Heat Quantities and Vapours

Heat Quantities and Vapours

Introduction
In our everyday life, we use heat for cooking food and very many other purposes. How can you measure the amount of heat the food absorbs? In addition, why do you think water is commonly used to cool machines or engines? If you want to cook food faster, would you use steam or boiling water? Why?
In this chapter, you will be able to explain heat capacity and latent heat and know their common applications.
6.1: Heat Capacity On a very hot day you realise that it is more conducive to stay in a grass
thatched house than one covered with iron sheets. Why do you think so?In addition, it is quite easier to stir hot soup using a wooden spoon than a metallic one. Why do these materials respond to heat differently?

Activity 6.1 Understanding heat capacity
Key question: Explain the meaning of heat capacity. What you need: Note book and pens, 2 thermometers. Caution: Be careful when heating to avoid burns. Initial question: What is heat?

What to do (In groups)?

1. Analyse Figure 6.1 and write the answers to the questions below in your note book.
2. Assume that:
   (a) Water in the beakers in Figure 6.1 (a) is supplied with the same amount of heat energy.

(b) Solids in Figure 6.1 (b) are of the same mass and at the same
temperature.

• The two activity set ups are made to stand for 5.
• Share with other groups.
  Conclusion and Application
• In which beaker will the temperature of the water rise faster? Why?
• What will happen to the temperature of water in the beakers when the heat source is removed? Which water will lose heat faster?
  7.Will the solids in Figure 6.1 (b) absorb heat at the same rate when heated? Explain.
  8.Which solid will hold more heat when the heat source is removed?
  9.What is meant by heat capacity of a material?
  1o.Suggest factors which you think affect heat capacity of materials.

Calculate the quantity of heat required to raise the temperature of an aluminium
metal block with heat capacity of 460 JK-I from 15 oc to 45 oc.
61.1: Specific Heat Capacity by method of mixtures
When comparing materials in terms of heat absorption, It is better to consider a unit mass of each. This relates to their specific heat capacities.
Activity 62 Determining specific heat capacity of materials by
Key question: Describe how to determine specific heat capacity of a solid by methods of mixtures

What you need: ICT resource, calorimeter (coffee cup), water, thermometer,
heat source, thread, a weighing scale, and iron block.
Initial question: What do you understand by specific heat capacity?
Caution: Be careful when dealing with heat to avoid burns.
What to do (In groups):

1. Conduct a research on specific heat capacities of substances.
2. Carry out an experiment to determine the specific heat capacity of the iron block.
3. Share your work with the rest of the groups in class.
   Conclusion and Application
   4.Define specific heat capacity of a material.
   5.State the used to determine specific heat capacities of liquids and solids.
   6.Explain the flow of heat when hot and cold substances are mixed together.
   7.Mention the assumptions made.
   8.Determine the specific heat capacity of the iron block
   9.Discuss applications of the high specific heat capacity of water.

Self Check

1. Two materials P and Q have specific heat capacities of 450 J/kg0C and 900 J/kg0C respectively. Which one of them is a better conductor of heat? Explain why?
2. A block of iron of mass 2 kg absorbs 19 kJ of heat, its temperature rises by 10 oc. Find the specific heat capacity of iron.
3. A piece of metal of mass 0.5 kg is heated to 100 oc and then placed in 0.4 kg of water at 10 oc. If the final temperature of the mixture is 30 oc. Calculate the specific heat capacity of the metal. Given that the specific heat capacity of water is 4200 J/kgK.
4. A copper block of mass 250 g is heated to a temperature of 145 oc and then dropped into a copper calorimeter of mass 250 g containing 250 g of water at 20 oc. Calculate the maximum temperature attained by the water. (S.H.C of water is 4200 J/kgK and copper is 400 J/kgK).
5. 650 g of water at 33 oc are poured into an aluminium pot of mass 250 g at 15 oc. If no heat is lost to the surroundings, find the final temperature of the system. Specific heat capacity of aluminium is 900 J.kg-10C-l

6.2: Latent heat
Whenever heat is supplied to a given substance, its temperature increases. Did you know that there is a certain state a substance will reach when addition of heat will not cause any changes in temperature? What is the heat doing to the substance at this point? What is this heat called?

