6.

Heat transfer  Absolute zero: “the temperature at which molecular

 Heat is a form of energy. When two bodies are in contact motion completely ceases.”
heat flows from body at higher temperature to body at  Factors effecting the rate of evaporation :
lower temperature till the lower temperature of both is (i) Temperature: increases with increase in temperature
same. (ii) S.a.: increases with increase in s.a.
 When a body is heated, its molecules move faster about (iii) Blowing air—renewal of air increases evaporation.
their means position and kinetic energy increases and (iv) Nature—some liquids like spirit, alcohol, petrol
with fall in temperature their k.e. decreases. evaporate easily.
 When a substance is heated  Evapouration → produces coolness, boiling produces
(i) It expands i.e. A change in size takes place hotness.
(ii) Change in temperature takes place.  Linear expansion: when a solid rod (metal) is heated
(iii) Change in state takes place. change in length takes place, which depends upon
 Change of state : “the process of change from one (i) Original length (l0)
state to another at a constant temperature is called (ii) Increase in temperature
change of state.” (iii) Material of rod.
 Solid on heating changes into liquid. Liquid on Let l0 be the original length at 0°c, when heated to t°c
absorbing heat changes to vapours some solids on final length becomes l increase in length (lt – l0) a l0 (t
heating directly change in vapours called – 0)
sublimation. Substance is called sublimate. Or
Solidification on cooling when a vapours change into solid. Coefficient of linear expension a which depends upon
Gas or material of rod.
Vapours on cooling changes to liquid also Lt– l0 = l0 α t
called liquifaction. Α = lt – l0 / l0 t = increase in length / original length × rise
 Melting: change of solid into liquid at constant in temperature
temperature. Fusion ⇒ freezing is change of liquid into  When a metal plate is heated, change in area takes
solid at constant temperature and change of solid into
liquid at a constant _ temperature is called fusion. place and the expansion is called superficial
 Evaporation: “change liquid to gas at alltemperatures” expansion.
it is surface phenomenon. “  When a solid of volume v0 is heated change in volume
 Vapourization : “change of liquid into vapours at fixed called cubical expansion takes place.
temperature”.  Α:β:γ=1:2:3
 Meting point: “is the temperature at which a solid starts
melting and remains constant till the whole of solid
melts.”
M.p. is same as freezing point.
M.p. of ice is 0°c or freezing point, of water is 0°c.
 Boiling point: “is the temperature of a liquid at
which it start, boiling i.e. Change into vapours or
gaseous state.”
B .p. Of pure water is 100°c.

~Sayantan Chakraborty
B. Short/long answer questions (i) volume:
Soids : have least volume.
Question 1. Liquids: have definite volume.
Gases: have maximum volume.
What is matter ? What is it composed of
Answer: (ii) shape:
Soldis: have definite shape.
Matter: anything around us is a matter.
Liquids : have no definite shape.
“anything that has mass, occupies volume and
Gases : have no definite shape.
can be felt by our senses.” It is composed
of‘molecules’.
Question 3.
Question 2. Distinguish between liquid and vapour (or gas) states of matter
on the basis of following factors
Name the three states of matter and distinguish them on the
basis of their (i) volume, and (ii) shape (a) Arrangement of molecules
Answer: (b) Inter-molecular separation
(c) Inter-molecular force, and
Three states of matter:
(d) Kinetic energy of molecules
(i) solid (ii) liquid (iii) gaseous
Answer:
Distinction between three states on the bases of .
Ans. Distinction between liquid and vapour on the bases of: Question 4.
What is evaporation ? Explain it on the basis of molecular
motion.
Answer:
Evaporation : “the change of liquid into its vapours at all
temperature from its surface is called evaporation.”
Explanation of evaporation on the bases of molecular motion:
Molecules of liquid have more spaces, less molecular force of
attraction and more k.e.
Than molecules of solids and can move through out the liquid.
While moving they can not escape the surface as they are
being pulled inside by other molecules as there are no
molecules above the surface. But when some molecules
acquire sufficient k.e. (threshold velocity), they over come the
attractive forces of other molecules and escape into the open
space above the liquid. These escaping molecules form the
vapour of the liquid and the process called evaporation
continues till all the liquid evaporates.

