Download Magic of Chemical Reactions 2. - mt

2. Magic of Chemical Reactions
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(A) Fill in the blanks :
Ice changing to water is an example of ....................... change.
When copper powder is heated, ....................... is formed.
The substances that undergo changes are called as ....................... .
Reactants are represented on ....................... .
Products are represented on ....................... .
Conversion of oil to fats requires ....................... as catalyst.
When vegetable oil is mixed with nickel as a catalyst, and heat it with
hydrogen gas ....................... are obtained on cooling.
The simple form of representation of chemical reaction in words is called
....................... .
If reactants or products are present as solution in excess of water, the
symbol used is ....................... .
According to ......................., the total mass of reactants is equal to the
total mass of products during a chemical reaction.
The chemical formula of POP is ....................... .
The chemical reaction during which hydrogen is lost is called as
....................... .
Corrosion can be prevented by using ....................... .
When acids and alkalis react together, ....................... and .......................
is formed.
The chemical reaction in which heat is evolved is called as .......................
reaction.
When oil and fats are oxidized or even allowed to stand for a long time,
they become ....................... .
Answers :
1. physical
2. copper oxide
3. reactants
4. left hand side
5. right hand side
6. Nickel
7. fats
8. word equation
9. aq
10. law of conservation of mass
11. (CaSO 4) 2.H 2O
12. oxidation reaction
13. anti-rust solution, coating surface by paint,
process like galvanizing and electro-plating
14. salt and water
15. exothermic reaction
16. rancid
Q.I
1.
(i)
(ii)
(iii)
(iv)
Ans.
(B) Match the following :
Column I
Column II
Reactants
Products
Fe + S
(a) NaCl + H2O
(b) 2CuO
CuSO4 + Zn
2Cu + O2
(c) ZnSO4 + Cu
HCl + NaOH
(d) FeS
(i - d, 4), (ii - c, 3), (iii - b, 1), (iv - a, 2).
2.
(i)
(ii)
(iii)
(iv)
Ans.
Column I
Gain of oxygen
a)
Digestion
(b)
Iodine, Camphor
(c)
NaOH + HCl  NaCl + H2O
(d)
(i - b), (ii - a), (iii - d), (iv - c).
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Column III
Types of reactions
Oxidation
Neutralization
Displacement
Combination
Column II
Chemical change
Oxidation
Balanced equation
Sublimatory substances
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Column I
Copper
Coper Sulphate
Silver chloride
Copper oxide
(i - d), (ii - b), (iii - c), (iv - a).
(a)
(b)
(c)
(d)
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Column II
Black colour
Peacock blue
White precipitate
Reddish brown
(C) State whether the following statements are true or false. If false
write the corrected statement :
Chemical change is a temporary change
False. It is a permanent change.
Camphor sublimes on heating.
True.
(g) indicates the physical state as solid.
False. (g) indicates the physical state as gas.
Conversion of quick lime to slaked lime is an example of displacement
reaction.
False. It is an example of combination reaction.
When left over edible oil is heated, its smell turns foul and develops
rancidity.
True.
(D) Find the odd man out :
Cu, Fe, Mn, CuSO4.
CuSO4 as it is a compound and rest all are elements.
Conversion of milk to curd, Rusting of iron, Digestion of food, Water turns
into ice.
Water turns into ice as rest all are examples of chemical change.
Action of heat on calcium carbonate, Action of light on silver bromide,
Action of light on silver chloride, Action of sodium hydroxide on hydrochloric
acid.
Action of sodium hydroxide on hydrochloric acid as it is a neutralization
reaction and rest are examples of decomposition reaction.
Fe + S  FeS, NH3 + HCl  NH4Cl, H2 + Cl2  HCl, CaCO3  CaO + CO2.
CaCO3  CaO + CO2, this is decomposition reaction and rest all combination
reaction.
, (g), (l), =.
=. This sign is not used in chemical equation.
SO2 = sulphur dioxide, H 2S = hydrogen sulphide, K 2CrO4 = potassium
dichromate, NaNO3 = sodium nitrate.
K2CrO4 as it is potassium chromate and not potassium dichromate.
Photosynthesis, Digestion, Respiration, Magnetization of iron nail.
Magnetisation of iron nail as it is a physical change and rest are chemical
change.
(E) Write the correlated terms :
Solid to liquid : Melting :: ....................... : Sublimation.
Solid to gas.
Reactants : Breaking of bonds :: Products: ....................... .
Formation of bond.
Physical change : Reversible :: Chemical change : ....................... .
Irreversible.
BaSO4 : ....................... :: Copper sulphide : Black precipitate.
White precipitate.
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(F) Classify the following into physical
Rusting of iron
2.
Copper sulphate dissolved in water
4.
Ripening of mango
6.
Magnetisation of iron nail
8.
Decaying of vegetables
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Production of energy in body.
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change and chemical change :
Water turns into ice
Bending of iron nail
Digestion of food
Formation of compound
Formation of blood
Physical change
Water turns into ice
1.
Copper sulphate dissolved in water 2.
Bending of iron nail
3.
Magnetisation of iron nail
4.
5.
6.
7.
Chemical change
Rusting of iron
Ripening of mango
Digestion of food
Formation of compound
Decaying of vegetables
Formation of blood
Production of energy in body
(G) Classify the following into exothermic and endothermic reactions :
Quick lime is added to water
2. Potassium chlorate is heated
Sodium dissolved in water
4. Mercuric oxide is strongly heated
Neutralization of NaOH and HCl 6. Calcium carbonate is strongly heated
Electrolysis of water
8. Zinc reacts with dil HCl
Ans.
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Exothermic reactions
Endothermic reactions
Quick lime is added to water. 1. Potassium chlorate is heated.
Sodium dissolved in water.
2. Mercuric oxide is strongly heated.
Neutralization of NaOH and HCl. 3. Calcium carbonate is strongly heated.
Zinc reacts with dil HCl.
4. Electrolysis of water.
Define the following :
Reactants.
Substances undergoing a chemical change are called as reactants.
Products.
The new substance formed as a result of chemical reaction is called as
product.
Word equation.
The simple form of representation of chemical reaction in words is word
equation.
Chemical equation.
The representation of chemical reaction with the help of chemical formulae
is a chemical equation.
Combination reaction.
When two or more substances combine (mixture or compounds) to form a
single product, then the reaction is called as combination reaction.
Displacement reaction.
When a more reactive element removes another element, having less
reactivity, from its compound, this reaction is called as displacement reaction.
MEMORISE :
Reactivity series- Arrangement of metals in a vertical column in the order
of decreasing reactivities is called reactivity series of metals. Here the