Activity 6.3 Understanding the meaning of latent heat
Key question: Explain the meaning of latent heat.
What you need: Note book and pens, ICT resource, ice cubes, beaker,
thermometer, source of heat, relevant textbooks, graph book.
Caution!: Take care when heating to avoid burns.
Initial question: What do you understand by latent heat?
What to do (In groups):

1. Place a beaker on the source of heat.
2. Pour the ice cubes in the beaker and put on the source of heat.
3. Using a thermometer, measure the temperature of the ice every after 2 minutes for 30 minutes.
4. Record the values of temperature and time in a suitable table.

5. Plot the results on a graph to show the variation of temperature and time.
   Conclusion and Application
6. What do you observe as you heat the ice?
7. Basing on the graph plotted in step (5) above, Identify the regions where the temperature is;
   (b) constant
   (a) increasing
8. Explain what is happening to the particles of water in the regions in step 7(a) and 7(b)?
9. Why is the temperature constant in the regions mentioned in step 7(b) yet heat is being supplied?
10. Why is “latent heat” also known as “hidden heat”?

6.2.2 Specific Latent Heat
Activity 6.4 Understanding specific latent heat Key question: Explain the types of specific latent heat.
What you need: ICT resource, relevant textbooks, ice cubes, water, copper, diameter,thermometer, stirrer, weighing scale.
Initial question: What is latent heat?
What to do (In groups):

1. Use the ICT resource or textbook and research about specific latent heat.
2. Plan and carry out the experiment using the materials provided to
   determine the specific latent heat of ice.
   *Conclusion and Application
3. Write down the meaning of specific latent heat.
4. Explain the types of specific latent heat.
5. What is the value of specific latent heat of fusion of ice?

6. What is the ice used for in Figure 6.2. Explain how it does its function.
7. What can happen if the steam from boiling water in Figure 6.2 (b) lands on your hands. Explain what causes the feeling.
8. Discuss other uses of the high specific latent heat of water.

6.3: Change of State
Earlier on you learnt about the different states of matter and these states are interchangeable. What causes a substance to change its state?
6.3.1 Processes involved in change of state
Activity 6.5 Reviewing the processes involved in change of state
Key question: Describe how change of state occurs.
What you need: Note book, and pen, ICT resources, relevant text books
Initial question: What is meant by a molecule and matter?What to do (In groups):

What to do (In groups):

1. Analyse the processes shown in Figure 6.3 and answer questions in your note book.
   conclusion application
2. Giving reasons, identify the states of matter X, Y and Z shown in Figure 6.3.

3.Identify the processes A to F.
4.When does the state of a substance change?
5.Comment about the temperature of a substance during change of state.
6.Explain what happens to the arrangement of particles of a substance when it changes from;
(b) liquid to solid (a) liquid to gas
6.3.2 Melting and Freezing
Melting and freezing are some of the common changes of state of matter which occur when a solid changes to liquid and a liquid changes to solid respectively. In activity 6.6, you will use stearic acid to learn more about these processes. Stearic acid is a long chain fatty acid which is found in
combined form in natural animal and vegetable fats.

Activity 6.6 Investigating melting and freezing of stearic acid

Key question: Describe the melting and freezing of stearic acid. What you need: ICT resource, relevant text books, stearic acid, thermometers, boiling tube, beaker, tripod stand, wire gauze, stop clock
and a heat source.

Initial question: What is freezing point?
What to do (In groups):

1. Use the set resource or textbook and research about an experiment to
   investigate;
   (a) melting of stearic acid.
   (b) freezing of stearic acid.
2. Plan and carry out the experiment using the materials provided.
   Conclusion and Application,
   3.What happens when you start heating the stearic acid?
   4.At what temperature does the stearic acid; start to melt?
   (a)start to melt
   (b) begin to solidify again?
   5.Discuss what happens to stearic acid when it reaches room temperature.
   6.Draw a heating and cooling curve to show melting and freezing of stearic acid.
   7.How will you obtain the total energy absorbed by stearic acid?
   8.Suggest any applications of stearic acid
   9.Write a short report showing the explanation of results.

1.In which region of the graph is the substance all;
(a) liquid
(b) vapour?
2.At what temperature will the substance;
a) start boiling?
b) start freezing?
c) finish melting?
3.What is happening to the kinetic energy of the particles between region BC and CD.
4.As the substance goes through BC, what happens to the distance between its molecules?
What is happening to the substance in the region DE.

6.4: Evaporation
Have you ever wondered why when you place wet clothes outside on the hang line they become dry? Where does the water go?
Now that you are aware that evaporation is change of state from liquid to gas, you also need to know the factors that facilitate it.

