements and compounds are represented by symbols and formulae
but not mixtures.
Ans:
An element is made of only one type of atoms. Hence it can be
represented by a symbol. A symbol is usually an abbreviation
for the name of an element. Compounds consist of two or
more elements combined chemically in a fixed ratio. Hence
it can be represented by a formula. The molecular formula of
a compound denotes the number of atoms of each element
present in the compound. A mixture is a substance, which
consists of two or more pure substances not chemically
combined with each other in a definite composition. Hence it
cannot be represented by a symbol or a formula.
(g)Chewing bread is a chemical reaction.
Ans:
Chewing the bread mixes it with saliva. The saliva contains
the enzyme amylase, which chemically changes the starch
molecules to smaller sugar molecules.
(h)Charring of sugar is a chemical change, while dissolving of sugar in
water is a physical change.
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TH—Chemistry For ICSE—7
Ans:
Yes, charring of sugar is a chemical reaction because imply the
partial thermal decomposition of sugar. Sugar decomposes
into Carbon black and water vapour when subject to heat. The
reaction can be expressed as follows:
C12H22O11
12C + 11H2O The dissolving of sugar in water is a physical change. The sugar molecules
just dispersed among the water. No new substance is formed. One can boil
off the water to collect the sugar again. All this shows that it is a physical
change.
Multiple choice
8. Choose the correct answer.
(a) The correct formula for ammonium sulphate is __________.
(ii) (NH4)2SO4
(b)Which of the following is an incorrect formula?
(i) NaCl2
(c) In one molecule of carbonic acid there are __________ .
(ii) 1 atom of C, 2 atoms of H and 3 atoms of O
(d)Which of these metals do not corrode?
(iii) Platinum
(e) Which is the correct symbol for copper?
(iii) Cu
(f) Two solutions are mixed together and the new solution becomes hot
with a change in colour. This shows that
(iv) a chemical change is taking place
(g)Jolly wants to experiment with sugar cubes. Which of the following
causes a sugar cube to only change physically, not chemically?
(ii) crushing the sugar cube and dissolving it in water
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3. Air and its Constituents
Subjective
1. Answer the following questions.
(a) Name the gases present in air.
Ans: Nitrogen (78 per cent) and oxygen (21 per cent) are the two
major constituents of air. Besides these gases, air contains
carbon dioxide which forms about 0.03 per cent of the volume
of air, and inert gases like argon.
(b) Name the inert gases. Why are they so called?
Ans:
The inert gases consist of helium (He), neon (Ne), argon (Ar),
krypton (Kr), xenon (Xe) and radon (Rn). Inert gases are so
called because they seldom bond with other gases. (c) What are the causes of air pollution?
Ans: Causes of air pollution are as follows:
• Smoke from automobiles, burning of coal, wood, etc.
• Release of carbon dioxide, ammonia, sulphur dioxide, nitrogen
dioxide, etc., from factories, refineries and power plants
• Release of dust from factories manufacturing cement and steel, mining
of coal and minerals, and burning of coal in thermal power plants
• Sewage and rotting of domestic waste, animal excreta and remains
of plants and animals
• Use of chemical sprays
• Volcanic eruptions emitting sulphur dioxide and hydrogen sulphide
(d)Name the gaseous pollutants of air.
Ans:
Some gaseous pollutants of air are carbon monoxide,
chlorofluorocarbons (CFCs), ammonia, sulphur dioxide,
nitrogen dioxide, etc.
(e) What is acid rain? How is it harmful?
Ans:
The precipitation of dilute solutions of mineral acids from
the atmosphere as rain, snow, fog and dew along with dust is
known as acid rain.
Effects of acid rain are as follows:
i. Acid rain destroys wildlife as the acid present in water causes damage
to animal and plant habitats. Acid rain pouring into lakes and ponds
destroys aquatic life.
ii. Acid rain causes harm to buildings and sculptures made of marble,
limestone, etc.
iii.Many old buildings, statues and temples have lost their lustre due
to acid rain.
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iv. Acid rain adds to the corrosion of metals, thus harming bridges and
other metallic structures.
v. Acid rain adversely affects vegetation by suppressing their growth.
It damages leaves of plants and trees.
(f) Write three ways to prevent air pollution.
Ans:
We can reduce air pollution to some extent by the following
ways:
i. Plantation of trees
ii. Avoiding burning of plastic and polythene bags as they produce toxic gases
iii.Setting up industries, automobile workshops, spray painting units, etc., away from residential areas
iv. Using lead-free petrol and compressed natural gas (CNG) in automobiles
(g)Give a few uses of oxygen.
Ans: Uses of oxygen
i. Both plants and animals need oxygen to respire.
ii.Oxygen is used for the generation of oxyacetylene flame
and oxyhydrogen flame. Oxyacetylene flame (temperature ~
3200°C) and oxyhydrogen flame (temperature ~ 2000°C) are
used for melting metals and for joining metal plates.
iii.Oxygen is also used in the manufacture of sulphuric acid and
nitric acid.
iv.It is used as a rocket fuel oxidant in the form of liquid oxygen
under the name LOX.
v.A mixture of nitrous oxide and oxygen is used as anaesthetic
in surgical operations.
vi.Oxygen is essential for combustion. In its absence, burning
(fire) is not possible.
(h)What is the standard test for oxygen?
Ans: The standard test for oxygen is that it rekindles a glowing
splinter. The glowing splinter burns brightly in oxygen.
(i) What is a catalyst?
Ans: A catalyst is a subtance that increases the rate of a chemical
reaction by reducing the activation energy, but which is left
unchanged by the reaction.
(j) Write the equations for the following chemical changes:
(i) Burning magnesium ribbon in oxygen produces magnesium
oxide.
2Mg + O2  2MgO
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(ii) Heated iron wire combines with oxygen forming iron oxide.
Fe3O4
3Fe + 2O2 tri-iron tetroxide
(iii)Phosphorus burns in oxygen to form phosphorus pentoxide.
2P2O5
P4 + 5O2 (iv)Copper reacts with oxygen to form copper oxide.

