Download cbse class – x science solutions

CBSE CLASS – X
SCIENCE SOLUTIONS
(CBSE DELHI)
SET – 3
SECTION – A
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7.
Name the part of our eyes that helps us to focus near and distant objects in quick succession.
Eyelens
The outer surface of a hollow sphere of aluminium of radius 50 cm is to be used as a mirror. What will
be the focal length of this mirror? Which type of spherical mirror will it provide?
f = 25 cm, convex
What is nature of the image formed by a concave mirror if the magnification produced by the mirror is
+ 4?
Virtual, erect & magnified
A charged particle enters at right angles into a uniform magnetic field as shown. What should be the
nature of charge on the particle if it begins to move in a direction pointing vertically out of the page due
to its interaction with the magnetic field?
Positive
On what basis is a chemical equation balanced?
A chemical equation is balanced on the basis of law of conservation of mass according to which mass
can neither be created nor be destroyed.
State two characteristic features of carbon which when put together give rise to a large number of
carbon compounds.
1.
Catenation i.e. self linking property of carbon
2.
Tetravalency i.e. tendency to form 4 bonds with same or different atoms
Write any two observations in an activity which may suggest that a chemical reaction has taken place.
Given an example in support of your answer.
A chemical change can be confirmed by any of the following observations:
™
Change in temperature
™
Evolution of gas
™
Change in colour
™
Formation of precipitates
™
Change in energy
For example: Calcium oxide reacts vigorously with water to produce calcium hydroxide. During this
process, a large amount of heat is evolved, which increases the temperature of the reaction mixture
system. This confirms that a chemical reaction has taken place.
CaO(s)
+
Calcium Oxide
H2O (l) ⎯
⎯→
Water
Ca(OH)2
Calcium hydroxide
Also, when calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide.
CaCO3(s)
Calcium carbonate
∆
⎯⎯→
CaO (s)
Calcium oxide
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+
CO2 (g)
Carbon dioxide
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In this reaction, calcium carbonate breaks down to form calcium oxide and carbon dioxide. Here,
evolution of the gas (carbon dioxide) confirms that a chemical reaction has taken place.
What are fossil fuels? Write two disadvantages of burning fossil fuels.
Fossil fuels are the fuels that are formed by the gradual decomposition of the remains of dead animals
and plants that got buried deep inside the earth millions of years ago eg. Coal, petroleum, natural gas,
etc.
Disadvantages of burning fossil fuels:
1.
The burning of fossil fuels produces large amounts of gases such as sulphur dioxide and
nitrogen dioxide. These gases when combine with rainwater result in acid rain which causes
disastrous affects.
2.
Fossil fuels such as coal leave a lot of ash after burning. This ash is mostly passed into rivers,
leading to their pollution. Such contaminated water causes serious health problems.
3.
The burning of fossil fuels produces smoke, leading to air pollution.
4.
The burning of fossil fuels releases carbon dioxide − which increases the temperature of the
earth and results in global warming.
(Any two points will serve the purpose)
Two coils A and B of insulated wires are kept close to each other. Coil A is connected to a
galvanometer while coil B is connected to a battery through a key. What would happen if
(i)
a current is passed through coil B by plugging the key, and
(ii)
the current is stopped by removing the plug from the key?
Explain your answer mentioning the name of the phenomena involved.
(i)
When current is made to pass through coil B, by plugging the key in, for that instantaneous
moment, the galvanometer connected to coil A shows a deflection, i.e., current passes through
coil A as well.
(ii)
When current is stopped through coil B by taking the plug of the key out, for that instantaneous
moment, the galvanometer connected to coil A again shows a deflection, this time, however, in
the opposite direction. That is, a current passes through coil A in the opposite direction.
The observations are made because whenever we change the current in the coil B, the magnetic field
associated with it and the coil A in its proximity changes. At the very instant of plugging the key in or
taking the plug out, there is an instantaneous flow of current and a change in the magnetic field
associated with the coil B. During this instantaneous moment, the current in coil B reaches from zero to
the maximum or from the maximum to zero. This changing magnetic field of coil B induces a current in
the coil A.
The phenomenon is known as electromagnetic induction.
What is a solenoid? Draw the pattern of magnetic field lines of a solenoid through which a steady
current flows. What does the pattern of field lines inside the solenoid indicate?
A solenoid is cylindrical shaped coil containing many turns.
I
N
S
I
I
+
V
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–
K
R
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11.
11.
The parallel field lines indicate uniform magnet
What is Hypermetropia (far sightedness)? Draw a ray diagram to show how this defect can be corrected
using a lens.