Question 5.

~Sayantan Chakraborty
Do all the molecules of a liquid take part in evaporation ? If not, molecules. Then other molecules come to the surface of the
explain your answer. liquid and acquire more k.e. and escape the surface. This
Answer: continues till all the liquid evaporates.
No, all the molecules of the liquid do not take part in
evaporation only those molecules near the surface of liquid Question 6.
which acquire sufficient kinetic energy (threshold velocity) No heat is supplied to a liquid during evaporation. How does
escape as they over come attractive forces of other then the liquid change into its vapours ?
Answer: Question 8.
Though no heat is supplied to the liquid but molecules near Why is cooling produced when a liquid evaporates ?
the surface of the liquid acquire sufficient kinetic energy by Answer:
collisions with other liquid molecules and with this k.e. they For changing liquid into vapours heat is needed this heat is
over come the attractive forces of other molecules and taken from the container or surroundings and temperature of
change into vapours. container or body itself fall and cooling is produced.

Question 7. Question 9.
Comment on the statement ‘evaporation is a surface Give reason for the increase in rate of evaporation of a liquid
phenomenon’. when
Answer: (a) Air is blown above the liquid
Change of liquid into vapours at all temperatures from the (b) Surface area of liquid is increased
surface is called evaporation. Evaporation takes place at (c) Temperature of liquid is increased.
surface in those molecules which are at surface and have Answer:
sufficient k.e. to over come attractive force due to inner (a) Blowing air on liquid surface increases rate of
surrounding molecules. evaporation : blowing air takes away with it the molecules of
liquid escaping out of the surface. To take their place, other
molecules escape out from the surface of liquid.
(b) Surface area increases the rate of evaporation: on
increasing the area of the surface, number of molecules
escaping out from the surface increases.
(c) Increase in temperature increases the rate of
evaporation: increase in temperature increases k.e.

(k.e = mv2). More and more molecules come to the surface

of liquid hence the rate of evaporation will increase with
increase in temperature.
Question 10. Boiling: “the change of liquid to vapours on
What is boiling ? Explain it on the basis of molecular motion? heating at a constant temperature is called
Answer: boiling”

~Sayantan Chakraborty
 energy increases and when average kinetic energy decreases
K.e = mv2 : more the speed of molecules more is the there is decrease of body temperature and hence at boiling
kinetic energy. Heating of the liquid increases the average point average kinetic eneigy increases.
k.e. of liquid molecules and molecules acquire sufficient k.e.
needed to overcome the force of attraction of other Question 13.
molecules. These molecules start leaving the liquid not only How is the heat energy supplied to a liquid used during boiling
at the surface but also near the walls of the containing vessel at a fixed temperature ?
and bubbles are seen on the walls of vessel. This causes the Answer:
agitation in the whole of the liquid and this is called boiling. When heat is supplied, temperature of liquid rises
continuously till the water starts boiling at 100°c. Once the
Question 11. water starts boiling, its temperature does not rise further,
Why does bubbles appear when a liquid is heated ? although heat is still being supplied. This heat is being used to
Answer: change each and every water molecule into vapour.
When liquid is heated formation of vapours takes place
which appear in the form of bubbles. Question 14.
Name two ways of change of liquid state to the vapour state and
Question 12. distinguish them.
What is the change in average kinetic energy of molecules of Answer:
a liquid during boiling at its boiling point ? Two ways of change of liquid state to vapour state are:
Answer: (i) Evaporation
Average kinetic energy is the measure of temperature of body. (ii) Boiling.
When temperature of body increases the average kinetic
Difference between evaporation and boiling:

Question 15.
What do you understand by thermal expansion of a substance ?
Answer:
The expansion of a substance on heating is called the
thermal expansion of the substance.

Question 16.
Give two examples of the substances which expand on heating.
Answer:
Iron, water expand on heating.

Question 17.

