Question

What causes vapor pressure?

Answer 1

There is always an equilibrium between the gaseous phase and the liquid phase, and such equilibrium is dependent on temperature. So the kinetic energy of the particles is the cause.

Explanation:

The saturated vapor pressure of water is something that is very well-tabulated. When water is raised in temperature to `100^@` `C`, its vapor pressure is equal to 1 atmosphere (this is why bubbles of vapor form directly in the liquid when boiling). Even involatile liquids (such as mercury metal) have a measurable vapor pressure.