most reactive metal is placed on top and less reactive is placed at the
bottom.So, potassium is the most reactive metal and gold is least reactive.
– Extra Information.
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Decomposition reaction.
When a single reactant has broken down to give a simple product is known
as decomposition reaction.
Double displacement reaction.
The reaction where a precipitate is formed by exchange of ions between
the reactants is called as double displacement reaction. OR
A chemical reaction in which constituents (radicals or ions)of two substances
mutually exchange their places to form two new substances is called as
double displacement reaction.
Redox reaction.
When oxidation and reduction takes place simultaneously in a given
chemical reaction, it is called as redox reaction.
Oxidation reaction.
The chemical reaction in which reactants gain oxygen to form corresponding
oxide. Similarly
A chemical reaction in which reactants lose hydrogen to form product is
called oxidation.
Reduction reaction.
The chemical reaction in which reactants gain hydrogen are reduction
reactions. Similarly
The reaction in which reactants lose oxygen atom to form product is called
reduction.
Neutralization reaction.
The reaction between acid and alkali to form salt and water is called as
neutralization reaction.
Corrosion.
The slow process of decay or destruction of metal due to the effect of air,
moisture, acids on it is called as corrosion.
Exothermic reaction.
The reaction in which heat is evolved is called as exothermic reaction.
Endothermic reaction.
The reaction in which heat is absorbed is called as endothermic reaction.
Balanced equation.
When number of atoms are same on the L.H.S. and R.H.S. of the chemical
equation, then the equation is said to be a balanced equation.
Q.III Give scientific reasons :
*1. Grills of doors and windows are always painted before they are used.
Ans. 1. Railings, iron bridges, grills of doors and windows, railway coaches,
steel furniture, and bodies of truck, cars, buses are made of iron.
2. To protect them from rusting, a coating of paint is applied. By doing
this, air and moisture cannot come in contact with the iron object and
hence rusting doesn’t takes place.
So, grills of doors and windows are always painted before they are used.
*2. Physical states of reactants and products are mentioned while writing
a chemical equation.
Ans. 1. The equation that gives more information about the chemical reaction
is known as more informative or information giving equation.
2. The chemical equation can be made more informative in three ways :
(a) By indicating the physical states of reactants and products.
(b) By indicating the heat changes taking place in the equation.
(c) By indicating the conditions under which the reaction takes place.
So, physical states of reactants and products are mentioned while
writing a chemical equation.
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Potassium ferrocyanide is stored in dark coloured bottles and kept away
from sunlight.
1. Potassium ferrocyanide, (molecular formula is K 4[Fe(CN)6].3H2O) is a
yellow coloured compound.
2. If this compound gets exposed to sunlight then it will undergo
decomposition reaction.
3. Therefore, potassium ferrocyanide is stored in dark coloured bottles
and kept away from sunlight.
Iron articles rust readily whereas steel which is also made of iron will
not undergo corrosion.
1. Iron is a widely used metal but if it used in the pure form, it may rust.
2. When a small amount of carbon (varying from 0.1% to 1.5% )is mixed
with iron, we get an alloy called steel.
3. This alloy of iron is hard and strong and is used for making nails,
screw, bridges, railway lines, ships, construction of buildings, vehicles.
4. An important property of alloy is that they resist corrosion.
So, iron articles rust readily whereas steel which is also made of iron
will not undergo corrosion.
MEMORISE :
An alloy is a homogeneous mixture of two or more metals( or a metal and
small amount of non-metal). If iron metal is alloyed with other metals
like chromium and nickel, we get stainless steel that does not rust at all.
– Extra Information.
Edible oil is not allowed to stand for a long time in an iron or tin container.
1. Fats and edible oil when allowed to stand for a longer time in iron or
tin container becomes rancid.
2. The condition produced by oxidation of fats and oils in food marked by
unpleasant smell and taste is called rancidity. When the oil is heated,
it starts frothing and smells foul.
3. If the food is cooked in rancid oil, it gives an unpleasant smell and
taste, making it unfit for consumption.
So, edible oil is not allowed to stand for a long time in an iron or tin
container.
Rusting of iron is a chemical change.
1. When iron is exposed to air, it gets oxidized by oxygen in air in the
presence of moisture to form reddish brown coloured hydrated iron
oxide. This process is called rusting of iron. The number of water
molecules in the rust varies.
4Fe (s)
+
3O 2(g)
+
2xH2O(l) 
2Fe 2 O 3 .H 2O (s)
Iron
Oxygen
Water
Hydrated iron oxide
2. The chemical properties of iron oxide are different from iron and oxygen.
3. We cannot obtain back iron from rust(iron oxide) by simple method i.e.
change is irreversible.
Hence rusting of iron is a chemical change.
Chemical change is always associated with exchange of energy.
1. Chemical reaction or changes may take place spontaneously by bringing
reactants in contact. This is usually accompanied by release of heat.
2. During a chemical reaction, the bonds between the atoms of the
reactants break. This is made possible by bringing reactants in contact
and then by supplying energy like heat, electricity or light. As a result,
products are formed.
Thus, a chemical reaction is always associated with an exchange of
energy.
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Conversion of ice into water is a physical change.
1. On heating, ice melts into water and on the other hand when water is
cooled, it freezes into ice.
2. In this change, only the physical property of water i.e state of water
changes as both ice and water chemically are water only. Therefore,
no new substance is formed.
Hence, conversion of ice into water is a physical change.
Formation of a compound is a chemical change.
1. Compound is a substance produced by the chemical reaction between
two or more elements in a fixed proportion.
2. Both physical and chemical properties of the compound are different
from the properties of its constituent elements.
3. A new substance is formed, which can not be converted into original
substance.
Hence formation of a compound is a chemical change.
Silver salts are kept in dark coloured bottles.
1. A chemical reaction that proceeds by absorption of light is called a
photochemical reaction.
2. Eg. When silver bromide is exposed to light, yellow colour of silver
bromide changes to grey colour.
Silver bromide is decomposed to form grey silver metal and bromine
gas is liberated.
Sunlight
+
Br 2(g)
2AgBr(s)

