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Pressure, Temperature and Change of State.

What keeps that beaker of water from boiling when it is at room temperature? If you say it's because it is not hot enough, sorry but you are wrong. The only thing that keeps it from boiling is the pressure of the air molecules pressing down on the surface of the water. When you heat that water to 212 degrees and then continue to add heat, what you are doing is supplying sufficient energy to the water molecules to overcome the pressure of the air and allow them to escape from the liquid state. If you took that beaker of water to outer space where there is no air pressure the water would flash into a vapour. If you took that beaker of water to the top of Mt. Everest where there is much less air pressure, you would find that much less heat would be needed to boil the water. (it would boil at a lower temperature than 100 degrees).

So water boils at 100 degrees at normal atmospheric pressure. Lower the pressure and you lower the boiling point. Therefore we should be able to place that beaker of water under a bell jar and have a vacuum pump extract the air from within the bell jar and watch the water come to a boil even at room temperature. This is indeed the case! Altering the amount of pressure on something can make it change state.

Imagine a gas is trapped inside a container which has a fixed size (its volume cannot change). If the gas is heated the particles will gain kinetic energy which will make them move faster. This means they will collide with the walls more frequently. This causes the force on the walls of the container to increase and so the pressure increases.

  • Fixed volume: adding heat = increases the pressure
  • Fixed volume: removing heat = decreases the pressure
  • Fixed volume: increasing the pressure = increase in heat
  • Fixed volume: dropping the pressure = decrease in heat

A liquid requires heat to be added to it in order for it to overcome the air pressure pressing down on its' surface if it is to evaporate into a gas. We just learned that if the pressure above the liquids surface is reduced it will evaporate easier. We could look at it from a slightly different angle and say that when a liquid evaporates it absorbs heat from the surrounding area. So, finding some fluid that evaporates at a handier boiling point than water (IE: lower) was one of the first steps required for the development of mechanical refrigeration.

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