~Sayantan Chakraborty
Describe an experiment to demonstrate the thermal expansion Answer:
in solids. Experiment: [gravesand’s ball and ring experiment]
(2) Temperature of rod. It is directly proportional to increase in
temperature.
(3) Nature of material of rod.
If l1 is original length of rod at t1 °c and on heating to t2 °c its
length becomes l2.
∴ Increase in
length of rod
= ( l2 – l1 )
and increase
in
temperature
( t2 – t1 )
∴ [l2 – l1] ∝ l1 (t2 – t1 )
But the increase in length of rod does not depend on whether
the rod is solid or hollow.

Take metallic ball and ring so that ball can pass through ring
Question 19.
easily. Now heat the ball. We see that heated ball cannot pass
Two iron rods – one 10 m long and the other 5 m long, are
through the same ring. This is because on heating the ball
heated to the same rise in temperature. Which will expand
increases in size. Now on cooling the ball again it can pass
more ?
through the ring. This experiment shows that solids expand on
Answer:
heating and contract on cooling. Lt – l0 ∝ l0
∴ The rod having greater length will expand more when rods
are heated to the same temperature.
Question 18. ∴ Rod with 10 m length will expand more.
State three factors on which depend the linear expansion of a
metal rod on heating.
Answer: Question 20.
Factors on which increase in length of a rod depend when Two identical rods of copper are heated to different
heated are: temperatures — one by 5°c and the other by 10°c. Which rod
(1) Length of rod l. Increase in length is directly proportional to will expand more ?
original length.
Answer: One rod of copper and another identical rod of iron are
Two rods with same length and material are heated heated to the same rise in temperature. Which rod will
to different temperature. Lt-l0 ∝ t| expand more? Give reason.
∴ Rod with higher temperature i. E. At 10°c will expand more.

Question 21. Answer:

~Sayantan Chakraborty
When two identical rods (same length) are of different material On heating the ball expands and increases in size and cannot
and heated to same rise in temperature, the copper rod will pass the ring when left on it. As the ball cools, it contracts, size
expand more than iron, since expansion depends on nature of becomes less than ring and passes the ring.
material.
(coefficient of linear expansion of copper 17 × 10-6 per °c > iron Question 24.
13 × 10-6 per °c) Explain the following:
(a) The telephone wires break in winter.
Question 22. (b) Iron rims are heated before they are fixed on the wooden
Two identical rods—one hollow and the other solid, are wheels.
heated to the same rise in temperature. Which will (c) The gaps are left between the successive rails on a railway
expand more ? track.
Answer: (d) A glass stopper stuck in the neck of a bottle can be removed
When two identical rods are heated to the same range of by pouring hot water on the neck of the bottle.
temperature, solid rod will expand more. (e) A cement floor is laid in small pieces with gaps in between.
V more heat is present in solid rod.
(a) Metals expand on heating (in summer) and contract on
Question 23. cooling (in winter). Therefore while putting up the wires
In the ball and ring experiment, ifthe ball after heating is left to between two poles, care is taken that they are kept tight while
laying them in winter as they sag in summer due to expansion.
cool on the ring for some time, the ball again passes through
the ring. Explain the reason. (b) Iron rims are made slightly smaller in diameter than the
Answer: wooden wheel and on heating wheel expands and can easily
slip over the wooden wheel and on cooling the rim contracts
and makes a tight fit over the wooden wheel.
(c) In summer due to considerable rise in In summer when temperature increases considerably the bridge
temperature, the gaps allow for the edxpansion of made of metal expands and rollers slide to allow for expansion
rails, otherwise the rail will bend side ways. otherwise the bridge may break the pillar.
(d) By pouring hot water on the neck of the bottle, the neck
expands and stuck glass stopper can easily be removed. Question 26.
(e) In summer when temperature rises small pieces of A metal plate is heated. State three factors on which
cement expand and to allow expansion gaps are left the increase in its area will depend.
between small pieces. Answer:
Three factors are:
Question 25. (i) Original area of plate a0.
Why is one end of a steel girder in a bridge kept on rollers (ii) Rise in temperature t.
instead of fixing it in pillar ? (iii) Nature of material of plate.
Answer:

~Sayantan Chakraborty
Question 27. Increase in volume = (vt — v0) [l3t — l30]
A cubical metal solid block is heated. How will its volume Increase in temperature = (t — o)°c = t°c
change ?
Answer: Question 28.
When a solid is heated, it expands in all directions. Describe an experiment to show that liquids expand on heating.
The volume of a cube also increases. Answer:
Let v0 be the volume of cube of side Experiment. Take round bottomed flask filled with water. Close
l0 at 0°c. I.e. V0 = l30 – v when its mouth with air-tight cork having delivery tube through it.
temperature increases to t°c each Mark the level of water. Let it be at a. Now heat the flask. As
side increases to vt water gets heated in flask, the level of water in the tube rises
∴ Vt = l3t
from a to b. This proves that liquids expand on heating.

