Chapter – Transfer of heat
Question-1:
What is matter? What is it composed of ?
Answer-1:
Anything that has mass, occupies space It is composed of molecules.
Question-2:
Name the three states of matter and distinguish them on the basis of their (i) volume, and (ii) shape
Answer-2:
THREE STATES OF MATTER:
(i) SOLID
(ii) LIQUID
(iii) GAS
(i) VOLUME:
SOLIDS: have a definite volume. LIQUIDS: have a definite volume. GASES: have no definite volume.
(ii) SHAPE:
SOLIDS: Have a definite shape. LIQUIDS: Have no definite shape. GASES: Have no definite shape
Question-3 :
Distinguish between liquid and vapour (or gas) states of matter on the basis of the following factors
(a) Arrangement of molecules
(b) Inter-molecular separation
(c) Inter-molecular force, and
(d) Kinetic energy of molecules
Answer-3:
DISTINCTION BETWEEN LIQUID AND VAPOUR ON THE BASIS OF:
(a)Arrangement of molecules
Liquid-Very loosely packed
Gas- wide separation
(b)Intermolecular separation
Liquid – less
Gas – wide separation
(c) Intermolecular force
Liquid – moderate
Gas- weak
(d) Kinetic energy-
Liquid- high
Gas- very high
Question-4:
What is evaporation? Explain it on the basis of molecular motion.
Answer-4:
The change of liquid into its vapour at all temperatures from its surface is called evaporation
When the molecules of liquid on surface collide with each other they acquire kinetic energy and they overcome the attractive forces of other molecules and change into vapour and also molecules of liquids on the surface absorb heat from the surroundings and change into vapour.
Question-5:
Do all molecules of a liquid take part in evaporation? If not, explain your answer.
Answer-5:
No, not all molecules of liquid do not take part in evaporation, those molecules on the surface which acquire sufficient kinetic energy escape the surface by overcoming forces of attraction of other molecules. .
Question-6:
No heat is supplied to a liquid during evaporation. How does then the liquid change into its vapours?
Answer-6:
When the molecules of liquid on surface collide with each other they acquire kinetic energy and they overcome the attractive forces of other molecules and change into vapour and also molecules of liquids on the surface absorb heat from the surroundings and change into vapour.
Question-7 :
Comment on the statement, 'evaporation is a surface phenomenon'.
Answer-7 :
Molecules of liquid on the surface absorb heat from surrounding ,over come force of attraction with other molecules and change into vapour.
Question 8:-
Why is cooling produced when a liquid evaporates?
Answer:-
To change liquid into vapour, heat is needed which is taken from surroundings and cooling is produced
Question-9:
Give reasons for the increase in rate of evaporation of a liquid when
i) Air is blown above the liquid
ii) Surface area of liquid is increased
iii) Temperature of liquid is increased
Answer 9:-
i) When air is blown above the surface of liquid, the rate of evaporation increases. The reason is that 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
ii) the rate of evaporation : On increasing the area of surface , the rate of evaporation increases. The reason being that on increasing the area of surface, number of molecules escaping out from the surface increases.
iii) The rate of evaporation cha increases with the increase in temperature of incr liquid The reason is that the energy of the molecules increases with increase in temperature. So more and more molecules come to the surface of liquid, hence the rate
Question-10
What is boiling? Explain it on the basis of molecular motion?
Answer-10:-
Change of liquid state to vapour state at fixed temperature is called boiling .
In boiling at fixed temperature by absorbing heat from an external source, all molecules throughout the liquid acquire sufficient kinetic energy to overcome the force of attraction of other molecules so they escape out from liquid in form of vapour.
Question 11:-
Why does bubbles appear when liquid is heated ?
Answer-11:-
On heating, liquid starts changing into its vapour state at the place where it is being heated. The liquid converts to gas which appears in the form of bubbles.
Question-12:
What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point?
Answer-12:
The kinetic energy increases.