2CuO (black coloured)
2Cu + O2 copper oxide
(k) Rusting is slow combustion. Do you agree with this statement? If
yes, give reason.
Ans: Yes, to some extent, the process of rusting can be called slow
combustion. The rusting of iron is an oxidation process. In this
iron using oxygen and water, gets oxidized and is rusted out. It
is a slow process which does produce heat at a very slow rate.
(l) How is oxygen in air renewed?
Ans: Plants and animals exchange gases—oxygen and carbon
dioxide—all the time. Animals take in oxygen and give out
carbon dioxide while breathing. Green plants take in carbon
dioxide from air (during the process of photosynthesis)
and release oxygen. Thus the two processes, breathing and
photosynthesis, help maintain a balance of oxygen and carbon
dioxide in air.
(m) How is carbon dioxide prepared in the laboratory?
Ans: Carbon dioxide gas is prepared in the laboratory by the action
of dilute hydrochloric acid on marble chips, which contain
mostly calcium carbonate.
Some marble chips are placed in a Woulfe’s bottle or a conical flask.
The flask is fitted with a thistle funnel and a delivery tube. Sufficient
quantity of water is poured through the thistle funnel such that all the
chips of marble and the end of the thistle funnel remain dipped in water.
The other end of the delivery tube almost reaches the bottom of the gas
jar. Dilute hydrochloric acid is poured through the thistle funnel. As
soon as hydrochloric acid comes in contact with the marble chips, the
reaction starts and carbon dioxide gas is evolved with effervescence.
For diagram refer to Fig. 3.11 Preparation of carbon dioxide.
(n)How does carbon dioxide help in extinguishing fire?
Ans: Carbon dioxide being heavier than air settles on the fire and cuts
off the supply of air that supports burning. This extinguishes
the fire.
(o)Complete and balance the following reactions.
(i) 4Na + 3CO2  2Na2CO3 + 3C
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heat 2KNO + O
(ii) 2KNO3 2
2
potassium potassium
nitrate
nitrate
(iii) CO2 + H2O H2CO3
carbonic acid
(iv) 2H2S + 3O2 2SO2 + 2H2O
hydrogen
sulphur water
sulphide
dioxide
(p) What is global warming?
Ans:
Owing to human activities, the percentage of greenhouse gases
in the atmosphere, especially carbon dioxide, has increased
gradually. This increase has led to more trapping of heat in
the atmosphere. This in turn leads to the increase in the Earth’s
temperature. This change in the temperature of the Earth is
called global warming.
(q)What is the importance of nitrogen for plants?
Ans:Plant proteins are made from simple nitrogen compounds
found in the soil. Plants of the legume family have small
swellings called nodules in their roots. They fix the nitrogen
in air and convert it into nitrogen compounds. Plants use these
compounds to make proteins.
(r) What is humidity?
Ans: The amount of water vapour in air is called humidity.
Objective
2. Fill in the blanks.
(a) The presence of harmful and undesirable components in air is called
pollution.
(b) In the preparation of oxygen by decomposition of hydrogen peroxide,
manganese dioxide (MnO2) is used as a catalyst.
(c) Oiling or greasing of machine parts that are movable keeps them
away from getting corroded.
(d)Combustion is a chemical reaction in which heat and carbon dioxide
are evolved.
(e) Plants and animals exchange the gases oxygen and carbon dioxide
with their surroundings all the time.
(f) Carbon dioxide + water + energy  glucose + oxygen
(g) All living organisms need nitrogen to make protein molecules.
(h) Helium is a chemical element with the symbol He.
(i) Krypton along with argon is used in most fluorescent lamps.
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(j) Ozone layer present in the atmosphere absorbs most of the harmful
ultra violet rays from the Sun.
Skill based
3. The diagram below represents the water cycle.
Cycle
Q
X
Y
P
Water
(a) Complete the water cycle by filling in the boxes X and Y.
Ans: Y: water vaour; X : clouds
(b)What processes do arrows P and Q represent?
Ans: P: evaporation; Q: condensation
4. Look at the adjacent figure. When you blow air into the test tube
containing lime water you find some changes happening. What is the
change and why does it happen?
Ans:
Carbon dioxide turns limewater milky because of the formation
of insoluble calcium carbonate.
Multiple choice
5. Choose the correct answer.
(a) The formula of baking powder is
(i) NaHCO3
(b)Which gas in air is responsible for diluting the activity of oxygen?
(iii) nitrogen
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(c) Which inert gas is inhaled along with oxygen by patients having
breathing problems?
(ii) helium
(d)The amount of water vapour present in air is called
(iii) humidity
(e) When baking powder reacts with vinegar, which gas is evolved?
(iii) carbon dioxide
(f) Which of the following is/are not a greenhouse gas(es)?
(iii) hydrogen
4. Acids, Bases and Salts
Subjective
1. Answer the following questions.
(a) What are acidic and basic substances?
Ans: An acid is a substance which increases the concentration of
hydrogen ions, when dissolved in water.
Those compounds which are usually oxides and hydroxides of metals
and behave chemically opposite to acids are bases.
(b)Write any two properties of acids and two of bases.
Ans: Acids:
i. Acids generally dissolve in water and release hydrogen ions.
ii. Their aqueous solutions conduct electricity.
iii. They turn blue litmus paper red.
Bases:
i. They are bitter in taste and soapy to touch.
ii. They turn red litmus paper blue.
(c) Write the names of any two
(a) organic acids : acetic acid and oxalic acid
(b) mineral acids : sulphuric acid and hydrochloric acid
(d)What are indicators? Write the names of two indicators that are
extracted from a plant part.
Ans: Indicators are substances which help find out the pH of a
solution. They show a change in colour, when brought in
contact with acids and bases.
Turmeric and Red cabbage juice
(e) How does litmus help in testing acids and bases?
Acids turn blue litmus red and bases turn red litmus blue.
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(f) You are provided with three clear and colourless liquids in three
different test tubes. How will you identify which test tube contains
an acid, a base or water?
Ans: The acidic and basic nature of the solution can be identified by
using litmus paper. A basic solution turns red litmus to blue
and an acidic solution turns the blue litmus to red.