 2Ag(s)
Silver bromide
Silver metal
Bromine gas.
So, Silver salts are kept in dark coloured bottles.
Q.IV
1.
Ans.
Distinguish between :
Physical change and Chemical change.
2.
Ans.
Exothermic reaction and Endothermic reacion.
Exothermic reaction
Endothermic reaction
1. The reaction where heat energy 1. The reaction where heat energy
is evolved or liberated is called
is absorbed is called as
as exothermic reaction.
endothermic reaction.
2. Temperature of the reaction 2. Temperature of the reaction
mixture rises.
mixture lowers.
3. Generally, the reactions are fast. 3. Generally, the reactions are slow.
4. Products are more stable.
4. Reactants are more stable.
5. These reactions take place at 5. These reactions take place at
lower temperatures.
high temperatures.
6. Eg.: Neutralization reaction.
6. Eg.: Calcium is strongly heated.
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Physical change
1. In this change, the composition
of substance does not change and
no new substance is formed.
2. There is a change
only in
physical properties.
3. This change is temporary.
4. The original substance can be
recovered by simple means.
5. Eg.: Steam is condensed.
Chemical change
1. In this change, the composition
of substance changes and a new
substance is formed.
2. There is a change in both physical
and chemical properties.
3. This change is permanent.
4. The original substance cannot be
recovered by simple means.
5. Eg.: Ripening of fruit, milk is
converted to curd.
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Answer the following in detail :
What do you observe when H2S gas is passed through cadmium chloride
solution, name the type of reaction?
When hydrogen sulphide gas is passed through cadmium chloride solution,
it gives a yellow precipitate of cadmium sulphide and hydrochloric acid.
CdCl 2(aq)
+ H 2S (g)
 CdS(l) + 2HCl(aq)
It is a double displacement reaction.
What do you understand by the term redox reaction? Explain with one
example.
1. When oxidation and reduction takes place simultaneously in a chemical
reaction, it is termed as redox reaction.
2. Eg.: When sulphur dioxide reacts with hydrogen sulphide, it forms water
and sulphur.
SO 2 + 2H2S
 2H2O + 3S 
In this reaction, SO2 is changing to S. This is removal of oxygen which
is called as reduction. So, SO2 is reduced to S. H2S is changing to H2O.
This is addition of oxygen which is called as oxidation. So, H2S is oxidized
to H2O.
What is corrosion? Do gold ornaments corrode, if not why?
1. The slow process of decay or destruction of metal due to effect of air,
moisture, acids on it is called as corrosion.
2. Gold is a yellow shining metal. Gold metal does not corrode because it
is a highly unreactive metal that remains unaffected by air, water
vapour, and other gases in the atmosphere. Gold does not tarnish and
retains its luster for years. Since gold does not corrode, gold ornaments
look new after years. break.
3. This is achieved by bringing reactants in contact with each other and
then by supplying energy in the form of either heat, light or electricity.
4. This results in breaking of bonds in the reactants and rearrangement
of atoms of reactants and formation of new bonds in the product.
5. Thus, a chemical reaction is always associated with exchange of energy.
State the neutralization.
1. The reaction between acid and base to form salt and water is called as
neutralization reaction.
Acid
+ Base
 Salt
+ Water
2. When an acid is treated with base, the base neutralizes the acid and
destroys its acidity. Since an acid and base neutralize each other's
effect, it is called as neutralization reaction.
3. Eg.: When hydrochloric acid reacts with sodium hydroxide, then a
neutralization reaction takes place to give salt and water.
NaOH(aq)+ HCl (aq)  NaCl (aq) + H2O (l)
MEMORISE :
We can carry out this reaction in the laboratory and phenolphthalein is
used as an indicator.
– Extra Information.
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What are the steps of writing a chemical equation?
1. The symbols or molecular formulae of the reactants are written on the
left hand side and products are on the right hand side.
2. Reactants and products are connected with an arrow () pointing
towards product side.
3. Whenever there are two or more reactants, a plus (+) sign is written
between each of them. Similarly, if there are two or more products, a
plus sign is written between them.
4. Certain reactions have to be carried out under specific condition such
as temperature, pressure, catalyst. These condition are mentioned on
the arrow pointing from reactants to product.
5. The physical states of reactants and products are also mentioned in a
chemical equation to make it more informative.
6. The notations like g,l,s,aq are written in brackets as subscripts along
with symbols/formulae of reactants and products.
 or (g) - Gas
(l)
- Liquid
(s)
- Solid
(aq)
- Aqueous solution