Question-13:
How is the heat energy supplied to a liquid used during boiling at a fixed temperature
Answer-13:
During boiling, the heat energy supplied is used to overcome force of attraction between liquid molecules, which changes its state from liquid to gaseous.
Question-14:
Name two ways of change of liquid state to the vapour state and distinguish them.
Answer-14:
Two ways of change of liquid state to the vapour state are:
Question-18:
State three factors on which depend the linear expansion of a metal rod on heating.
Answer-18:
Factors on which linear expansion of rod depends:
(i) original length of rod
(ii) increase in temperature
(iii) Nature of material of rod
Question-19:
Two iron rods - one 10 m long and other 5 m long, are heated to the same rise in temperature. Which will expand more?
Answer-19
The iron rod of 10 m will expand more.
Question-20:
Two identical rods of copper are heated to different temperatures - one by 5°C and the other by 10°C. Which rod will expand more?
Answer-20:
Two rods with the same length and material are heated to different temperatures.
Rod with higher temperature i.e 10 degree centigrade will expand more.
Question-21:
One rod of copper and another identical rod of iron are heated to the same rise in temperature. Which rod will expand more? Give reason.
Answer-21:
The copper rod will expand more as coefficient of linear Expansion of copper is more than iron.
Question-22:
Two identical rods - one hollow and the other solid, are heated to the same rise in temperature. Which will expand more?
Answer-22:
When two identical rods are heated at the same temperature, they will expand by the same amount on heating as the material of the rods are the same.
Question-23:
In the ball and ring experiment, if the ball after heating is left to cool on the ring for sometime, the ball again passes through the ring. Explain the reason.
Answer-23:
On heating, the solid ball expands and increases in size and cannot pass through the ring. The ball after heating is left to co on the ring for sometime, the ball again passes through the ring, because it contracts in size and passes through the ring.
Question-27
A metal plate is heated. State three factors on which increase in its area will depend.
Answer-26 :
Factors on which increase of metal plate area depends are:
(i) Original area
(ii) Rise in temperature
(iii) Nature of material of plate.
Question-27:
A cubical metal solid block is heated. H will its volume change?
Answer-27:
When a solid is heated, it expands in directions. When a cubical metal solid block is heated the volume of the cube increases.
Question-29:-
State one application of thermal expansion of liquids
Answer-29 :
A mercury thermometer is based on the application of thermal expansion of liquids.
Question-31:
An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into water. Explain the reason.
Answer-31 :
On heating, the air in the bottle expands and escapes the water in the form of bubbles.
Question-32 :
Which of the following will expand more, when heated to the same temperature: (a) solid (b) liquid and (c) gas?
Answer-32 :
Gas will expand more as the intermolecular forces of attraction in gas molecules is less and kinetic energy is maximum.
Question-34
100 ml of each of the following is heated from 10⁰C to 50 ⁰C. Which will expand mote: a) water b) Benzene c) Alcohol?
Answer-34:
Benzene will expand the most because coefficient of cubical expansion of Benzene is more than water and alcohol.
Question-35:
Water is heated from 0 degree centigrade to 4 degree centigrade. Will it expand?
Answer-35:
No, water does not expand on heating from 0 ⁰C to 4 ⁰C.
Question-36:
What do mean by anomalous behaviour of water?
Answer-36:
When water is cooled from 4 ⁰C to 0 ⁰C , instead of contraction Expansion takes place.This is anomalous behaviour of water.
Question-37:
How does the density of a substance (solid, liquid and gas) change on heating?
Answer-37:
Density =Mass x volume
When a substance is heated its volume increases and so the density decreases. In solids, increase in volume is negligible and hence decrease in density too. In liquids and gases, as the temperature increases, volume increases and therefore density decreases considerably.
Question-38: An iron washer is heated.
(a) State the effect on its mass.
(b) State the effect on its internal diameter
(c) State the effect on its external diameter.
Answer:-
a) No effect on mass
b) Internal diameter increases
c) External diameter increases
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