6.4.1 Factors affecting Evaporation
Activity 6.7 Investigating factors which affect evaporation
Key question: Explain the factors affecting evaporation
what you need: ICT resource, relevant text book, note book and pens.
Initial question: What is evaporation?

What to do (In groups):

1. Make research about evaporation.
   Conclusion and Application
   2.Write down the meaning of evaporation.
   3.Explain how evaporation takes place.
   4.State the properties of evaporation.
   5.Figure 6.5 shows clothes hanged on a line at night.

6. Will the clothes get dry?
7. Under what conditions will they dry or not?
   6.4.2: Cooling effect of evaporation (evaporative cooling)
   Activity 6.8 Investigating the cooling effect of evaporation Key question: Describe the cooling effect of evaporation.
   What you need: Methylated spirit and cotton wool, ICT resource.
   Initial question: Why does sweating cool our bodies down?
   What to do (In groups):

1.Pour a few drops of methylated spirit in cotton wool.

2.Smear the spirit on your skin.
3.How did you feel when you smeared methylated spirit on your skin?
4.What happened to the spirit on your body?
5.What was the cause of the effect felt in step (4) above?
6.Basing on your observations, describe how evaporation causes cooling.

7. Explain what will happen to the temperature of the person in Figure 6.6 after sweating?
   Activity 6.9 Investigating the parts of refrigerator
   Key question: Describe the parts of refrigerator
   What you need: ICT resource.
   Initial question: What is a refrigerator.
   What to do (In groups):
8. Figure 6.7 shows parts of a refrigerator;

(a) Identify the parts labelled A to E and their functions.
(b) Describe the working principle of a refrigerator.

6.5: Vapours

2. Give any other applications of cooling effect of evaporation in daily life
   We always see vapour rising from the water surface most especially when cooking, what do you know about vapours? How are they formed? In this section, you will learn more about vapours.

6.5.1: Saturated and Unsaturated Vapours
Activity 6.10 Understanding saturated and unsaturated vapour Key question: Explain vapours in terms of particle theory.
What you need: ICT resource, relevant physics textbooks.
Initial question: What is a vapour?
What to do: In groups;

1. Use the ICT and textbook to research about;
   (a) Saturated vapour
   (b) Unsaturated Vapour
2. Study Figure 6.8 and answer the questions (4) to (9) below in your notebook.

3. Share your work with other groWps in class.
4. State the processes that take place in the jar in Figure 6.8 (a) when water is heated.
5. Discuss the behaviour of water particles as the flask is being heated.
6. When are particles of water and vapour said to be in equilibrium?
7. Under what condition is a vapour;
   (a) unsaturated
   (b) saturated
8. Explain the term saturated vapour pressure.
9. What is the name of the water formed on surfaces such as that on the body of the car in Figure 6.8 (b)? Explain how it is formed?

6.5.2: Boiling and Vapour pressure
Investigating the relationship between boiling and vapour
Activity 6.11
pressure
Key question: Explain how boiling occurs.
What you næd: ICT resource, relevant textbooks, syringe, water (70 oc), beaker
Initial question: What is boiling point?
What to do: In groups;

1. Make research about boiling and vapour pressure.
   Conclusion and Application
   2.Write down the meaning of boiling and vapour pressure.
   3.How do bubbles form during boiling?
   4.What happens to the bubble when it reaches the surface of water? Explain.
   5.How does boiling take place?
   6.Discuss the factors affecting boiling point.
   7.How is boiling different from evaporation?
   8.Why is cooking using a pressure cooker faster than the usual saucepan?
   9.Explain why cooking food on top of mountains takes longer?

Chapter 6 Summary
In this chapter you have learnt;

• The ability of materials to lose or gain heat (heat capacity) depends on the mass, nature of material and temperature difference.
• Specific heat and the applications of the high specific heat capacity of water.

  Latent heat as the heat involved in change of state in substances and
  does not cause any temperature changes in the substances but simply
  breaks intermolecular forces of attraction.
  Specific latent heat and applications of the high specific latent heat of water.
  The processes involved during change of state such as evaporation, melting, freezing and many others.
  Evaporation and the cooling effect of evaporation.
  Saturated and unsaturated vapours and the conditions when the vapours form.
  Boiling takes place when the vapour pressure is equal to the atmos-
  pheric pressure.