HEAT Notes
Uploaded by
Bharati GulajkarHEAT Notes
Uploaded by
Bharati GulajkarHEAT
temperature: In the SI system, the unit of temperature is kelvin,
The unit of heat is joules.
the heat energy contained in a body is Q = m × C × ∆ t
Q1) Write True or False for each statement
(a) Evaporation is rapid on a wet day.
(b) Evaporation takes place only from the surface of liquid.
(c) All molecules of a liquid take part in the process of evaporation.
(d) Temperature of a liquid rises during boiling or vaporization.
(e) All molecules of a liquid take part in boiling.
(f) Boiling is a rapid phenomenon.
(g) All solids expand by the same amount when heated to the same rise in temperature.
(h) Telephone wires are kept tight between the two poles in winter.
(i) Equal volumes of different liquids expand by the different amount when they are
heated to the same rise in temperature.
(j) Solids expand the least and gasses expand the most on being heated.
(k) A mercury thermometer makes use of the property of expansion of liquids on
heating.
(l) Kerosene contracts on heating.
Answer:
(a) False (b) True (c) False (d) False (e) True (f) True
(g) False (h) True (i) True (j) True (k) True (l) False
Q2) Fill in the blanks
(a) Boiling occurs at _______.
(b) Evaporation takes place at ______.
(c) The molecules of liquid _____ heat from surroundings in evaporation.
(d) Heat is _____ during boiling.
(e) Cooling is produced in ______.
(f) A longer rod expands ____ than a shorter rod on being heated to the same
temperature.
(g) Liquids expand _____ than the solids
(h) Gases expand _____ than liquids.
(i) Alcohol expands _____ than water.
(j) Iron expands _____ than copper.
Answer:
(a) Boiling occurs at a fixed temperature.
(b) Evaporation takes place at all temperatures.
(c) The molecules of liquid absorb heat from surroundings in evaporation.
(d) Heat is absorbed during boiling.
(e) Cooling is produced in evaporation.
(f) A longer rod expands more than a shorter rod on being heated to the same
temperature.
(g) Liquids expand more than the solids
(h) Gases expand more than liquids.
(i) Alcohol expands more than water.
(j) Iron expands less than copper.
Q4) Select the correct alternative:
(a) In evaporation
1. All molecules of liquid begin to escape out
2. Only the molecules at the surface escape out
3. The temperature of liquid rises by absorbing heat from surroundings.
4. The molecules get attracted within the liquid
(b) The rate of evaporation of a liquid increases when
1. Temperature of liquids falls
2. Liquid is poured in a vessel of less surface area
3. Air is blown above the surface of liquid
4. Humidity increases
(c) During boiling or vaporization
1. All molecules take part
2. Temperature rises
3. No heat is absorbed
4. The average kinetic energy of molecules increases.
(d) The boiling point of a liquid is increased by
1. Increasing the volume of liquid
2. Increasing the pressure, on liquid
3. Adding ice to the liquid
4. Decreasing pressure on liquid
(e) Two rods A and B of the same metal, but of length 1m and 2m respectively are
heated from 0 degree celsius to 100 degree celsius. Then
1. Both the rods A and B elongate the same
2. The rod A elongates more than the rod B
3. The rod B elongates more than the rod A
4. The rod A elongates but the rod B contracts
(f) Two rods A and B of the same metal, same length, but one solid and other hollow are
heated to the same rise in temperature. Then
1. The solid rod A expands more than the solid rod B
2. The hollow rod B expands more than the solid rod A
3. The hollow rod B contracts but the solid rod A expands
4. Both the rods A and B expand the same
(g) A given volume of alcohol and the same volume of water are heated from the room
temperature to the same temperature then
1. Alcohol contracts but water expands
2. Water contracts but the alcohol expands
3. Water expands more than alcohol
4. Alcohol expands more than water
(h) The increase in length of a metal rod depends on
1. The initial length of the rod only
2. The rise in temperature only
3. The material of rod only
4. All the above three factors
(i) The correct statement is
1. Iron rims are cooled before they are placed on the cart wheels
2. A glass stopper gets tighten on warming the neck of the bottle
3. Telephone wires sag in winter but become tight in summer
4. A little space is left between two rails on a railway track
Answer:
(a) 2. only the molecules at the surface escape out
(b) 3. air is blown above the surface of liquid
(c) 1. all molecules take part
(d) 2. increasing the pressure, on liquid
(e) 3. the rod B elongates more than the rod A
(f) 4. both the rods A and B expand the same
(g) 4. alcohol expands more than water
(h) 4. all the above three factors
(i) 4. A little space is left between two rails on a railway track
Q5: What is evaporation? Explain it on the basis of molecular motion.
Answer:
The change of liquid into its vapor at all temperatures from its surface is called
evaporation.
The intermolecular spaces in liquids are more and the molecular force of
attraction is less which makes them move throughout the liquid.
They cannot escape the surface of liquids because of less kinetic energy, when
heated they acquire sufficient kinetic energy and they overcome the attractive
forces of other molecules. On escaping the molecules form the vapor of the
liquid.
Q6: Do all molecules of a liquid take part in evaporation? If not, explain your answer.
