|Picture 2.4 The particles in a liquid can move around. Some of them have enough KE to escape from the surface and form a vapour. This is in dynamic equilibrium with the liquid.|
| || ||The particles in a liquid are free to move around, although they are still weakly bonded to other particles. Once again, the total internal energy of the liquid is randomly distributed amongst the particles some as kinetic energy and some as elastic potential energy. As the temperature rises, the average total energy of the particles increases until, once again, they can break free of the bonds that are holding them into the liquid. A particle that has enough energy will escape from the liquid. Eventually, when all the particles are able to gain sufficient energy, the liquid will turn into a gas.|
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| ||Why does a liquid boil?|
| || ||As the average total energy of the liquid particles rises, some of them can break free of the surface. These particles form a vapour above the liquid surface. We say they have evaporated. They might return to the liquid or they might remain as a vapour. At a given temperature, there is a dynamic equilibrium between the vapour particles and the liquid particles. |
As the liquid temperature increases, more particles will join the vapour. This will increase the vapour pressure. Eventually, the vapour pressure will match the surrounding, atmospheric pressure. At that point the liquid will boil.
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