Hypermetrpia is a defect where the near point of the eye recedes, so one faces problem in seeing
nearby objects even though he may be able to view distant objects.
Actual near point
(25 cm)
12.
12.
Mention any four limitations in harnessing wind energy on a large scale.
Wind energy farms can only be installed in regions where wind is available throughout the year.
•
The minimum wind velocity required is 15 km/hr to maintain the required speed of turbine.
•
Establishment of wind energy farms requires a large area of land. For a 1MW generator, the farm
needs 2 hectares of land.
•
High cost of installation.
•
Since tower & blades are exposed to the vagaries of nature like rain, sun, storm and cyclone,
they need a high level of maintenance .
•
13.
13.
Back–up facilities like storage cells are required to take care of the energy needs during a period
when there is no wind.
At what distance should an object be placed from a lens of focal length 25 cm to obtain its image on a
screen placed on the other side at a distance of 50 cm from the lens? What will be the magnification
produced in this case?
Given that,
Focal length f = 25 cm
Image distance, v = 50 cm
Form the lens formula,
1 1 1
– = (where u = object distance)
v u f
or,
1 1 1
= –
u v f
=
1
1
–
50 25
Therefore,
1 −1
=
u 50
Or, u = –50 cm
Magnification, m =
v
u
50
− 50
= -1
The object must be placed 50 cm away from the lens, on the other side to produce a magnification of
–1.
=
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14.
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15.
15.
Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells but
different number of electrons in their outermost shell. It was found that elements A and G combine to
form an ionic compound. This compound is added in a small amount to almost all vegetable dishes
during cooking. Oxides of elements A and B are basic in nature while those of E and F are acidic. The
oxide of D is almost neutral. Based on the above information answer the following questions:
(i)
To which group or period of the periodic table do the listed elements belong?
(ii)
What would be the nature of compound formed by a combination of elements B and F?
(iii)
Which two of these elements could definitely be metals?
(iv) Which one of the eight elements is most likely to be found in gaseous state at room temperature?
(v)
If the number of electrons in the outermost shell of elements C and G be 3 and 7 respectively,
write the formula of the compound formed by the combination of C and G.
(i)
The elements A tp H belong to the third period of the periodic table and to the following groups.
A belongs to group 1
B belongs to group 2
C belongs to group 3
D belongs to group 14
E belongs to group 15
F belongs to group 16
G belongs to group 17
H belongs to group 18
(ii)
Compound.
(iii)
A and B
(iv) Element H
(v)
CG3
No chemical reaction takes place when granules of a solid, A, are mixed with the powder of another
solid, B. However when the mixture is heated, a reaction takes place between its components. One of
the products, C, is a metal and settles down in the molten state while the other product, D, floats over it.
It was observed that the reaction is highly exothermic.
(i)
Based on the given information make an assumption about A and B and write a chemical
equation for the chemical reaction indicating the conditions of reaction, physical state of
reactants and products and thermal status of reaction.
(ii)
Mention any two types of reactions under which above chemical reaction can be classified.
(i)
Solid A is assumed to be MnO2 and solid B to be Aluminium (Al).
When the mixture of MnO2 and Al is heated, manganese (Mn) is produced in molten state and
aluminium oxide (Al2O3) floats over it. Mn is produced in the molten state as a lot of heat energy
is released in the process.
The balanced chemical equation involved is:
∆
3MnO 2( s ) + 4 Al ( s ) ⎯⎯→
3n (l) + 2 Al 2 O 3( s ) + Heat
(ii)
16.
16.
The types of reaction under which the above reaction can be classified are:
(a)
Redox reaction
(b)
Displacement reaction
Name the functional group of organic compounds that can be hydrogenated. With the help of suitable
example explain the process of hydrogenation mentioning the conditions of the reaction and any one
change in physical property with the formation of the product. Name any one natural source of organic
compounds that are hydrogenated.
‘alkenes’ and ‘alkynes’
Hydrogen can be added to unsaturated hydrocarbons in the presence of nickel to yield saturated
hydrocarbons. In this reaction, nickel acts as a catalyst.
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alkene
17.
alkene
On hydrogenation, the unsaturated fatty acids which are liquid at room temperature become saturated
and change to solid.
A natural source of organic compound that is hydrogenated is vegetable oil.
Derive the expression for the heat produced due to a current ‘I’ flowing for a time interval ‘t’ through a
resistor ‘R’ having a potential difference ‘V’ across its ends. With which name is the relation known?
