Combined Science
Physics
Cambridge IGCSE
Physics
Cambridge IGCSE
TOPIC 2: THERMAL PHYSICS
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 boiling?
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When water is heated, the particles gain kinetic energy. 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:
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.
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
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.