- Precipitate.
Take finely powdered zinc and allow it to react with CuSO4. Then take
zinc granules and carry out the same reaction. Which reaction takes
place faster ?
1. The reaction is as follows :
When zinc reacts with copper sulphate solution to give a water soluble
compound zinc sulphate and reddish brown deposit of copper.
+ CuSO4(aq)
 ZnSO 4(aq)
+ Cu (s)
Zn (s)
Zinc
Copper sulphate
Zinc sulphate
Copper
2. In solid reactants, the rate of a reaction depends upon size of particles.
Smaller the size of particles, quicker is the reaction. This is because,
in case of large solid reactants, only the molecules on surface are
available for reaction. But if the reactants are finely powdered, the
total surface area on all the smaller particles become much more and
hence more number of molecules become available for reaction.
3. So, the reaction takes place faster with powdered zinc than zinc granules.
We feel fresh while on a morning walk in natural surroundings. Why
don’t we feel the same after 10 a.m. and during rush hours ? Write a
chemical reaction to support your statement.
1. During early morning more amount of O 2 is available for cellular
respiration. So, glucose is completely oxidise to release energy.
C 6 H 12 O 6 + O 2  CO 2 + H 2 O + Energy (38 ATP)
2. As the day progresses the level of O2 decreases as compare to early
morning this is due emission of other gases under this condition glucose
releases only 2 ATP (energy) molecules.
3. Since, more amount of energy is released in presence of O2 and less
amount of energy is released due to insufficient oxygen.
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
Q.VI
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3.
Ans.
MEMORISE :
The rate of reaction is change in concentration of reactants and products
in unit time. The rate of reaction also depends on :
Nature of reactants, Size of particles, Concentration of reactants,
Temperature and effect of catalyst.
A catalyst is used to bring about a chemical reaction at low temperature
and at a faster rate that reduces the production cost and increases
– Extra Information.
profitability.
Answer the following questions in one sentence each :
What are the different types of reactions?
There are four different types of reactions that are :
1. Combination reaction
2.
Displacement reaction
3. Decomposition reaction
4.
Double displacement reaction.
Carbohydrates like rice, potato, sago etc are major sources of energy in
our diet. During digestion, these are broken down to glucose. Glucose
combines with oxygen in our body and provides energy.
C 6H 12O 6(aq) + 6O 2(g)
 6CO 2(g) + 6H2O(l) + Energy
1. Is this an exothermic reaction?
Since energy is released, it is an exothermic reaction
When edible oil is converted to fats, what reaction is it? Express it in
words.
1. Conversion of edible oils to fats is called as hydrogenation reaction.
2. When H2 gas is passed through edible oil in presence of Nickel which
acts as a catalyst at high temperature. It gets converted into fats, this
reaction is called as hydrogentation reaction.
Edible oil
4.
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5.
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+
Hydrogen gas
Ni/