The solution that changes the red litmus blue is a basic solution.
Now, between the two solutions, that which turns the blue litmus
red, is an acidic solution.
The remaining solution is distilled water.
(g)What is an alkali?
Ans:
Water soluble basic hydroxides are called alkalis. Sodium
hydroxide (NaOH), potassium hydroxide (KOH), ammonium
hydroxide (NH4OH) and calcium hydroxide [Ca (OH)2] are
soluble in water, hence are alkalis.
(h) Why are antacid tablets given to cure indigestion or acidity?
Ans:
Antacid tablets or syrups containing magnesium hydroxide
helps neutralize the acidity inside the stomach and relieves one
from the pain and discomfort.
(i) Bee stings are cured by rubbing a mild base on the affected area?
Give reason for the use of a base in this case.
(j) Which acid is found in
(a) lemon : citric acid (b) apple :maleic acid
(c) curd : lactic acid (d) tomato:oxalic acid
(k) Acids contain hydrogen. Water too contains hydrogen but it is not
an acid. Give reason.
Ans:
Acids contain hydrogen which can be replaced. On reacting
with acids, metals like zinc (Zn), magnesium (Mg), iron (Fe) and
sodium (Na) replace the hydrogen of the acid which evolves
as a gas. Metals combine with acidic radicals to form salts.
(l) Define
(a)Neutralization: The reaction between an acid and a base to produce salt and water is called neutralization reaction.
(b) Salt: Salts are compounds obtained by neutralization of acids with bases.
acid + base  salt + water
(m)
How will you show that carbon dioxide is evolved when an acid reacts with bicarbonate?
Ans: Acids react with carbonates and bicarbonates to give salts,
carbon dioxide and water.
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NaHCO3 + HCl  NaCl
+ H2O + CO2
Sodiumsodium
bicarbonate chloride
The reaction of a carbonate salt with an acid can be easily identified by
the effervescence produced due to liberation of CO2. Carbon dioxide can
be tested by passing it through limewater, which turns milky.
(n) Which of the following is a weak acid – HCl, H2CO3, HNO3.
H2CO3
(o) How are salts prepared?
Ans:
Salts are compounds obtained by neutralization of acids with
bases. A salt constitutes a positive ion and a negative ion.
Salts are prepared:
By the action of acids on some metals
+
H2
2HCl + Mg 
MgCl2
magnesium
chloride (salt)
By reaction of alkalis with acids
HCl + NaOH  NaCl
+
H2O
(salt)
(p)What are the natural sources of salts? Write names of two salts found
in nature.
Ans:
A number of salts can be obtained from seawater. Rocks also
contain many salts. Some salts commonly found in nature and
their uses are given in Table 4.7.
(q) There are 5 solutions with pH values equal to 2, 5, 7, 10 and 12.
Arrange them in order of their increasing acidity.
Ans: 12, 10, 7, 5, 2,
2. Balance the following neutralization reactions.
(a) Ca(OH)2 + 2HCl  CaCl2 + 2H2O
Na2SO4 +2H2O
(b) 2NaOH + H2SO4  (c) CaO + 2HCl  CaCl2 + H2O
(d) 2NH4OH + H2SO4  (NH4)2SO4 +2H2O
3. Classify the following acids as organic acids and inorganic acids.
Organic acids
Inorganic acids
lactic acid, tartaric acid, oxalic carbonic acid, phosphoric acid,
acid, formic acid
sulphuric acid, nitric acid,
hydrochloric acid, nitrous acid
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4. Complete the following reactions and balance them.
(a) 2NaCl + H2SO4  Na2SO4 + 2HCl
(b) KNO3 + H2SO4  KHSO4 + HNO3
(c) CaSO heat CaO + SO
3
2
(d) ZnCO3 + H2SO4  ZnSO4 + H2O + CO2
(e) NaHCO3 + HCl  NaCl + H2O + CO2
(f) 2CH3COOH + 2Na  2CH3COONa + H2
(g) NH4OH + HCl  NH4Cl + H2O
NaNO3 + H2O
(h) NaOH + HNO3  CuSO4 + H2O
(i) CuO + H2SO4  (j) Zn(OH)2 + 2HCl  ZnCl2 + 2H2O
Objective
5. Fill in the blanks.
(a) Acids are sour in taste and bases are bitter in taste.
(b) Acids turn blue litmus red and bases turn red litmus blue.
(c) Litmus solution is extracted from lichens.
(d) Phenolphthalein gives pink colour in basic medium but it is colourless
in acidic medium.
(e) Ions formed when hydrochloric acid ionizes in water are H+ and Cl–.
(f) Ions formed when ammonium hydroxide ionizes in water are
NH4+ and OH–.
(g) The salt of nitric acid is called aqua fortis and spirit of niter.
(h) The reaction between an acid and a base gives salt and water.
(i) The reaction of a metal with an acid evolves hydrogen.
(j) The pH is a number used to indicate the concentration of hydrogen
ions in a solution.
(k) Gold, silver and copper do not displace hydrogen from acids.
6. Write True or False for the following statements. Also rewrite the false
statements correctly.
(a) All acids are strong and corrosive. False
Mineral acids are strong and corrosive.
(b)Sodium hydroxide is corrosive in nature. True
(c) All acids turn blue litmus red. True
(d)Bases show no colour change with phenolphthalein. False
Acids show no colour change with phenolphthalein.
(e) Neutralization occurs only when a strong acid reacts with a strong
base. True
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(f) Water is acidic in nature. False
Water is neutral in nature.
7. Match the following acids to their radicals.
(a) Sulphuric acid
(b)Hydrochloric acid
(c) Phosphoric acid (d) Nitric acid
(iii) SO42–
(iv) Cl–
(ii) PO43–
(i) NO3–
Skill based
8.(a) Three solutions A, B and C are tested with blue litmus paper. A
showed no change of colour; B changed the litmus to red; C also showed
no change of colour.
Which of the following statements are true and which false?