How much heat will an instrument of 12W produce in one minute if it is connected to a battery of 12V?
Consider a resistor of resistance R across which a potential difference V is applied as shown in the
figure.
A
R
B
I
V
According to the Ohm’s Law, the current I passing through the resistor R is given by
V = IR
…. (i)
Suppose that a constant current I passes through the resistor for a time ‘t’. Then the total charge that
flows through R in time ‘t’ is given by
q = It
… (ii)
Now, if current is passing from A to B (opposite to the flow of electrons), the end A is at a higher
potential than B and this potential difference V is equal to the amount of work done in bringing a unit
‘+ve’ charge from B to A. In other words, if a unit charge flows from A to B, energy equal to ‘V’ will be
consumed from the battery. Similarly, if charge ‘q’ flows from A to B, energy dissipated across resistor,
i.e. heat produced, is given as:
H = Vq
Using (i) and (ii)
H = I2Rt
or
H = VIt
or
H=
V2
t
R
Here P = 12 W
V = 12 V
t = 60 s
H=P×t
= 12 × 60
= 720 J
OR
Explain with the help of a labelled circuit diagram how you will find the resistance of a combination of
three resistors, or resistance R1, R2 and R3, joined in parallel. Also mention how you will connect the
ammeter and the voltmeter in the circuit when measuring the current in the circuit and the potential
difference across one of the three resistors of the combination.
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Consider a parallel combination of three resistors R1, R2 and R3 as shown below:
I
R1
+A –
R2
1
I1
L
+A–
+A3– R3
I3
+
+
V
E
M
M
2
I2
RP
L
I
–
≈
+
–
V
–
A
+
–
K
+
E
–
K
Sum of the currents across individual branches of a parallel circuit is equal to the net current
drawn by the parallel combination circuit.
Clearly, I = I1 + I2 + I3
… (i)
As voltage across each resistor is V, we can write the relations for the currents in different resistors R1,
R2 and R3. Using Ohm’s Law
I1 =
V
,
R1
I2 =
V
,
R2
I3 =
V
R3
Using above equations in (i), we get
⎛ 1
1
1 ⎞
⎟
I = V ⎜⎜
+
+
R2
R3 ⎟⎠
⎝ R1
… (ii)
Now, again consider a resistor ‘RP’ across which if we apply some voltage V and the same current
I is generated through RP, then RP will be equivalent resistance of the parallel combination of R1, R2
and R3. From fig. 4(b), using Ohm’s law:
V = IRP
or I =
V
RP
… (iii)
On comparing equations (ii) and (iii)
⎛ 1
V
1
1 ⎞
⎟
= V ⎜⎜
+
+
⎟
RP
⎝ R1 R 2 R 3 ⎠
or
18.
1
1
1
1
=
+
+
R P R1 R 2 R 3
Ammeter will always be connected in series to the resistor through which we wish to measure current
and Voltmeter would be connected parallel to the resistor across which we wish to measure the
potential difference.
Write the names and symbols of two most reactive metals belonging to group 1 of the periodic table.
Explain by drawing electronic structure how either one of the two metals reacts with a halogen. With
which name is the bond formed between these elements known and what is the class of the compound
so formed known? State any four physical properties of such compounds.
OR
What is meant by refining of metals? Name the most widely used method of refining impure metals
produced by various reduction processes. Describe with the help of a labelled diagram how this method
may be used for refining of copper.
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18.
Sodium (Na) and potassium (K)
2,8,1
2,8
2,8,7
2,8
2,8,8
As the ions of sodium and chlorine are oppositely charged, they attract each other by strong
electrostatic forces. The reaction between sodium and chlorine can be represented as follows:
Na
2, 8,1
× ×
×
× Cl ×
××
2,8, 7
−
⎡ ×× ⎤
Na + ⎢ ו Cl ×× ⎥ ⎯
⎯→ NaCl
⎣ ×× ⎦
The bond formed between these elements is known as ionic or electrostatic bond.
Sodium chloride belongs to the class ionic compounds.
Physical properties of ionic compounds are:
1.
Ionic compounds exist in solid state
2.
Ionic compounds have high melting and boiling points
3.
Ionic compounds are soluble in polar solvents (water) and insoluble in non-polar or organic
solvents (kerosene and petrol).
4.
Ionic compounds conduct electricity in their molten or aqueous state.
OR
Refining is the process of obtaining pure metal from its impure sample.
Electrolytic refining is the most widely used method for refining impure metals produced by various
reduction processes.
Electrolytic refining of copper
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