Vapor pressure

Every substance is a collection of atoms and/or molecules held together by mutual attraction. The temperature of that substance is a measure of the kinetic energy these molecules have - the faster they move and vibrate, the higher the temperature. Depending on the temperature and the pressure, the molecules may pack together tightly as a solid, may pack loosely as a liquid, or may freely move around as a vapor. These different states of matter are called phases, and the transition from one phase to another involves the absorption or release of large amounts of energy in the form of heat.

Molecules can escape from a solid or liquid substance to form a gas. When they escape from a solid, this is called sublimation; escape from a liquid is called evaporation.

Molecules manage to escape by having enough energy to get through a barrier at the surface of the substance. This barrier is created by the fact that the attractive force between molecules is directed inwards at the surface of a substance. Inside the body of the substance, the attractive forces are arrayed in all directions, and cancel themselves out.

This is a very strong force in liquids and is called surface tension, meaning that liquids act as if they have a "skin" on their surface holding them in. Surface tension causes liquids to form a meniscus against the side of a glass (the curved bit 'sticking' to the sides), and makes globules of mercury scoot around like ball bearings. Surface tension is strong enough to allow many insects to walk on water!

It takes a lot of energy to vaporize a liquid, far more than to just heat it up, and the extra energy is needed to overcome the attraction between the molecules in the liquid. The stronger the attractive force between the molecules of a liquid, the harder it is to vaporize. Once molecules leave the liquid or solid, their rapid motion adds to the pressure of the environment. Each substance produces a well-defined amount of pressure at a given temperature, known as its vapor pressure. The total pressure of the surrounding environment is made up of the pressure contributed by all the substances present.

Do not confuse vapor pressure with smell! It is true that scents and smells come to us by way of airborne molecules, but a strong smell does not imply a high vapor pressure. The strongest smelling substances of all are a group of chemicals called mercaptans, which include the famous "eau de skunk". As powerful as the stink is, though, it takes very, very few of these molecules to empty a room. The tiny number of molecules required to overpower your nose exert a negligible vapor pressure. More to the point of distilling, the almost odorless ethanol has a higher vapor pressure than the heavier fusel alcohols that appear late in a run, and which smell awful.

Net force inwards

Net force zero ce ^H

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