False
(i) Solutions A and C are bases. (ii)Solution B is acidic. True
(iii) Solution A is neutral but C is acidic. False
(iv)All the three solutions A, B and C are basic. False
(b) A piece of cloth with haldi stains was dipped in two solutions—A and
B. The stains on the cloth showed a change of colour to reddish in A
but not in B. Which of the following statement is correct?
(i) Solution A is basic.
(ii) Solution B is acidic.
(iii) Both the solutions A and B are neutral.
(iv) None of the statements is correct. True
(c) A solution has no effect on litmus solution but it conducts electricity.
Is it an acid or a base?
Ans:
It is a salt solution.
9. Write A for acidic, B for basic and N for neutral.
Toothpaste Distilled water N Shampoo B
Sugar solution Toilet cleaner A
Mouth wash Coca cola A
Soap solution B
N
B
B
10. State the colours of the indicators below in acidic and basic solutions.
Acidic
Basic
Indicator
Litmus
Red
Blue
PhenolphthaleinColourless Pink
Methyl orange Red
Yellow
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Multiple choice
11. Choose the correct answers.
(a) Choose the correct statement from the following.
(ii) Alkalis are soluble bases.
(b) What is the formula of the compound formed by the neutralization
of calcium hydroxide by hydrofluoric acid?
(ii) CaF2
(c) Which is true in the case of sugar solution?
(iv) It does not conduct electricity.
(d)In a neutralization reaction, cations of an acid react with anions of a
base to produce
(i) salt (e) Which compound liberates ammonia gas when heated with aqueous
sodium hydroxide?
(ii) Ammonium chloride
(f) Which of the following has oxalic acid in it?
(iii) Tomato (g)Which gas is released when acids react with metal carbonates or
bicarbonates?
(i) Carbon dioxide (h)Zinc reacts with sulphuric acid to produce a salt, and a gas is evolved.
Name the gas.
(ii) H2
(i) Vitamin C is
(iii) ascorbic acid (j) Sodium hydroxide (NaOH) is a
(i) diacidic base (ii) monoacidic base ()
(iii) triacidic base (iv) nonacidic base
(k) The pH of a solution is 8.2. It is a
(i) base ()
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5. Useful Metals and Nonmetals
Subjective
1. Answer the following questions.
(a) What are the advantages of bronze over copper?
Ans:
When copper is alloyed with tin, bronze is formed. Bronze is
much harder than copper.
(b)Does copper also rust like iron?
Ans: Copper tarnishes to dark brown before it turns green. Rust is
formed by the oxidation of iron. The iron reacts with oxygen
in moist air to form rust (Fe2O3.xH2O).
When copper is exposed to the atmosphere, it oxidizes and the bright
copper surface takes on a dull tan tarnish. After a few years this
tarnish gradually changes to dark brown or black. At a later stage
the corrosion products of copper turn green due to the formation of
copper sulfate, carbonate and chloride salts in varying concentrations. (c) List three advantages of making alloys giving examples.
Ans: Alloys are made by mixing metals in molten state. The original
properties of a metal are changed when it is alloyed with
another metal. For example, copper is a useful metal but it
is soft and not suitable for making coins and other objects.
To make it hard and resistant to corrosion, it is alloyed with
tin and the alloy formed is known as bronze. Bronze is much
harder than copper. Iron is alloyed with chromium and nickel
to yield stainless steel which is corrosion resistant and strong.
Aluminium alloys like Duralumin and Magnalium are light, strong
and corrosion resistant.
(d)Why is diamond so expensive?
Ans: Diamond owes its economic value to its hardness and great
brilliance.
(e) Why cannot we make ornaments with pure gold? What do we do to
solve this problem?
Ans:
Pure gold is very soft and so ornaments cannot be made from
it. Copper, silver or both are added to make it stronger and
shape it into ornaments.
(f) Which properties of iron make it an important building material?
Ans: Iron is the world’s cheapest and most useful metal. It is hard
and durable. Hence, iron is one of the most important building
materials. It is used in the construction of buildings, bridges,
dams and other structures.
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(g) Why is zinc used for galvanizing iron?
Ans:
Zinc is largely used to protect iron from rusting. The iron sheets
or wares, after cleaning, are dipped in molten zinc to form a
thin coating of zinc. This process is called galvanization, and
the product is galvanized iron. Galvanized iron sheets in
corrugated form are used in roofing. These sheets are also used
in making trunks.
(h)Why is aluminium used for making aeroplanes and not iron or steel?
Ans: Aluminium is a lightweight, silver-coloured metal that can be
made into almost any shape. Unlike iron aluminium does not
rust, and it resists wear from weather and chemicals. Hence
aluminium is used for making aeroplanes and not iron or steel.
(i) Why is mercury used in thermometers?
Ans:
Mercury is a silver-coloured metal. Unlike other metals, it is
a liquid at room temperature. On being heated and cooled
respectively, mercury expands and contracts evenly. It also
remains liquid over a wide range of temperatures and does
not stick to the glass. These properties have prompted its use
in thermometers.
(j) What is the importance of iodine in our body? What problems can
its deficiency cause in our body?
Ans:
In the human body, the thyroid gland located in the neck, uses
iodine to produce a hormone called thyroxin. This hormone
controls the rate of physical and mental development of a body.
Iodine deficiency can hinder growth and can also cause goitre,
an enlargement of the thyroid gland. To prevent the deficiency
of iodine, we should use iodized salt in our food.
(k)Why do we use helium and not hydrogen to fill weather balloons?
Ans:
Helium is a very light noble gas. It is used to fill weather balloons.
Though hydrogen weighs less than helium but it is not used to
fill weather balloon because unlike hydrogen catches fire easily.
(l) What are the advantages of stainless steel over iron?
Ans:
Iron is alloyed with chromium and nickel to yield stainless steel.
Unlike iron stainless steel is corrosion resistant. Moreover it is
stronger than iron.
Objective
2. Fill in the blanks.
(a) Metals have a natural shine.
(b) Metals are generally solid at room temperature.
(c) Property of a substance to be drawn into wire is called ductility.
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(d) Metals are good conductors of electricity.