Answer: No, not all molecules of liquid do not take part in evaporation, those molecules
which acquire sufficient kinetic energy escape the surface by overcoming forces of
attraction of other molecules. This continues till all the liquid evaporates.
Q7: No heat is supplied to a liquid during evaporation. How does then the liquid change
into its vapours?
Answer: When the molecules liquid collides with each other they acquire kinetic energy
and they overcome the attractive forces of other molecules and change into vapours.
The particles of water on the surface absorb heat from the surroundings and change
into vapour.
Q8: Why is cooling produced when a liquid evaporates?
Answer: To change liquid into vapours, heat is needed which is taken from
surroundings and temperature of the container or body itself falls and cooling is
produced.
Q9) Give reasons for the increase in rate of evaporation of a liquid
(a) When air is blown above the liquid.
(b) When surface area of liquid is increased
(c) When temperature of liquid is increased.
Answer:
The factors that decide the rate of evaporation are:
Temperature
Surface area exposed
Partial pressure of liquid in the air above it.
(a) When air is blown above the surface of liquid, it will take away the liquid carrying air
particles from the air above the liquid, resulting in decrease in humidity and increase in
rate of evaporation.
(b) On increasing the surface area, the number of molecules in contact at the surface of
liquid increases, and evaporation takes place rapidly.
(c) The increase in temperature increases the kinetic energy of the molecules, they
escape the force of attraction of molecules and evaporate faster.
Q10: Why does bubbles appear when a liquid is heated?
Answer: 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.
Q11: What is the change in average kinetic energy of molecules of a liquid during
boiling at its boiling point?
Answer: The kinetic energy is the measure of temperature of the body, when
temperature increases the average kinetic energy also increases. At boiling point,
average kinetic energy increases.
Q12: Why there is no temperature change during boiling?
Answer: During boiling, the heat energy supplied is used in increasing the energy of the
water molecule, which changes its state from liquid to gaseous.
Q13: Write difference between
Answer: (i) Boiling (ii) Evaporation
Boiling Evaporation
Heat is supplied Heat is absorbed from the surroundings
Fast process Slow process
Starts from bottom Starts from surface
Sound is produced Silent process
Heating effect Cooling effect
At a fixed temperature At all temperatures
Q14: State three factors on which depend the linear expansion of a metal rod on
heating.
Answer: Factors on which linear expansion of rod depends: (Lt − L0) α L0
(i) Length of rod
(ii) Temperature of rod
(iii) Nature of material of rod
Q15: 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: Expansion depends on the nature of material, when two identical rods
are of different material and heated to the same rise in temperature, the copper
rod will expand more than the iron rod as copper is a better conductor of heat
than iron as its heat capacity is less than the iron.
Q16: Two identical rods - one hollow and the other solid, are heated to the same rise in
temperature. Which will expand more?
Answer: 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.
Q17: In the ball and ring experiment, if the ball after heating is left to cool on the ring
for some time, the ball again passes through the ring. Explain the reason.
Answer: On heating, the solid ball expands and increases in size and cannot
pass through the ring. The ball after heating is left to cool on the ring for some
time, the ball again passes through the ring, because it contracts in size and
passes through the ring.
Q18) Explain the following:
(a) The telephone wires break in winter.
(b) Iron rims are heated before they are fixed on the wooden wheels.
(c) The gaps are left between the successive rails on a railway track.
(d) A glass stopper stuck in the neck of a bottle be removed by pouring hot water on
the neck of the bottle.
(e) A cement floor is laid in small pieces with gaps in between.
(f) Why is one end of a steel girder in a bridge kept on rollers instead of fixing it in
pillar?
Answer:
(a) Most telephone wires are made out of copper, which contracts with drop in
temperature and hence breaks in winter if it becomes too tight between the poles.
(b) Iron rims are made smaller in diameter than the wooden wheel, so on heating the
rims expand (due to superficial expansion) and can easily slip over the wooden wheel
and contract on cooling and fit tight over the wooden wheel.
(c) The gaps are left between the successive rails for making space for their expansion
when there is rise in temperature in summers.
(d) A glass stopper stuck in the neck of a bottle can be removed by pouring hot water
on the neck of the bottle, because the neck expands on pouring hot water due to
superficial expansion.
(e) Small pieces of cement expand in summers. So, to allow expansion, gaps are left
between small pieces.
(f) When temperature increases, the metal bridge expands and the rollers slide to allow
for expansion otherwise the bridge may break the pillar.
Q18: A metal plate is heated. State three factors on which increase in its area will
depend.
Answer: Factors on which increase of metal plate area depends are:
(i) Original area
(ii) Rise in temperature
(iii) Nature of material of plate.
Q19: State one application of thermal expansion of liquids.
Answer: A mercury thermometer is based on the application of thermal expansion of
liquids. The bulb of the thermometer is kept in contact with a hot body, and mercury
expands and level of mercury rises in the tube.
Q20: What is anomalous expansion of water?
Answer: Substances expand on heating, and their density decreases. Water does not
expand between 0°C to 4°C instead it contracts. It expands above 4°C. This means
water has maximum density at 4°C. This is called anomalous behaviour of water.
Q21: How does the density of a substance (solid, liquid and gas ) change on heating?
Solution: Density = Mass/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.
Q22: 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
(d) State the effect on its density.
Answer:
(a) Mass does not change in heating.