Boiling Points

Liquids are held together by weak interactions between their molecules. To transition to the gas phase, a molecule must gain enough energy from heat or other sources to overcome these interactions.

Let's consider a closed container of a pure liquid at room temperature. Some of the liquid phase molecules at the surface gain enough energy to evaporate, and some of that vapor loses energy and condenses back to liquid. When the rates of evaporation and condensation are the same, the liquid and vapor reach equilibrium. At this point, the space above the liquid is saturated with vapor, and there is no net change in the amount of liquid and vapor in the container. The pressure exerted by the vapor is called the vapor pressure.

Now, consider an open container of liquid. Here the vapor can escape, keeping the area above the liquid from being saturated. Thus, evaporation could continue until there is no liquid left. As the temperature of a liquid increases, so does the evaporation rate, and therefore, so does the vapor pressure. If enough heat is added, molecules deeper in the liquid start vaporizing, which we see as bubbles of vapor forming.

This is called boiling, and it starts when the va