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By the end of this chapter, you will be able to:
Keywords
7.1: Introduction
In the previous chapter, you looked at heat and some of its effects on objects. Can you recall them?
Have you ever wondered why:
Electric wires, say on a hot day, sag?
Milk pours out as it starts to boil?
In this chapter, you will be able to explain how heat causes the expansion of solids, liquids and gases and explore their application.
7.2: Expansion and Contraction
Figure 7.1: An increase in heat energy causes particles to vibrate with bigger movements, resulting in the expansion of matter
When the temperature decreases, the movement of particles slows down, and there is a decrease in the space between particles. A decrease in the space between particles means that the material contracts. In other words, there is a decrease in the volume (space) it occupies.
Activity 7.1 Expansion of Solids, Liquids and Gases Demonstrating expansion and contraction of a material
What you need
A soda straw, a lit candle
a) Touch a straw at both ends.
b) Pass it above the fire of a lit candle over and over for about 5 minutes.
c) Stretch the straw.
d) Note the difference in the deformation of the straw.
e) Place it down for some minutes for it to cool down.
f) Stretch it again and note its level of deformation.
When there is an increase in temperature of particles of a material, they increase in kinetic energy (movement) and thus increase in space between particles. This leads to expansion in the volume of that material (straw).
Assignment 7.1 Researching and preparing presentation on expansion and contraction
a) In pairs, carry our research on how particle theory can be used to explain how heat changes the volume and density of solids, liquids and gases.
b) Using your findings, prepare a presentation. (You can prepare a Powerpoint presentation) and present to the whole class.
7.3: Expansion in Solids
Activity 7.2 Demonstrating expansion in solids
What you need
A metal ring attached to a wood material, a metallic ball that just passes through the ring attached to a chain, heat source such as a Bunsen burner.
What to do
a) In groups, pass the metal ball through the ring.
b) Place the metal ball over the heat source to heat it gently for sometime.
c)Attempt to pass the metal ball through the ring. Metal ball G Chain Wood material Ring
Figure 7.2: Ball and ring experiment
d) Note and record your observations.
e) Explain your observations in (a), (b) and (c).
f) What do you conclude from this experiment?
When solids are heated, their particles vibrate more rapidly, pushing each other apart slightly, causing the solid to expand.
The amount of increase in volume depends on the type of solid.
Figure 7.3: Suspension bridge in Jinja, Uganda and gaps that are left to avoid cracking of bridges during hot times/seasons
Similarly, a decrease in temperature cause solids to contract by varying degrees.
Even though the amount of expansion and contraction in solids may seem quite small, it can have serious implications in certain situations, such as the construction industry.
For example, power lines need to be hung from poles at the correct tension (tightness). If the tension is too tight, the power lines will contract and break in cold weather; if the tension is too loose, they will expand and hang too close to the ground in hot weather. Figure 7.4 shows power lines with the correct tension.
Figure 7.4: The tension of these power lines must allow for expansion and contraction of the metal wires
Apart from the applications of expansion of solids mentioned above, expansion is also widely used in the bimetallic strip.
A bimetallic strip is a strip consisting of two strips of metals of different expansivities laid side by side in tight contact, e.g. brass and steel.
Figure 7.5: A bimetallic strip when cold and hot Figure 7.5 (i) shows a bimetallic strip when cold.
When it is heated, brass having a higher expansivity than steel, it expands more, causing the strip to bend towards the steel as shown in 7.5 (ii). Bimetallic strips are widely used in thermostats for temperature control like in cookers and flat irons.
Activity 7.3 Drawing and describing thermostat
a) In groups, using the internet and other textbooks, draw and describe how a thermostat is used in controlling an electric heater.
b) Suggest other applications of expansion and contraction of materials.
Although expansion in solids has numerous applications as shown, it also has some undesirable effects. For instance, expansion of solids is responsible for cracking of continuous concrete road surfaces or buckling and tearing of long continuous metal sheets or rails.
Under hot weather, expansion occurs and these structures, if fixed, buckle, as shown in Figure 7.6 (a). In cold weather, contraction occurs and the structures, if restrained, crack or tear.
To avoid these types of damage, short pieces with gaps in between are used. e.g. railway lines are constructed with gaps, as shown in Figure 7.6 (b).
Figure 7.6: Railway lines
7.4: Expansion in Liquids
When liquids are heated, their particles move around faster, pushing each other apart slightly, and causing the liquid to expand.
As with solids, the amount of increase in volume depends on the type of liquid, but in general, liquids expand more than solids when heated.
his is because the particles in liquids are not held in a fixed arrangement, as they are in solids, so this increased freedom of movement results in greater expansion.
Activity 7.4 Demonstrating expansion of liquids
This is because the particles in liquids are not held in a fixed arrangement, as they are in solids, so this increased freedom of movement results in greater expansion.
Activity 7.4 Demonstrating expansion of liquids Hot water bath
What you need Round bottom flask, narrow glass tube, cork, water, heat source
What to do
In groups, fill a round bottom flask with water to the brim.