Or experiment :take a laboratory celsius thermometer suspend

it with the help of a stand. Note the level of mercury thread in it.
Let it be at a. Now take hot water in a beaker and as shown, set
up the arrangement. The mercury thread rises to b by gaining
temperature.
This shows that liquids expand on heating.
Question 29. because air inside the flask expands with rise in temperature.
State one application of thermal expansion of liquids. This proves that gases expand on heating.
Answer:
Application of thermal expansion of liquids : an important
application is mercury thermometer or alcohol thermometer.
When the bulb of the thermometer is kept in contact
with a hot body the mercury expand and the level of
mercury rises in the capillary tube.

Question 30.
Describe an experiment to showthat air expands on heating.
Answer:
Take a glass fl bottomed flask. Close its mouth with a cork
having capillary tube containing indicator visible in the tube as Question 31.
shown. Make it air tight. Place the flask in hot water. After few An empty glass bottle is fitted with a narrow tube at its mouth.
minutes, we see the indicator moving up. This happens The open end of the tube is kept in a beaker containing water.

~Sayantan Chakraborty
When the bottle is heated, bubbles of air are seen escaping Which of the following will expand more, when heated to
into water. Explain the reason. the same temperature : (a) solid (b) liquid and (c) gas ?
Answer: Answer:
When the bottle is heated, air in it expands and (c) Gas will expand more as the inter-molecular force of
escapes the water in the form of bubbles. attraction in gas molecules is least and k.e. is maximum.

Question 32. Question 33.

Describe an experiment to show that same volume of different
liquids heated to same
Rise in temperature expand by different amounts. Question 34.
Answer: 100 ml of each of the following liquid is heated from 10°c to
Cubical expansion of different liquids is different: 50°c. Which will expand more : (a) water (b) benzene (c)
alcohol ?
Answer:
100 ml. Of each means same volume of each liquid heated
10°c to 50°c means same rise in temperature.
Hence, benzene will expand more water will expand least.

Question 35.
Water is heated from 0°c to 4°c. Will it expand ?
Answer:
Substances expand on heating.
But water—contracts with heated from 0°c to 4°c. After
Experiment: take four identical glass flasks each fitted with a that i.e. Above 4°c water starts expanding.
narrow glass tube through a cork at its mouth. Fill flask a with
water, b with kerosene, c with alcohol and d with benzene. So
Question 36.
that volume of each is same i.e. To the same level and their
levels are visible above hot water bath. Put enough hot water in What do you mean by anomalous behaviour of water ?
Answer:
hot-water-bath. So that each flask is in the hot water. After
some time we will see that different liquids rise to different Substances when heated expand. Their density [ ]
levels. Water expands the least and benzene the most. This decrease. But in case of water: when water is heated from 0°c
shows that different liquids of same volume expand by different to 4°c it contract and density increase, heating water above
amount. 4°c starts expanding and density of water decrease.
This means water has maximum density at 4°c. This is called
anomalous behaviour of water.
Question 37. Answer:
How does the density of a substance (solid, liquid and gas) Density = mass / volume
change on heating ?

~Sayantan Chakraborty
In case of solids, when temperature is increased, increase
in volume is very small and decrease in density is not
appreciable. In case of liquids and gases, as the
temperature increases, volume increases by an appreciable
amount and therefore decrease in their density is quite
considerable.

Question 38.
An iron washer is heated. State the effect on its (i) mass,
(ii) internal diameter, (iii) external diameter, and (iv)
density.
Answer:
(i) Mass remains same on heating.
(ii) Internal diameter increases.
(iii) External diameter increases.
(iv) Density decreases.

~Sayantan Chakraborty