Why evapourate at a low pressure?

Answer 1

Reducing the air pressure allows the molecules to escape

Answer 2

Liquids are made up of molecules. In any normal situation, the temperature of a liquid is above absolute zero, so the molecules in it have some random kinetic energy that the temperature represents. Some molecules in the liquid will be almost still, but others will have quite a lot of energy. When a really energetic molecule moves quickly enough away from the surface of the liquid, it can escape the liquid entirely. The more molecules that reach the liquid's "escape velocity," the higher the liquid's vapour pressure. Think of this in terms of the liquid being inside a cylinder. As long as there is still some of the liquid present inside that cylinder, there is a certain tendency, on average, for molecules in that liquid to make the transition to the air above the liquid. This translates to a certain number of molecules moving into the air per unit liquid surface area per unit time. That's the explanation for how a vapour builds up over a liquid. That vapour is basically a gas. The number of molecules above the liquid that tend to escape in a given period of time over a given liquid area determines the 'vapour pressure' of that liquid at its current temperature. The vapour pressure in the cylinder exerts a force against the air above it. If the liquid vapour pressure is far lower than the pressure of the air around it, the rate of evapouration is slow. As the gas surrounding the liquid reduces in pressure, there is less resistance against the vapour, so the vapour is able to force itself away from the liquid to a greater extent. Finally, as the gas pressure around the vapour is reduced to the point where the vapour pressure is equal to that gas pressure, the evapouration will continue at a pace sufficient to maintain that vapour pressure above the liquid. The gas pressure won't be able to be reduced below that vapour pressure until all the liquid is gone. At the point where the liquid is completely gone, the molecules in the liquid are all just part of the gas in the chamber inside which you're varying the pressure. Since it's the temperature of the liquid that determines the vapour pressure, for that vapour pressure to stay constant, the temperature of the liquid has to also stay constant. Evapouration will cool a liquid, so as we evapourate the liquid, we will have to add heat to it to keep the liquid temperature constant. Evapouration cools a liquid because it is the molecules with the most energy that escape, which lowers the average energy in the remaining liquid. Finally, the pressure of a gas is made up of the partial pressure of all the constituent chemical species that make up the gas. For instance, if you have a mix of 80% nitrogen and 20% oxygen at 100 kPaA, the partial pressure of the nitrogen is 80 kPa and the partial pressure of the nitrogen is 20 kPa. The partial pressure of any gas can't exceed the vapour pressure of that gas at the temperature of the gas. For instance, water can't exist in air to a proportion greater than the ratio of water's vapour pressure at the ambient temperature to the atmospheric pressure. If that ratio is exceeded, the excess water vapour will condense into liquid.

Answer 3

Evaporation at a low pressure occurs because the reduced pressure lowers the boiling point of the liquid, allowing it to vaporize more easily. This process is commonly used in vacuum distillation to separate compounds with different boiling points.

Answer 4

no one actually cares