(e) Alloys are the homogeneous mixture of metals and nonmetals.
(f) The property of a substance to be drawn into thin sheets is called
maleability.
(g) 22 carat gold means it contains 22 parts of gold and 2 parts of copper
or silver.
(h) Stainless steel has mostly iron mixed with chromium and nickel.
(i) Argon is used with nitrogen to fill electric bulbs.
3. Answer in one word.
Iodine
(a) Deficiency of which element causes goitre? Silver
(b) Which metal is used in photography?
Sodium
(c) Which metal can be cut into pieces?
Magnesium
(d) Which metal is used in fireworks?
(e) Which metal is used to construct communication towers? Iron
Chlorine
(f) Which element is used to purify water?
(g) Name a nonmetal which is a good conductor of electricity. Graphite
Magnalium
(h) Which alloy is used for making aeroplane bodies?
Bronze
(i) Which alloy is used for making statues?
(j) Which metal is used in galvanization of articles made of iron?
Zinc
(k) Which gas along with nitrogen is filled in electric bulbs? Argon
(l) Which gas is essential for synthesis of proteins in living organisms? Nitrogen
(m) Which metal is used in making of thermometers? Mercury
(n) Which gas is filled in weather balloons? Helium
(o) Name the nonmetal that is the hardest known substance. Diamond
Skill based
4. On burning a piece of charcoal, a gas evolves. How is it possible to
know whether the gas is acidic or basic in nature?
Ans: On burning a piece of coal, a gas is evolved. If we hold a wet litmus
paper in the gas, we will find it turn red. The gas is carbon dioxide
which when comes in contact with wet litmus turns it red. This
shows that carbon dioxide forms acid when it reacts with water.
5. Why cannot we store pickles in metal containers?
Ans: Metals react with acids and bases to give respective salt and liberate
hydrogen. Pickles being acidic will react with metal. Hence they
should not be stored in metal containers.
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6. Why do we use copper and not silver or graphite for electrical wiring
in our houses?
Though both silver and copper are good conductors of electricity,
copper is preferred over silver to be extensively used in electrical
wiring and electrical transformers. This is because it is inexpensive.
Multiple choice
7. Choose the correct answers.
(a) Which metal is used in electronic industry for making printed circuit
boards?
(iv) Copper
(b) Which metal vapours are used in fluorescent bulbs and tubes?
(iii) Mercury
(c) Which metal is used to protect from exposure to radioactive radiation?
(i) Lead
(d) Which elements are present in stainless steel?
(iii) Iron, nickel, chromium
(e) Which elements are present in bronze?
(ii) Copper and tin
(f) Which elements are present in brass?
(i) Copper and zinc
(g)Which alloy is used for making surgical instruments?
(iii) Stainless steel
(h) Which element is filled in food cans to stop bacterial growth?
(iii) Nitrogen
(i) Which is the odd one out in the following?
(iv) Aluminium
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6. Useful Compounds and Mixtures
Subjective
1. Answer the following questions.
(a) Write a short note on the uses of fertilizers.
Ans: Fertilizers are added to the soil to increase its fertility and
encourage a healthy growth of plants. Some fertilizers are made
from organic wastes such as manure or sewage.
Many others are manufactured from certain minerals or synthetic
compounds produced in factories. Mineral fertilizers supply three
main elements to the soil—nitrogen, phosphorus and potassium
(NPK) — that are essential for plant growth.
(b)How does copper sulphate act as a pesticide?
Ans: Blue vitriol is hydrated copper sulphate (CuSO4.5H2O). It
is used in large quantities as a fungicide and as a timber
preservative.
(c) Why is natural gas considered a clean fuel?
The main constituent of natural gas is methane (CH4). It burns easily
and does not produce smoke. Hence natural gas considered a clean
fuel.
(d) Write short notes on:
(i) Borosilicate glass
Borosilicate glass is a heat-shock resistant glass, better known
by such trade names as Pyrex and Klimax. It contains roughly
80 per cent silica, 4 per cent sodium oxide, 2 per cent alumina
and 13 per cent boric oxide.
Such glass is about 3 times heat-shock resistant as soda lime
glass and is excellent for use in chemical and electrical purposes.
Products such as ovenware and glass pipelines are made from
this glass.
(ii) Laminated safety glass
Laminated safety glass is made by combining alternate layers
of plastic material and flat glass. The outside layer of glass
may break when struck by a hard object but the plastic layer is
elastic and so it stretches. The plastic holds the broken pieces
of glass together and keeps them from flying in all directions.
Laminated glass is used where broken glass might cause serious
injuries, as in car windscreens.
(iii) Photochromatic glass
Photochromatic glass has the property to get darker when
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exposed to sunrays and to restore its colour in the absence of
sunlight. Photochromatic glass is used for windows, sunglasses
and instrument controls.
(iv) Fibre glass
Fibre glass is so made that each fibre is a fine but solid rod of
glass, often less than one-twentieth the width of a human hair.
These tiny strands can be loosely packed together in wool-like
mass for heat insulation. They can also be used like wool or
cotton fibres to make glass yarn, tape, cloth and mats. Fibre
glass is used for electrical insulation, chemical filtration and
firefighters’ suits. Combined with plastic, fibre glass is used to
make car bodies. Fibre glass is also a popular curtain material
because it is washable and fire-resistant.
(e) How are plastics important in our life?
Plastics are man-made materials that can be shaped into almost any
form. They are one of the most useful materials ever created. Plastics
that are as rigid as steel or as soft as cotton have been developed.
Plastics are used in making several objects ranging from car bumpers
to squeezable bottles to soft fabrics.
Some advantages of plastic material like polythene over natural
materials are:
• It is cheap and can be produced on a large scale.
• It is unbreakable, corrosion free, tough and flexible.
• It is lightweight and can be easily transported.
• It can be shaped as desired.
(f) How are detergents different from soaps?
