Physics
Cambridge IGCSE

TOPIC 2: THERMAL PHYSICS

# 2.2b Changing StateMelting, Boiling and Evaporation

In section 2.2a you will have learned that the particles in solids, liquids and gases all have different arrangments. But what happens to the particles when they change state, during melting or boiling?

 melting boiling

Figure 1. Particles during a change of state
Julio Miguel A Enriquez and Monica Muñoz -Wiki Learning Tec de Monterrey. CC BY-SA 4.0

### Temperature changes

As discussed in section 2.1b (temperature and K.E.), temperature is a measure of the kinetic energy of the particles in a substance. If we supply heat energy at a constant rate, the average particle K.E. increases and so does the temperature. However, when a change of state happens, it takes energy to break the bonds. This means that the energy supplied is going into breaking bonds (and increasing their potential energy as they are pulled apart). During this change of state, the temperature does not increase.

This is shown in the following graph:

Figure 2. Temperature during a change of state

Note that the same shape of graph is found when a liquid turns to a gas, with all of the energy supplied going into breaking the bonds between particles in the liquid.

In addiiton, we could reverse the process, and observe a constant temperature as a substance condenses back into a liquid, or solidifies from liquid to solid. The particle bonds are reforming during this time. Energy that is always lost to the surroundings is coming from the bonds snapping back together and releasing energy, meaning the particles remain at the same temperature with the same kinetic energy.

### Standard Practical: Obtaining a temperature–time graph to show the constant temperature during a change of state

This is a typical investigation into this constant temperature change as shown in figure 2:

You will need:

• A boiling tube half filled with naphthalene, a waxy substance used in moth balls. it is solid at room temperature.
• Temperature sensor (or thermometer)
• Data logger (or stop watch).
• A heat source e.g. Bunsen burner
• A large beaker of water

Method

Place the boiling tube containing naphthalene in the beaker of cold water as shown in figure 6. Heat the water until the naphthalene has melted and has reached 90 0C or above. Then put the temperature sensor in the liquid naphthalene as shown, turn off the heat source and begin recording the temperature.
As the naphthalene cools and freezes again, you should be able to see a flat region in the graph, just like in figure 2 above. However, this time the temperature is falling as it cools, so the graph is reversed as shown in figure 4.

Figure 3: Heating then cooling naphthalene

Figure 4. Expected cooling curve results

Note that you could leave the temperature sensor in the frozen naphthalene, and repeat the experiment to obtain a graph with the temperature increasing as it is heated.
You can see from the graph that the freezing (and melting) point of naphthalene is about 80 0C.

Note that the results are ideal, and in practice it can be quite hard to get a completely flat line on the graph - the napthaline at the edge of the tube cools faster so heat energy is not evenly distributed throughout the substance.

### Water

The Celcius scale of temperature (also called 'centigrade') was fixed using the freezing point and boiling point of pure water (at standard atmospheric pressure). You should be able to explain that this means water always freezes at 0°C and boils at 100°C: This was how this scale was invented!

Figure 5. The celcius scale based on the freezing and boiling points of water

## Evaporation

Evaporation is probably the most confusing part of a change of state. When water is heated, the particles gain kinetic energy as described above. Eventually they have so much energy that they can break the bonds holding the liquid in place, and the liquid water turns to a gas (steam) at 100°C or higher. This is called 'boiling'.

However in water at room termperature, not all particles have exactly the same energy. Due to random collisions, some have more energy and some have less. The average kintetic energy tells us how hot the liquid is, even though there is quite a range of particle speeds.

The faster moving particles on the surface have enough energy to break the bonds and leave the liquid. This is the process of evaporation -  a slow loss of faster moving particles from the surface of a liquid.

Figure 6: Evaporation in Water
Hawesthoughts - Public domain

The end result of the process shown in figure 6 above is that:

• a) the volume of the liquid decreases over time, with water vapour entering the air
• b) the remianing liquid has slower moving particles - it has cooled down.

This is how we cool down when we sweat. Small droplets of liquid form on our skin when we are too hot. The liquid starts to evaporate, with faster more energetic particles escaping, leaving the liquid with the slower, colder particles. This then cools our skin.

### Factors affecting evaporation

There are several key variables that affect evaporation:

a) Temperature: The higher the initial temperature of the liquid, the more likely it is that faster particles can escape. The rate of evaporation increases.

b) Surface area: Evaporation happens at a liquid's surface. Therefore, the larger the surface area, the faster the rate of evaporation. A dish holding water has a faster rate of evaporation than a tall, thin glass.

c) Wind: If air blows across the surface, it carries the particles that have escaped away from the surface, leaving less particles above. This actually decreases the pressure of vapour particles back down on the liquid, and allows more to escape. This again is evident when we sweat. We cool down much quicker when it is windy, and actually being wet through on a windy day can cause us to get dangerously cold through evaporation. It is why it is always a good idea to wear a waterproof jacket on cold, wet and windy days!

Figure 7: Stay dry and stay warm!
Andy Køgl  Unsplash

### What is the difference between boiling and evaporation?

When water boils at 100°C, all of the particles have a high kinetic energy, and the average kinetic energy is enough to break the bonds keeping the water as a liquid. Particles escape through the surface until all of the water has turned to steam. Boiling is therefore a rapid process of water turning to steam (water vapour in the air).

During evaporation, only a few of the particles randomly have enough energy to break free of the liquid.  Particles can escape from the surface but evaporation will therefore happen at a much slower rate. Evaporation is a slow process of water turning to water vapour. The remaining particles in the liquid will be colder, as only the hot, fast moving particles escape. During boiling, this cooling effect does not happen as all of the particles have enough energy to escape.

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