Seal the mouth of the flask with a cork which carries a narrow glass tube projecting through it.
Place the flask with its contents in a hot water bath.
Note and record your observations on the liquid level in the narrow glass tube.
What can you deduce from this experiment?
Share your findings in class.
Comparison of expansion of different liquids
Different liquids have different rates of expansion.
Activity 7.5 Comparing the expansion of different liquids
What you need Liquids; water, alcohol, ether, benzene, four round bottom flasks, four corks carrying narrow glass tubes, a water bath (trough with hot water), heat source
What to do
Fill the flasks with the liquids to the same height/ level.
a) Seal each flask with a cork carrying a narrow glass tube.
b) Place the flask in a water bath (trough with water)
Figure 7.7: Different flasks with different contents
Place the water bath on a heat source and heat the water while stirring.
Note and observe the levels of the liquids in the narrow w tubes.
Share your findings in class.
As with solids, the expansion and contraction of liquids needs to be allowed for in certain situations.
For example, car radiators are connected to an overflow tank to allow for the expansion of coolant as it is heated by the engine.
Activity 7.6 Researching and explaining applications of expansion of liquids
In groups, research and explain other applications of thermal expansion of liquids.
7.5: Anomalous Expansion of Water
Liquids expand on heating and contract on cooling. But water exhibits an exceptional behaviour. Consider 1 kg of water at 0°C. When it is heated from 0°C, it contracts rather than expanding, up to 4°C. The volume of water is minimum at 4°C. Beyond 4°C, water starts expanding. This behaviour of water between 0°C and 4°C is called
anomalous expansion of water.
It is observed that the volume of water decreases as its temperature rises from 0°C to 4°C, and then its volume starts increasing after 4°C. At 4°C, the volume of water is minimum and, therefore, its density is maximum.
If we plot another graph, taking temperature on the x-axis and density of water on the y-axis, the graph in Figure 7.9 below is obtained
Consequences of anomalous expansion of water
In cold countries, the temperature of the atmosphere starts falling well below 0°C in winter. When the temperature falls, the water on the surface of a pond or a lake, initially above 4°C, begins to give heat energy to the atmosphere. So, the temperature of the water on the
surface decreases, and it contracts.
If the water on the surface contracts, its density increases. Therefore. it sinks to the bottom. This process continues till the temperature of all the water in the pond or lake falls to 4°C. Now, as the temperature of the water on the surface decreases below 4°C, it starts expanding instead of contracting, and its density decreases. So, the water does not sink, but remains on the surface.
When the temperature of the surrounding falls below 0°C, the temperature of the water on the surface of the lake or pond also decreases to 0°C, and it gets converted into ice. As ice is a bad conductor of heat, the ice on the surface does not allow heat to pass from the water to the atmosphere. Thus, the water below the layer of ice remains at 4°C. As a result, creatures like fish, aquatic animals and plants can survive.
In winter, water pipelines sometimes burst. This is because water starts expanding when its temperature falls below 4°C. It exerts pressure on the pipe carrying it. The pressure is so huge that the pipe bursts. Sometimes, water enters the long narrow gaps or cracks in rocks.
When the temperature of the surrounding falls below 4°C, the water starts to expand. As there is no place for the water to expand, it exerts huge pressure on the rocks and, as a result, the rocks break. Similarly, when the temperature of the atmosphere falls below 4°C, the water in plant nerves or capillaries expands. Due to the expansion, the plant nerves or capillaries burst and cause the death of the plant. 7.6: Expansion in Gases
When temperature of gases increase, their particles move with greater speed and energy, resulting in greater space between them, causing
the gas to expand.
Activity 7.7 Demonstrating expansion of gases
What you need
A round bottom flask fitted with glass tube, water, trough, a retort stand with a clamp, a source of heat e.g a bunsen burner. What to do
a) In groups, fill a trough with clean water.
b) Pass a glass tube through a cork and fix the cork into a round bottom flask.
Clamp the round bottom flask on a retort stand.
Dip the tube in the water in the trough.
Heat the flask by directing a Bunsen burner flame to it for a
short time.
Observe the water level in the tube.
Allow the flask to cool while the tube is still in the water.
Observe what happens.
Gases expand much more than solids and liquids when heated. This is because their particles are far apart and have low forces of attraction. Gas molecules, therefore, move almost independently of each other and consequently a gas fills any container in which it is placed.
Activity 7.8 Discussing expansion of gases
In groups, discuss why it is not advisable to leave a car in an open place on a sunny day.
Gases respond significantly to changes in temperature. Therefore, many safety considerations must be taken when handling gases in containers, especially since they are often already stored under high pressure.
For example, it is extremely dangerous to heat containers that store gas, such as gas cylinders, due to the risk of explosion.
You can ask our super AI Physics teacher below questions of Physics s1-s4 and get answers
Assignment
ASSIGNMENT : Chapter 7: Expansion of Solids, Liquids and Gases – Sample Activity MARKS : 10 DURATION : 1 week, 3 days