Fats
What was the colour of precipitate formed ?
When you add potassium chromate (K2CrO4) to barium sulphate solution
(BaSO 4).
+ BaSO4
 BaCrO4
+ K 2SO 4
K 2 CrO 4
Potassium
Barium
Barium
Potassium
chromate
sulphate
chromate
sulphate
Name the precipitate.
Barium chromate (BaCrO4) which is yellow in colour.
Write balanced chemical equation.
K 2 CrO 4
+ BaSO4
 BaCrO4
+ K 2SO 4
According to you, is it a displacement reaction or double displacment
reacion ?
It is a double displacement reaction.
Q.VII Balance the following chemical reactions with the help of balanced
equations :
+ Cl 2
 HCl
1. H2
+ Cl 2
 2HCl
Ans. H 2
2. NH 3
+ HCl
 NH4Cl
Ans. The equation is already balanced, so it is a self balanced equation.
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ZnS
2ZnS
+
+
Ans.
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

ZnO
2ZnO
+
+
SO2
2SO2 
Heat
CaCO3 
CaO
+ CO2
The equation is already balanced, so it is a self balanced equation.
H2S
+ O2
 H2O
+ SO2
2H2S + 3O2
 2H2O + 2SO2 
AgNO3 + H2S
 HNO3 + Ag 2S
2AgNO3 + H 2 S
 2HNO 3 + Ag2S 
NaOH + CO2
 Na2CO3 + H2O
2NaOH + CO 2
 Na 2CO 3 + H 2 O
Identify the following reactions as combination, decomposition,
displacement, double displacement reaction.
1. H2 + Cl 2
 2HCl
Combination reaction.
Heat
2Hg
+

Thermal decomposition reaction.
3. NH 3
+ HCl
 NH4Cl
Combination reaction.
4. BaCl2 + H2SO4  BaSO4
Double displacement reaction.
5. Zn
+ 2HCl
 ZnCl2
Displacement reaction.
6. CuSO4 + Fe
 FeSO4
Displacement reaction.
2. 2HgO
Ans.
O2
3O2
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O2 
+
2HCl
+
H2 
+
Cu
Heat
7. 2KClO 3 
2KCl
+
Thermal decomposition reaction.
8. 2KI
+ Cl 2
 2KCl
+
Displacement reaction.
9. C
+ O2
 CO2 
Combination reaction.
10.CaO
+ H2O
 Ca(OH)2 +
Combination reaction.
3O2 
I2
heat
MEMORISE :
To get the molecular formula of a compound:
Eg. : Sodium chloride
Symbol
Na
Cl
Valency
1
1
Criss cross Na 1
Cl 1
i.e. NaCl
– Extra Information.
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Write the formulae of the following :
Name of compound
Potassium chromate
Iron sulphide
Copper oxide
Hydrogen sulphide
Sulphur dioxide
Carbonic acid
Plaster of Paris
Potassium nitrate
Calcium carbonate
Quick lime or calcium oxide
Calcium hydroxide or slaked lime
Silber bromide
Calcium sulphide
Barium sulphate
Ethyl alcohol
Methane
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SCIENCE & TECHNOLOGY
Formula
K 2 CrO 4
FeS
CuO
H2S
SO 2
H 2CO 3
(CaSO 4) 2.H 2O
KNO 3
CaCO3
CaO
Ca(OH) 2
AgBr
CaS
BaSO4
C 2H 5OH
CH 4
1. Sulphur dioxide reacts with hydrogen sulphide
When sulphur dioxide reacts with hydrogen sulphide to give sulphur and
water.
SO 2(g)
+ 2H 2S (g)

3S (g)
+ 2H2O(l)
Sulphur dioxide
Hydrogen sulphide Sulphur
Water
2. Copper is exposed to air.
When copper is exposed to air, copper gets oxidized to form black coloured
copper oxide.
2Cu(s) + O 2(g)
 2CuO(s)
Copper
Oxygen
Copper oxide
3. Iron sulphide reacts with sulphuric acid.
When iron sulphide reacts with sulphuric acid to give ferrous sulphate
and hydrogen sulphide gas is liberated.
FeS (s) +
H 2SO 4(aq)
 FeSO 4(aq)
+
H 2S (g)
Ferrous sulphide Sulphuric acid Ferrous sulphate Hydrogen sulphide
4. Iron reacts with sulphur.
When iron reacts(heated) with sulphur to give iron sulphide.
  FeS (s)
Fe (s)
+ S (s)