Detergents and soaps clean soiled materials. Detergents have certain
advantages over soaps. The most important being their ability to clean
effectively in hard water. Hard water contains certain minerals and
most soap cannot be used to launder in it. These soaps react with the
minerals and form soap curd which does not dissolve. It becomes
difficult to remove it from fabrics and other surfaces. Detergents do
not leave such deposits. Also, they penetrate soiled areas better than
soaps do. In addition, detergents dissolve more readily in cold water.
(g) What do you understand by
(i) Pigment—Paint consists of one finely grounded substances
called pigments. Pigments determine the colour of the paint
and provide it with certain other properties as well.
(ii) Paint vehicle Paint consists of one or more material called paint
vehicle. A paint vehicle carries the pigment and binds it to the
surface. Paint vehicles are composed of one or more resins and
a solvent.
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(iii) Paint thinner
A paint thinner is an ingredient that turns the paint liquid. It is
also called Solvent.
(h) Name two medicinal plants. Write the medicine extracted from each
and purpose for which it is used.
Quinine is a bitter substance obtained from the bark of the Cinchona
tree. It is used to treat malaria because it has fever-reducing, antimalarial, painkilling and anti-inflammatory properties.
The stem of turmeric in powdered form is used with milk for early
recovery of fractures and a remedy for throat infection.
Chemical reactions in our body take place in solutions. Explain.
For a reaction to occur, molecules must collide. Collision happens
better in a fluid medium.
2. Differentiate between:
(a)Limestone and Plaster of Paris
Limestone is commonly found in marine environments, and
they are classified as sedimentary rocks. Marine water receives
calcium from land, and shells of sea animals, coral, skeletal
structures of sea animals, etc. When these are accumulated in
the form of calcite they form limestone. Some of the common
limestones are chalk, coquina, lithographic limestone, oolitic
limestone, fossiliferous limestone, tufa, etc. Limestone
is commonly used as an ingredient for cement and glass
manufacturing, hence an essential construction material. Plaster of Paris is a white powder which when mixed with
water forms a paste that turns hard in a few minutes. The paste
is used for surgical casts, for beautifying false ceilings and
for many other purposes. Plaster of Paris is made by heating
gypsum (CaSO4.2H2O), a stone composed of calcium sulphate
and water.
(b)
Antibiotics and analgesics
Antibiotics are chemical substances produced by certain
microorganisms. Antibiotics are generally obtained from
bacteria and fungi. Most antibiotics are used to fight various
diseases caused by harmful microorganisms. A few are used to
fight certain cancers. Antibiotics are also used to treat infectious
diseases in animals.
Analgesics are drugs used to relieve pain without causing
unconsciousness. Aspirin, a mild analgesic, relieves headache
and muscle pain. A more powerful analgesic is codeine which
is generally prescribed to reduce severe pain caused by injuries
and serious burns.
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(c) Thermoplastics and thermosetting plastics
Thermoplastic can be rolled into thin sheets or moulded into
desired shapes. It softens on heating but regains its properties
on cooling. This property allows it to be moulded into any shape
without loss of quality. Polythene is a thermoplastic. Polythene
bags are used for packaging milk, refined oil and many other
items. Polythene is also used as a waterproofing material.
Thermosetting plastics are a class of plastics that do not soften
on heating once set. They retain the original shape in which they
were moulded. They are harder and stiffer than thermoplastics.
They are good electrical insulators. Bakelite, a thermosetting
plastic, is used for making plugs, switches, telephone cases and
electrical fittings.
(d)Silicon and silicone
Silicon is a chemical element with the symbol Si and atomic
number 14. It is a tetravalent metalloid. Over 90% of the
Earth’s crust is composed of silicate minerals, making silicon
the second most abundant element in the Earth’s crust (about
28% by mass) after oxygen. Elemental silicon also has a
large impact on the modern world economy. Silicon is used
in the steel refining, aluminium-casting, and fine chemical
industries. Silicon is used in semiconductor electronics—in the
manufacture of integrated circuits, transistors and solar cells.
Hence, the basis of most computers, a great deal of modern
technology depends on silicon.
Silicone is a polymer that includes silicon together
with carbon, hydrogen, oxygen, and sometimes other elements.
Typically heat-resistant and rubber-like they have numerous
industrial uses.
(i) Nonstick pans are coated with silicones.
(ii) Silicone is sometimes used to protect electronic components
from environmental influence.
(iii) Silicone oils and greases serve as permanent lubricants.
(iv) Silicone rubber is used to seal oven doors and tank engines.
Objective
3. Give one word for each of the following.
(a) Commonly used gaseous fuel: Natural gas
(b) Material added to cement to stabilize it: Gypsum
(c) Type of glass used in eyeglasses, cameras, telescopes and microscopes:
Optical glass
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(d) A common painkiller derived from plants: Seeds of opium poppy
(e) Gas present in soda water: Carbon dioxide
(f) A nutrient that provides us energy: Carbohydrate
(g) A metal that exists in liquid state at room temperature: Mercury
(h) A nonmetal that is a good conductor of electricity: Graphite
(i) A substance that is added to soil to help plants grow: Fetilizer
(j) A nutrient that helps us grow: Protein
4. (a) Give the chemical names of the compounds with the following
common names:
: CaSO4. H2O
(i) Plaster of Paris
(ii) Bleaching powder : CaOCl2
(iii) Baking soda
: NaHCO3
(iv) Washing soda
: Na2CO3.10H2O
: CaSO4.2H2O
(v) Gypsum (vi) Epsom : MgSO4·7H2O
(vii) Blue vitriol
: CuSO4.5H2O
(viii) Green vitriol : FeSO4.7H2O
(ix) Chile saltpeter
: NaNO3
(x) Lime : CaO
(xi) Limestone : CaCO3
(xii) Potash alum
: K2Al2 (SO4)4·24(H2O)
(xiii) Slaked lime : Ca(OH)2
(xiv) Common salt
: NaCl
(xv) Milk of magnesia : Mg(OH)2
(b) Classify the following substances as fertilizers and pesticides:
Fertilizers: ammonium nitrate, urea, ammonium phosphate, potassium nitrate
Pesticides: sulphur powder, BHC, melathine, copper sulphate
(c) Give the name of one substance for each of the following type.
