Chapter 10 Thermal Properties of Matter

Chapter 10 Thermal Properties of Matter

Learning Outcomes

After completing this chapter, students should be able to:

1. describe a rise in temperature of a body in terms of an increase in its internal energy (random thermal energy)

2. describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature

3. explain the difference between boiling and evaporation

10.1 Internal Energy page 160 1. Emphasise to students that the internal energy of a body consists of two parts: kinetic energy and

potential energy.

2. The increase in kinetic energy of the molecules will cause the temperature of the body to increase. The kinetic energy of the molecules is also known as the random thermal energy.

3. State the increasing speed of the molecules as the temperature rises as a fact but try to justify it. E.g. as temperature rises, Brownian motion becomes more violent.

4. The increase in potential energy of the molecules will cause the body to change states. That is, to change from solid to liquid, or liquid to gas.

5. Emphasise to students not to confuse with temperature of a body and the internal energy possesses by a body. The temperature of a spark is very high, about 2000 0c. That is the energy per molecule of spark is very high. Because of the few molecules per spark, however, the internal energy is very small.

6. Temperature is a measure of the energy content of a body. It is a measure of the average kinetic energy of the molecules in a body.

7. Explain the difference between heat and temperature:

For example, two litres of boiling water has twice as much total molecular kinetic energy compared to one litre of boiling water, but the temperature of the two different amount of water are the same because the average kinetic energy per molecule is the same in each.

Twice the amount of thermal energy is needed to boil two litres of water compared to one litre. Furthermore you need to supply heat to boil water but the temperature does not increase. So temperature is not heat.

Heat is the energy that flows between two bodies that have different temperatures.

8. Emphasise that a rise in temperature is a consequence of the transfer of thermal energy (cause and effect).

Answers to Section Review questions page 160

1. The internal energy of a body is the combination of the total kinetic energy (due to motion) and potential energy (due to intermolecular forces) of the molecules in a body.

2. The molecules in gases possess only kinetic energy. The potential energy is zero.

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Chapter 10 Thermal Properties of Matter

10.2 Melting, Boiling and Evaporation page 160 Melting and Freezing page 161 1. Melting is the reverse process of freezing and vice versa. Melting point and freezing point refer to the

same temperature. Melting occurs when a solid absorbs energy and changes to liquid. Freezing occurs when liquid gives out energy and changes to solid. -

2. Cool a test -tube of naphthalene and plot a graph of temperature against time. Notice the shape of the graph and where the naphthalene becomes solid. This is the melting point.

3. The behaviour of water is different from that of many substances. When it is subjected to high pressure, the melting point of ice is lowered. This is because high pressure causes its volume to decrease, thereby enabling it to change to water. For most substances, however, an increase in pressure raises their melting (freezing) points. This is because the molecules are closer together in their solid state than in their liquid state, and high pressure prevents them from melting easily.

4. Some of the rocks in the interior of the Earth are hot enough to melt at normal atmospheric pressure, but most of them remain in the solid state because of the huge pressure. If the pressure is released, like when a volcano erupts, most rocks melt and form lava.

Answer to Think Time question page 162

Not suitable. At - 50°C, the mercury will be in solid state.

Boiling and Condensation page i63 1. Boiling is the reverse process of condensation and vice versa. Boiling point and condensation point

refer to the same point. Boiling occurs when a liquid absorbs energy and changes to gas. Condensation occurs when gas gives out energy and changes to liquid.

2. Measure the temperature of water as it is being heated. Continue measuring it as it boils .

3. The reason as to why there is no increase in temperature during boiling, even though latent heat is absorbed, is given in Section 11.4. A major portion of the latent heat is used to do work against the attractive forces between the liquid molecules. Less than 10% of the latent heat is actually used to do work against the external pressure when vapour is formed and expanded.

4. When an astronaut orbits round the earth, there is practically no atmospheric pressure. Hence an astronaut working outside his spacecraft must wear a pressurized spacesuit that encloses the air he breathes. This suit must cover his whole body or his body would swell and his blood would boil.

5. High pressure applied to liquid will oppose expansion and therefore oppose boiling. It follows that if the pressure is higher than normal atmospheric pressure, the liquid will boil at higher temperature.