Iron
Sulphur
Iron sulphide
5. Plaster of Paris is mixed with water.
When Plaster of Paris is mixed with proper quantity of water, it shows a
remarkable property of setting into a hard mass called gypsum.
(CaSO4)2, H2O + 3H2O  2(CaSO4, 2H2O) + Heat energy
Plaster of Paris
Gypsum
6. Cement with sand and gravel reacts with water at new construction
sites.
Cement with sand and gravel reacts with water to form concrete that
imparts strength to the building. This is an exothermic reaction.
3CaO.Al2O3
+ 6H2O(l)  3CaO.Al2O3.6H2O + Heat
Tricalcium aluminate
Concrete
7. Calcium oxide (quick lime) is mixed with water.
When calcium oxide reacts with vigorously with water to form calcium
hydroxide(slaked lime). Since a large amount of energy is released, it is
an exothermic reaction.
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CaO (s)
+
H2O (l)
 Ca(OH) 2
+ Heat
Calcium oxide
Water
Calcium hydroxide
8. Potassium nitrate is mixed with water.
When potassium nitrate is mixed with water, potassium nitrate dissolves
in water. There is absorption of heat (endothermic reaction) and so the
temperature of the solution falls.
KNO 3(s)
+
H2O (l)
 KNO3(aq)
+ Heat
Potassium nitrate Water
Potassium nitrate
9. Sodium hydroxide is mixed with water.
When sodium hydroxide is mixed with water, sodium hydroxide dissolves
in water. Since heat is liberated (exothermic), the temperature of the
solution rises.
NaOH (s)
+
H2O (l)
 NaOH(aq)
+ Heat
Sodium hydroxide Water
Sodium hydroxide
10.Glucose combines with oxygen during the process of respiration.
When glucose combines with oxygen in the cells of our body during the
process of respiration to form carbon-dioxide and water along with the
production of energy.
Since energy is released, it is an exothermic reaction.
C 6H 12O 6(aq) +
6O 2(g)
 6CO 2(g)
+ 6H2O(l) + Energy
Glucose
Oxygen
Carbon dioxide Water
11.Calcium carbonate is heated.
When calcium carbonate is heated, it decomposes to form calcium oxide
(quick lime) and carbondioxide gas. As heat is absorbed, it is an endothermic
reaction. Since the decomposition is brought by heat, it is also called as
thermal decomposition.
Heat 
CaCO 3(s)
CaO (s)
+ CO 2

Calcium carbonate
Calcium oxide Carbon dioxide
12.Sugar (sucrose) is heated strongly.
When sucrose is heated strongly, it breaks down to give black coloured
carbon and water. As heat is absorbed, it is an endothermic reaction.
Since the decomposition is brought by heat, it is a thermal decomposition.
Heat 
C 12 H 22 O 11(s) 
12C (s) + 11H 2O(g)
Sugar
Carbon
Water
13.Silver bromide is exposed to light.
When silver bromide is exposed to light, yellow colour of silver bromide
changes to grey colour.
Silver bromide is decomposed to form grey silver metal and bromine gas is
liberated.
This reaction is used in black and white photography during the process of
developing.
Sunlight
2AgBr(s)
+ Br 2(g)

 2Ag(s)
Silver bromide
Silver metal
Bromine gas
14.Silver chloride is exposed to light.
When silver chloride is exposed to light, white colour of silver chloride
changes to grey colour.
Silver bromide is decomposed to form grey silver metal and chloride gas is
liberated.
This reaction is used in black and white photography during the process of
developing.
Sunlight
2AgCl(s)
+ Cl 2(g)

 2Ag(s)
Silver chloride
Silver metal
Bromine gas
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15.Calcium sulphide reacts with dilute hydrochloric acid.
When calcium sulphide reacts with dilute hydrochloric acid to give calcium
chloride and hydrogen sulphide gas is liberated.
CaS (s)
+
2HCl(l)

CaCl 2(s)
+
H2S(g)
Calcium sulphide Hydrochloric acid Calcium chloride Hydrogen sulphide
16.Zinc reacts with copper sulphate.
When zinc reacts with copper sulphate, zinc being more reactive than
copper displaces copper to give colourless zinc sulphate and reddish brown
deposit of copper. So, it is a displacement reaction. The blue colour of
copper sulphate fades due to formation of colourless zinc sulphate.
CuSO 4(s)
+ Zn (s)
 ZnSO 4(aq)
+ Cu (s)
Copper sulphate
Zinc
Zinc sulphate Copper
17.Iron reacts with copper sulphate.
When iron reacts with copper sulphate, iron being more reactive than
copper displaces copper to give light green colored solution of iron sulphate
and reddish brown deposit of copper. So, it is a displacement reaction. The
blue colour of copper sulphate fades due to formation of light green solution
of zinc sulphate.
CuSO 4(s)
+ Fe (s)
 FeSO 4(aq)
+ Cu (s)
Copper sulphate
Iron
Iron sulphate Copper
18.Copper chloride reacts with potassium iodide.
When copper chloride reacts with potassium iodide, it gives a brown coloured
precipitate of cupric iodide and potassium chloride.
This reaction is a double displacement reaction.
2CuCl2(aq)
+ 2KI(aq)

CuI2  +
2KCl
Copper chloride
Potassium iodide
Cuppric iodide Potassium chloride
19.Silver nitrate reacts with sodium chloride solution.
When silver nitrate solution reacts with sodium chloride solution it gives a
white precipitate of silver chloride and sodium nitrate.
This reaction is a double displacement reaction.
AgNO3(aq)
+ NaCl (aq)

AgCl 
+ NaNO3(aq)
Silver nitrate
Sodium chloride
Silver chloride Sodium nitrate
20.Barium sulphide reacts with zinc sulphate solution.
When barium sulphate reacts with zinc sulphate solution, it gives a white
precipitate of barium sulphate and zinc sulphide.
This reaction is a double displacement reaction.
BaS(aq)
+
ZnSO 4(aq)

BaSO4 
+
ZnS (aq)
Barium sulphide
Zinc sulphate
Barium sulphate
Zinc sulphide
21.Aluminium is burnt in air.
When aluminium is burnt in air, it gets oxidized to form aluminium oxide
or alumina.
4Al (s)
+
3O 2(g)
 2Al 2O 3(s)
Aluminium oxygen
Aluminium oxide
22.Sodium metal reacts with ethyl alcohol.
When sodium metal reacts with ethyl alcohol to give sodium ethoxide and
hydrogen gas is liberated. The hydrogen atom of the hydroxyl (OH)group is
replaced by a sodium atom, forming sodium ethoxide.
2C 2H5OH(l) +
2Na (s)  2C 2H 5ONa(g)
+ H2 
Ethyl alcohol
Sodium
Sodium ethoxide
Hydrogen
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MEMORISE :
Sodium ethoxide is a strong base. It is commercially available as a dry
yellow solid, or as a solution in ethanol.
– Extra Information.
23.Hydrogen gas is passed over carbon.
When hydrogen gas is passed over carbon, it gives methane.
+ 2H 2(g)
 CH 4(g)
C (s)
Carbon
Hydrogen
Methane
24.Hydrogen gas is passed over copper oxide.
When hydrogen gas is passed over copper oxide, it gives a pinkish brown
coating of copper.
CuO(s)
+ H 2(g)
 Cu (s)
+ H2O (l)
Copper oxide
Hydrogen
Copper
Water
25.Ozone is subjected to heat and light.
When ozone is subjected to heat and light, it gives oxygen gas and nascent
oxygen.
Light, 
O2
+ [O]
O 3 