(i) Fertilizer: Urea (NH2.CO.NH2)
(ii) Pesticide: Blue vitriol is hydrated copper sulphate (CuSO4.5H2O)
(iii) Antacid: Digene
(iv) Analgesic : Aspirin
(v) Synthetic fibre: Rayon
(vi) Thermosetting plastic: Bakelite
(d) Give a name for the substance which
(i) Relieves body pain : Analgesics
(ii) Kills harmful microorganisms in the body : Antibiotics
(iii) Protects our body from diseases: Vaccines
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(e) Give names of
(i) the class of organic compounds present in petrol, LPG and CNG
: Hydrocarbons
(ii) the main constituent of bio gas: methane
(iii) three different varieties of glass: Soda lead glass, Soda lime glass,
Borosilicate glass
(iv) two substances used in making of soaps : oils or fats and sodium
hydroxide
(v) two substances in making of Portland cement : Limestone or
Calcium carbonate (CaCO3) and Clay or silica (SiO2)
(vi) main constituent of glass: silica (or silicon dioxide), soda ash
(sodium carbonate) and limestone (calcium carbonate)
Skill based
5. Plastic is a wonderful invention, yet problematic. Comment.
6. Can you substitute thermosetting plastics with thermoplastics or do the
reverse. Give reasons for your answer.
7. Can we use soaps in place of detergents or do the reverse. Give reasons
for your answer.
Multiple choice
8. Choose the correct answer.
(a) Which of the following is not a synthetic fibre?
(ii) cotton
(b) The main constituent of natural gas is:
(iii) methane
(c) When gypsum is heated, what is left behind?
(iv) Plaster of Paris
(d)Which gas mixed with nitrogen is filled in electric bulbs?
(i) argon
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7. Water
Exercises
Subjective
1. Answer the following questions.
(a) Water is a compound. Justify.
Ans:
A compound is formed when two or more elements combine
together in a fixed ratio. Water is a chemical compound
composed of hydrogen and oxygen. Chemical formula of
water is H2O. This implies, a water molecule has one atom of
oxygen bound to two atoms of hydrogen. Water has none of the
characteristics of either Oxygen or Hydrogen. All this shows
that water is a compound.
(b)What is the difference between surface water and underground
water?
Ans:
Surface water is present on the surface of the Earth in three
different forms: (i) Rainwater (ii) River and lake water
(iii) Seawater
Some of the rainwater percolates through the top layer of the Earth
and collects above the solid impervious layer of rocks as underground
water. Underground water can be drawn by wells or tubewells.
Sometimes it comes up naturally to the surface in the form of springs.
(c) Write briefly about the significance of water for living organisms.
Ans:
Water is essential for the survival of all living organisms. Plants
use water to prepare food by the process of photosynthesis.
Minerals dissolved in water are transported to all parts of a
plant. Aquatic plants and animals use oxygen and nutrients
dissolved in water for their survival. Water is also essential for
germination of seeds. Our body is about 70 per cent water by
weight. Water is essential for digestion of food and circulation
of digested food by the blood to different parts of the body. All
chemical reactions in living organisms take place in the solution
form, i.e., substances involved in the reactions are dissolved in
water.
(d)What is potable water? What are its characteristics?
Ans:
Water used for drinking purposes is termed potable water.
Potable water should be pure and clean. It should be colourless,
odourless and free from harmful bacteria. Potable water should be
free from suspended particles and any disease-causing germs.
(e) What is the importance of aeration during purification of water?
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Ans: During aeration germ free water is exposed to air or sprayed
through fountains. Oxygen from air dissolves in water.
Dissolved oxygen helps further purification of water by killing
some types of bacteria that may have survived chlorination.
This also removes unwanted odour of water.
(f) Define eutrophication.
Ans: Eutrophication is a process in which excessive richness of
nutrients in water bodies, frequently due to run-off from the
land, causes a dense growth of plant life. Most of these nutrients
come from fertilizers draining off farmlands or from detergents
or sewage. These nutrients fertilize weeds. As a result more
algae and small plants respire and grow. They also die and
decay. All these processes consume oxygen present in the water
body. In extreme cases, the river or lake becomes so eutrophic
that all oxygen dissolved in water is consumed so that no life
survives in the water body.
(g)Explain anomalous expansion of water.
Ans:
When water changes into ice, the density of ice is about 9/10 that
of water. Thus ice, the solid form of water, is lighter than water.
This is contrary to the general observation that a substance in
the solid state is heavier than that in the liquid state. This is
the anomalous property of water. Ice is less dense than water,
therefore it floats on water.
(h)What will happen if clothes are washed with hard water?
Ans: Hard water is the water that has high dissolved mineral content,
primarily calcium and magnesium cations (Ca2+, Mg2+) and
sometimes other dissolved compounds such as bicarbonates
and sulphates. Hard water can make cleaning more difficult
and can reduce the lifespan of household appliances. Soaps
and detergents do not form lather in hard water; rather it forms
thick scum. A large amount of soap is required in washing of
clothes in hard water.
(i) Describe chemical pollution of water.
Ans: Industries discharge a large amount of harmful chemicals as
waste. Much of this chemical waste is discharged directly into
water bodies in the vicinity. Rainwater flowing from farmlands
into streams carries off chemical fertilizers and pesticides
causing chemical pollution of water. This upsets various
physical and biological processes occurring naturally in water.
(j) How does addition of washing soda soften hard water?
Ans:
Permanent hardness of water can be removed by using washing
soda (sodium carbonate–Na2CO3.10H2O). On adding a solution
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of washing soda to water, the chlorides and sulphates dissolved
in water precipitate as carbonates. These precipitates can be
removed by filtration to obtain soft water.
(k) Write a short note on thermal pollution.
Ans:
Some industries utilize large quantities of water to cool
equipment being used in the factories. Heat from the equipment
makes the water hot. When this hot water is discharged into a
river or lake, it causes thermal pollution. Rise in the temperature
of water causes harm to plant and animal life. Thermal pollution
also reduces the amount of dissolved oxygen in water.
(l) List four measures to control water pollution.