6. One application of the effect of increased pressure on boiling is in aerosol sprays, where a propellant liquid is mixed with, say, the insecticide. The pressure in the container is kept high, so that the propellant does not vaporize. When the button is pressed, a valve opens and the pressure is released. At the reduced pressure, the propellant quickly vaporises. When discussing the working principle of an aerosol can, emphasize that many of the propellant liquids used are chlorofluorocarbons (CFCs). The release of CFCs into the atmosphere has been slowly depleting the ozone layer of the atmosphere, which acts as a shield to reduce harmful radiation from outer space. Advise students to buy only those aerosol products that are CFC-free.

Evaporation and Boiling - page 164 1. Ask students: 'what is the working principle of mist fan?' Answer: Mist fan provides high achievement

of outdoors cooling when it is positioned at the intended cooling area by improving the air ventilation and evaporation rate in the surrounding.

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Chapter 10 Thermal Properties of Matter

Mist fan

2. Explain the difference between boiling and evaporation.

3. Leave various dishes of water in direct sunlight. Record their maximum temperature. Do they ever reach 100°C?

4. Rate of evaporation increases when • Temperature of the liquid increases • Temperature of the surrounding increases • Area of the exposed surface increases • Low humidity of the surrounding air • Air is in motion • The external pressure is low • The liquid is volatile

5. The cooling effect of evaporation can be demonstrated by rubbing alcohol on the hand.

6. When our bodies overheat, our sweat glands produce perspiration. Evaporation of perspiration cools us and helps us to maintain a stable body temperature.

7. The working principle of the split unit air-conditioners is the same as the window unit. For the split unit air-conditioners, the blower and the compressor are separated whereas for the window unit, the blower and compressor are built inside the same box.

Answers to Section Review questions . page 166

1. Freezing point

2. Condensation point

3. It changes from liquid to gas.

4. No. When the water in the tube reaches 0 °C, i.e. when the temperatures inside and outside the tube are the same, no more energy can be extracted from the water. Hence the water cannot be frozen.

5. Refer to page 164 Figure 10.6

Answers to Misconception Analysis . page 167

l. True 2. True 3. False. Increase in temperature is associated with the increase in kinetic energy 4. True 5. True 6. True 7. False. Cooling is caused by decrease in the average kinetic energy of the liquid molecules. 8. True

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Chapter 10 Thermal Properties of Matter

Answers to Multiple Choice Questions page 168

1.A 2. A 3. D 4.D

Answers to Structured Questions page 168

1. (a) Evaporation causes cooling. For the sweat to evaporate, it needs energy. The energy is obtained from the thermal energy of the body, so the person feels cooler.

(b) Evaporation occurs. The water absorbs thermal energy from the surrounding and changes from liquid state to vapour state. The process of evaporation takes place gradually.

2. The energy supplied is used to (i) increase the kinetic energy and potential energy of the particles in the solid in stage AB. (ii) do work against the attractive forces between the particles in the solid jn stage BC (melting). OR

increase the potential energy of the particles but not the kinetic energy of the particles. (iii) increase the kinetic energy and potential energy of the particles in the liquid in stage CD. (iv) do work against the attractive forces between the particles in the liquid stage DE (boiling). OR

increase the potential energy of the particles but not the kinetic energy of the particles. Note: a small portion of energy is used to do work against the atmospheric pressure when the vapour expands.

Answers to Critical Thinking Questions page 168

1. The statement is not valid. The total internal energy contains in an object on its temperature as well as the number of molecules in the object. For example, a beaker of water at 50°C is colder than a drop of water at 100 °C but the total internal energy of the water in the beaker is higher the drop of water.

2. Energy is absorbed by ice to change to the liquid state. (It provides the latent heat needed for melting the ice.)

3. No. When the water in the tin reaches 100 °C, no more energy can be absorbed from the surrounding boiling water. Hence, the water in the tin cannot be boiled.

4. The water vapour in the air loses energy to the cold liquid so that condensation takes place.

5. As the water on the skin evaporates, thermal energy is taken from the body to provide the latent heat of evaporation needed. Loss of thermal energy from the body makes one feel cold. If the person wears wet clothes, the evaporation will take place for a longer period of time, so the person will feel cold for a longer time.

Extension page 168

Refer website:

http://en.wikipedia.org/wikilWater_resources_oCSingapore http://www.pub .gov.sg/NEWater_files/index.htrnl

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