Ozone
Oxygen
Nascent oxygen
MEMORISE :
Nascent oxygen is a single oxygen atom, a short-lived free radical of oxygen.
Stratospheric ozone protects us from harmful ultra-violet radiation from
the Sun. However, in the troposphere, ozone is dangerous to our health.
Its not true that we don't want any ozone in the troposphere. We need low
levels in the air to form hydroxyl radicals (OH) to clean the air of harmful
chemicals. However, high concentrations of ozone are harmful to our
health and damage plants. The highest levels of ozone in the atmosphere
are in the stratosphere, in a region also known as the ozone layer between
about 10 km and 50 km above the surface (or between about 6 and 31
miles). Here it filters out photons with shorter wavelengths (less than
320 nm) of ultraviolet light, also called UV rays, (270 to 400 nm) from the
Sun that would be harmful to most forms of life in large doses.
– Extra Information.
26.Barium sulphate reacts with carbon.
When barium sulphate reacts with carbon, barium sulphate gets reduced
to barium sulphide and carbon gets oxidized to carbon monoxide.
BaSO4(aq)
+
4C (s)
 BaS 
+
4CO(g)
Barium sulphate Carbon
Barium sulphide
Carbon monoxide
27.When an iron rod is exposed to air.
When an iron rod is exposed to air, it gets oxidized by oxygen in air in the
presence of moisture to form reddish brown coloured hydrated iron oxide.
This process is called rusting of iron. The number of water molecules in
the rust varies.
4Fe (s) + 3O 2(g)
+ 2xH2O(l)
 2Fe 2O 3.xH 2O (s)
Iron
Oxygen
Water
Hydrated iron oxide
28.Sodium hydroxide reacts with hydrochloric acid.
When sodium hydroxide reacts with hydrochloric acid, it undergoes
neutralization reaction to give sodium chloride and water.
NaOH(aq)
+
HCl (aq)

NaCl (aq)
+
H2O (l)
Sodium hydroxide Hydrochloric acid Sodium chloride
Water
Base
Acid
Salt
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Positively charged radicals or
basic radicals or cations
Monovalent cations
H+ Hydrogen ion
Na+ Sodium ion
K+ Potassium ion
Ag+ Silver ion
NH4+ Ammonium ion
Cu+ Cuprous ion (I)
Divalent cations
Mg+2 Magnesium ion
Ca+2 Calcium ion
Cu
+2
Cupric ion (II)
Fe+2 Ferrous ion (II)
Zn+2 Zinc iron
Negatively charged radicals or acidic
radicals or anions
F– Fluoride ion
Cl– Chloride ion
Br– Bromide ion
I– Iodide ion
OH– hydroxide ion
HCO3 bicarbonate ion
Divalent anions
O–2 Oxide ion
CO32
Carbonate ion
SO42 sulphate ion
S–2 sulphide ion
Trivalent cations
Trivalent anions
Al Aluminium ion
Fe+3 Ferric ion (III)
N–3 nitride ion
PO43 phosphate ion
Cr+3 chromium ion
BO43 borate ion
+3
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ACTIVITY BASED QUESTIONS
ACTIVITY : 2.1
Q.
Take a table spoon full of copper powder in a china dish. Heat it strongly.
What changes do you observe ?
ACTIVITY : 2.2
Q.
Take a small piece of aluminium wire and burn it. Record your
observations. Remove a tungsten wire from a fused bulb. Try to ignite
it. What do you observe ?
ACTIVITY : 2.3
Q.
Add potassium chromate (K2CrO4) to barium sulphate (BaSO4) solution.
Note your observations.
ACTIVITY : 2.4
Q.
Add a small quantity of dil.H2SO4 to FeS very carefully, (under supervision
of your teacher). Touch the test tube. Also experience the odour. What
do you observe ?
ACTIVITY : 2.5
Q.
Ans.
Heat camphor in a porcelain dish. Keep a funnel plugged with cotton
over the dish before you heat. What do you observe when camphor
burns ? What happens when the apparatus is cooled ?
Complete the table with reference to the activities (2.1 to 2.5).
Activity
Change in
colour
Evolution of
gas
Change in
Change in
temperature
state
2.1
Black
Yes
Yes
No
2.2
Black
Yes
Yes
Comparous
powder
2.3
Black
Hydrogen
sulphide
No
Yes
2.4
Substances
Black
Yes
Yes
2.5
Sublime
Carbon smoke
ACTIVITY : 2.6
Q.
Ans.
40
Take a small piece of used paper. Weight it on an electronic balance
which will be readily available in your nearby grocery shop. Burn the
piece of paper and now weigh the ash. What do you observe about the
initial and final weighs? What can you infer ?
Suppose the weight of small piece of used paper is 2 gms. After burning the
piece of paper the weight of ash remains the same i.e. 2 gms.
So, we observe that the weight remains the same. This is due to law of
conservation of mass.
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The total mass of the reactant is equal to the total mass of the products
during a chemical reaction. Therefore chemical equation needs to be balanced.
ACTIVITY : 2.6
Q.
Ans.
Rehana and Reshma were having fun with colours in their drawing
class. Rehana mixed few drops of red colour with white colour and got
pink colour. Reshma mixed yellow with red colour to get orange shade.
They also prepared violet by mixing red and blue and grey by mixing
black and white colours. When you mix “VIBGYOR” colours, what colour
do you get ?
When VIBGYOR colours are mixed you get one colour that is white if the
colours are faint as in rainbow.
ACTIVITY : 2.8
Q.
Ans.
Take a small quantity of detergent powder or washing soda in your
hand and add a little water to it. What do you feel ? Is there any change
in temperature when water is added to detergent ?
When we add water to detergent powder or washing soda in our hands, it is
slippery to touch. Yes, there is change in temperature, temperature increases.
ACTIVITY : 2.9
Q.
Ans.
Take 100 ml of distilled water in two polythene bottles. (This prevent
heat loss.) Note the temperature of water in both bottles. Add about 5
gm of potassium nitrate (KNO3) to one bottle stir well. Note temperature
of the solution. Add 5 gm of NaOH to the other bottle. Note the solution
temperature. What do you observe in both the cases ? What inferences
can you draw ?
At the start, temperature of distilled water in two bottles was suppose
60ºC. When we add KNO3 to one of the bottles, temperature of solution
falls down to 50ºC, this is because there is absorption of heat during the
reaction.
When we add NaOH to other bottle, there is rise in temperature as there
is evolution of heat during this reaction.
ACTIVITY : 2.10
Q.
Ans.
Marry was helping her mother to make brown sugar for pudding. After
sometime she obtained a black mass instead of brown with burning
smell. What do you think might have happened ?
Brown sugar is made by heating the sugar, but due to excessive heating
the sugar burnt to form black coloured carbon.
ACTIVITY : 2.11
Q.
Ans.
Take a spatula full of silver bromide (Your teacher will help you get it.)
on a watch glass. Note its colour. Place the watch glass in sunlight.
Observe the colour after some time.
When we take silver bromide on a watch glass its colour is pale yellow.
When we place this watch glass in sunlight the colour of silver bromide
turns grey.
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ACTIVITY : 2.12
Q.
Ans.
Take two completely used cells out of a wall clock, transistor, tape
recorder, etc. Remove the material from inside. You will get a can that
is made of zinc. Press and make it flat. Dip the zinc strip in copper
sulphate solution (A). Let there be copper sulphate solution in another
beaker B for comparison. Keep another plain zinc strip also for
comparison. Remove the zinc strip after about 5 minutes. Compare
1. Intensity of blue colour of CuSO4 in beaker A and beaker B.
2. The colour of the zinc strips.
1. Intensity of blue colour of CuSO4 in beaker A fades away and intensity
of blue colour of CuSO4 in beaker B remains the same.
2. The colour of the zinc strip turns brown.
ACTIVITY : 2.14
Q.
Ans.
During festivals all of you have fun while bursting crackers. When
crackers burn, you will find an ash residue after they are burnt. What
reaction has taken place ?
The reaction which has taken place while burning crackers is oxidation
reaction. Crackers are made up of phosphorus and magnesium powder
which gives the sparkling light. On burning they form an ash consisting of
oxides P4O10, MgO and Mg3N2.
ACTIVITY : 2.15
Q.
Ans.
Have you ever peeped into the store room of your home ? If you have,
you must have seen some old and out of use articles made of iron. They
are usually covered with a reddish layer. Do you know what red layer is
called ?
The reddish brown colour is deposited over the surface of iron is because
of oxidation i.e. formation of Fe2O3.H2O.
ACTIVITY : 2.16
Q.
Ans.
42
After you have your dinner tonight, wash your own plate with soap/
detergent. What colour change is observed when soap/detergent is
applied ? Can you name the type of reaction ?
The yellow oily left over stains turn red/orange because of neutralization.
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QUESTIONS & ANSWERS (TEXBOOK PAGE NO. 13)
1.
(a) Name the reactants and products in equation.
C12H 22O 11(s) + H 2O(aq)
Ans.
dil.HCl