Ans: The following steps can help control water pollution:
i. Industrial waste should be treated properly before it is released
into rivers and lakes. Harmful chemical substances should be
converted into harmless compounds before release.
ii.Purification of rivers, streams, ponds and lakes should be a
joint effort taken up by industries and the government. One
such example is the Ganga Purification Project.
iii.Sewage in all towns and cities should be treated properly before
it is released into water bodies.
iv.Fertilizers, insecticides, detergents, etc., should not be used in
excess.
2. Differentiate between:
(a) Freezing point and boiling point of water
Pure water freezes at 0°C and changes its state from liquid (water) to
solid (ice). Ice melts at 0°C. Hence, 0°C is referred to as the freezing
point of water or the melting point of ice.
Boiling point: The boiling point of pure water is 100°C. When water
is heated, its temperature begins to rise and gradually water starts
boiling.
(b) Temporary hardness and permanent hardness
Water containing calcium bicarbonate and magnesium bicarbonate
has temporary hardness. These salts can be removed from water by
simple methods such as boiling and filtration to obtain soft water.
The water which contains dissolved calcium chloride, calcium
sulphate, magnesium chloride and magnesium sulphate has
permanent hardness.
Permanent hardness of water can be removed by using washing soda,
caustic soda or by ion exchange method.
(c) Chemical pollution and thermal pollution
When this hot water is discharged into a river or lake, it causes
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thermal pollution. Rise in the temperature of water causes harm to
plant and animal life. Thermal pollution also reduces the amount of
dissolved oxygen in water.
When much of this chemical waste is discharged directly into water
bodies from industries and farmlands it causes chemical pollution
of water. This upsets various physical and biological processes
occurring naturally in water. These natural processes, which use
oxygen dissolved in water, help to turn wastes harmless.
(d)Chlorination and aeration
Chlorination is a procees of making the water germ free. Chlorine
or bleaching powder (a disinfectant) when added to filtered water
kills bacteria. This treatment helps to prevent waterborne diseases
like cholera, typhoid and dysentery.
In the process of purification of water Chlorination is followed
by Aeratio. In this process the germ free water is exposed to air or
sprayed through fountains. Oxygen from air dissolves in water, and
dissolved oxygen helps in further purification of water by killing
some types of bacteria that may have survived chlorination. This
also removes unwanted odour of water.
3. Complete the following equations:
(a)
MgCO3 + H2 O + CO2  Ca(HCO3)2
(b) Ca(HCO3)2 heat CaCO3 + CO2 + H2O
(c) CaCl2 + Na2CO3  CaCO3 + 2NaCl
(d) MgSO4 + Na2CO3  MgCO3 + Na2SO4
Objective
4. Fill in the blanks.
(a) Water has taste due to the presence of soluble salts.
(b) Water freezes at 0°C and ice melts at the same temperature.
(c) At 0°C, the density of ice is 0.9g/cc.
(d) Permanent hardness of water is due to dissolved chlorides and
sulphates of calcium and magnesium.
(e) Removal of hardness of water is called softening.
(f) Temporary hardness of water can be removed by boiling.
(g) At high altitudes, water boils at higher temperature.
(h) The chemical reactions in living organisms take place dissolved in
water.
(i) Plants need water for the process of photosynthesis.
5. Write True or False for the following statements. Also rewrite the false statements correctly.
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(a) Water contracts (volume reduces) when it freezes. False
Actually, water expands (gets less dense) when it freezes.
(b) Water has a high surface tension. True
(c) Condensation is water coming out of air. True
(d) More substances can be dissolved in milk than in water. False
More substances can be dissolved in water than in milk.
(e) Rainwater is the purest form of water. True
(f) Changing of water vapour into liquid is called evaporation. False
Changing of water vapour into liquid is called condensation.
(g) Hard water is generally harmful to health. False
Hard water is generally not harmful to health.
(h) Water freezes at 32°C and boils at 212°C at sea level. False
Water freezes at 32°F and boils at 212°F at sea level.
(i) Pure water has a pH of 7, which is neither acidic nor basic. True
(j) Water boils quicker at Himalayas than at plains. True
Skill based
(a)(i) Ice melts and changes to water when it gains heat.
(ii) Water evaporates and changes to water vapour when it gains heat.
(iii) Water boils and changes to steam when it is heated to its boiling
point.
(iv) Water freezes and changes to ice when it is cooled to 0°C.
(v) Water vapour condenses and changes to water when it is cooled.
(b) A beaker of boiling water is being heated continuously. A fall in
temperature of water would be caused by:
(i) stirring the water
(ii) adding some ice to the water
(iii) pouring tap water into the beaker
(iv) removing some boiling water from the beaker
Tick () the correct choice.
(ii) and (iii) only
Multiple choice
7. Choose the correct answers.
(a)Which of the following is the purest form of water?
(iii) rainwater (b) Which of the following processes are not a part of purification of
water at the waterworks?
(iv) boiling
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(c) The boiling point of water is
(ii) 100°C
(d) Water has the least volume and maximum density at
(iii) 4°C (e) Temporary hardness of water is caused due to dissolved calcium
bicarbonate and
(iii) magnesium bicarbonate
(f) By which of the following processes can permanent hardness of water
be removed?
(i) adding washing soda (g)The density of ice is __________ water, therefore it floats on water.
(ii) less than
(h)Water that does not form lather with soap easily is
(i) hard water
(i) The chief sources of water pollution are
(i) industrial waste
(ii) sewage
(iii) agricultural chemicals
(iv) all of these
(j) Which salts cause permanent hardness in water?
(iii) calcium and magnesium sulphates
(k) What percentage of water is present in human body?
(ii) 70%
(l) How much Earth’s surface is covered with water?
(iv) 75%
(m) Desalination is a process that
(iii) removes salt from sea water
(n) Which of the following processes is used for desalination?
(iv) Reverse osmosis
(o) Which substance is used for softening the hard water?
(ii) Sodium carbonate
(p)Washing soda is
(iii) Sodium carbonate
(q) Temporary hardness in water is caused by
(i) bicarbonates of calcium and magnesium
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