323 K, alcoholic
In the given reaction :
Reactants :
C 12 H 22 O 11(s) +
(Sugar)
Products :
C 6 H 12 O 6(s) +
Glucose
C6H12O6 + C6H12O6
H 2O (aq)
Water
C 6 H 12 O 6(s)
Fructose
(b) What is the name of the process in equation ?
n(H2C = CH2)
Ans.
high temp. ( )


high pressure
 H

 |
 C 
 |
 H

H 
| 
C 
| 
H 

n
Ethylene
Polyethylene
The name of the process is POLYMERIZATION.
QUESTIONS & ANSWERS (TEXBOOK PAGE NO. 16)
1.
Ans.
2.
Ans.
Paste of substance ‘A’ is used to decorate the ceiling of your home.
(a) Name the substance A and write its formula.
(b) Write the reaction of substance A with H2O. State the name of the
product (B).
(c) Where and how is ‘B’ useful ?
(a) The substance A is plaster of Paris and its formula is 2 CaSO4.H2O.
(b) The reaction of A with H2O is
2CaSO 4.H2O + 3H2O  2CaSO4.2H2O
Plaster of Paris
Gypsum (B) + Heat
(c) Gypsum is the raw material used in manufacturing cement.
Will it be possible for you to decompose water by heat or light energy ?
If you pass current from a 6 volt battery is decomposition of water
possible ?
If you take pure water it is not possible to decompose water by heat or light
energy or using a 6 volt battery. But if you add a few drops of acid to water
then it is possible to decompose acidulated water by passing current from
a 6 volt battery.
QUESTIONS & ANSWERS (TEXBOOK PAGE NO. 19)
1.
Ans.
When copper articles are exposed to light and moisture. Do they undergo
corrosion ?
Copper oxidises to form black copper oxide. Further copper oxide slowly
reacts with CO2 in air to form copper carbonate which is green in colour.
Thus, copper articles are exposed to light and moisture, they undergo
corrosion to form the green coating on its surface.
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2.
Ans.
What happens to silver ornaments ?
Exposure to light has no action on silver ornaments. But if the air is
contaminated with H2S, the silver is blackened due to the formation of a
thin film of black silver sulphide (Ag2S) on the surface. Thus, black coating
is formed on silver ornaments.
3.
What according to you is the reason for aluminium utensils regaining
their original shine when green leafy vegetables are boiled in them ?
Aluminium utensils become dull after a long time due to oxidation of Al to
Aluminium oxide (Al 2 O 3 ). Leafy vegetables contain chlorophyll and
chlorophyll has a capacity to produce reducing agent. When leafy vegetables
are boiled in dull aluminium utensils, Al 2O3 is reduced to Al and thus
regain their original shine.
Ans.
4.
Ans.
5.
Ans.
6.
Ans.
7.
Ans.
44
Visit an oil merchant, request him to give you 100 ml of left over edible
oil from the tin. Heat it in a container. Note your observation.
When leftover edible oil is heated, it undergoes oxidation and starts frothing
up and its smell turns foul.
Find out the names of some natural antioxidants. Now a days many
antioxidants are available in pharmacies. Can you list some brand
names ?
Antioxidants are the compounds which prevent the oxidation or deterioration
of food. Naturally occurring many fruits contain antioxidants such as apple,
strawberry, etc., dry fruits like walnut, almonds, etc., vegetable like
capsicum, papaya, tomato, carrots, etc. -carotene and lycopene are natural
antioxidants.
Many antioxidants are available in pharmacies which contain BHA
(Butylated hydroxy anisole), BHT (Butylated hydroxy toluene).
Kurkure, chips, fryums, etc. are stored in air tight containers. Is that
enough to prevent oxidation ?
No. It helps to slow down oxidation.
What more steps are taken to prevent oxidation of edible articles ?
To prevent oxidation of the edible articles, the bags used to wrap the edible
articles are flushed with an inert gas such as nitrogen.
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HOTS QUESTIONS & ANSWERS
1.
Ans.
2.
Ans.
3.
Ans.
4.
Ans.
Meena and Rakesh cleaned utensils used for cooking with the help of
ash and soap respectively. Who must have got the best results ? Why ?
Rakesh must have got the best results after cleaning the utensils by scap.
Edible oils are used while cooking. They are organic compounds made up
of alcohols and carboxylic acids. They are neutralized by alkaline soaps.
Ash cannot bring this kind of result.
When washing soda is rubbed on wet palms warmness is experienced.
Is the reaction exothermic or enothermic ? Why ?
When washing soda dissolves in water there is evolution of heat which we
feel on our palm, it is exothermic reaction.
Imagine you are preparing halwa for the first time and you forget to
turn off the gas after the preparation was over. What would happen ?
Why ?
The sugar in the halwa get overheated and breakdown to give a simple
product C + H2O. The halwa would turn in to black mass with burning
smell.
When copper articles are exposed to light and moisture. Do they undergo
corrosion ?
(a) Copper oxidises to form black copper oxide.
(b) Further copper oxide slowly reacts with CO 2 in air to form copper
carbonate which is green in colour.
(c) Thus, copper articles are exposed to light and moisture. They undergo
corrosion to form the green coating on its surface.
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GLOSSARY
Galvanizing
Electro-plating
Ester
Carboxylic acid
Alcohol
Gravel
Sago
Precipitate
Nascent oxygen
Catalyst
Aqueous solution
Voltameter
Ions
Oxidizing agent
Reducing agent
Formula
Symbol
: Process of depositing a thin layer of zinc metal on iron
objects to prevent corrosion.
: Electroplating is a plating process in which metal ions in a
solution are moved by an electric field to coat an electrode.
: Esters are usually derived from an inorganic acid or organic
acid in which at least one -OH (hydroxyl) group is replaced
by an -O-alkyl (alkoxy) group, and most commonly from
carboxylic acids and alcohols.
: Carboxylic acids are organic acids characterized by the
presence of at least one carboxyl group. The general formula
of a carboxylic acid is R-COOH, where R is some monovalent
functional group.
: In chemistry, an alcohol is any organic compound in which
a hydroxyl functional group (-OH) is bound to a carbon
atom, usually connected to other carbon or hydrogen atoms.
: Gravel is a rock that is of a specific particle size range.
Specifically, it is any loose rock that is larger than 2 mm
(0.079 in) in its smallest dimension (about 1/12 of an inch)
and no more than 64 mm (2.5 in).
: Sago is a starch extracted from the pith of sago palm stems.
: Solid insoluble substance sometimes produced when 2
solutions are mixed or a gas is passed into a solution
: It is a single oxygen atom, a short-lived free radical of
oxygen.
: A substance whose mere presence affects the rate of
chemical reaction without undergoing itself any chemical
change.
: Solution in which solvent is water.
: A scientific instrument used for measuring quantity of
electricity. It should not be confused with a voltmeter which
measures electric potential. An alternative name is
coulometer.
: An atom or molecule in which the total number of electrons
is not equal to the total number of protons, giving it a net
positive or negative electrical charge.
: The substance that gives oxygen or removes hydrogen for
oxidation.
: The substance that gives hydrogen or removes oxygen for
reduction.
: Symbolic representation of molecule of a substance.
: Abbreviation of full name of an element.
AMAZING FACTS
 Each time lightning strikes, some Ozone gas is produced, thus
strengthening the Ozone Layer in the Earth's atmosphere.
 Hot water freezes faster than cold water (the Mpemba effect).

46
SCHOOL SECTION
S.S.C.
Marks : 30
CHAPTER 2 : MAGIC OF CHEMICAL REACTIONS
Duration : 1 hr.
SCIENCE
Q.I
1.
2.
3.
4.
Q.I
1.
2.
3.
4.
Q.I
1.
[A] Fill in the blanks :
The chemical reaction during which hydrogen is lost is called as
....................... .
The chemical reaction in which heat is evolved is called as
....................... reaction.
Products are represented on ....................... .
When copper powder is heated, ....................... is formed.
2
[B] Match the columns :
Column A
Gain of oxygen
Digestion
Iodine, Camphor
NaOH + HCl  NaCl + H2O
2
(a)
(b)
(c)
(d)
Column B
Chemical change
Oxidation
Balanced equation
Sublimatory substances
[C] State whether True or False :
Conversion of quick lime to slaked lime is an example of displacement
reaction.
Chemical change is a temporary change.
1
Q.I
1.
2.
[D] Find odd man out :
, (g), (l), =.
Conversion of milk to curd, Rusting of iron, Digestion of food, Water
turns into ice.
1
Q.II
1.
[A] Give scientific reasons : (Any Two)
Grills of doors and windows are always painted before they are
used.
Edible oil is not allowed to stand for a long time in an iron or tin
container.
Conversion of ice into water is a physical change.
4
[B] Distinguish between :
Physical change and Chemical change.
2
2.
2.
3.
Q.II
1.
... 2 ...
Q.III
1.
2.
3.
4.
[A] Balance the following chemical reaction : (Any Three)
S + H2SO4  H2O + SO2
NH4Cl + Ca(OH)2  CaCl2 + H2O + NH3
Fe2O3 + CO  Fe + CO2
ZnS + O2  ZnO + SO2
3
Q.III
1.
2.
3.
[B] Identify the following reaction :
Heat  2Hg + O 
2HgO 
2
CuSO4 + Fe  FeSO4 + Cu
CaO + H2O  Ca(OH)2 + heat
3
Q.IV Explain the following chemical reactions with the help of
balanced equation : (Any Three)
1.
Calcium carbonate is heated.
2.
Silver bromide is exposed to light.
3.
Iron reacts with copper sulphate.
4.
Silver nitrate reacts with sodium chloride solution.
6
Q.V
1.
[A] Answer the following in detail : (Any One)
What is the importance of a chemical equation? OR
What information is conveyed by a chemical equation, give an
example.
State the neutralization.
4
[B] Define the following :
Redox reaction.
Corrosion.
2
2.
Q.V
1.
2.
Best Of Luck 