Liquids are held together by van der Waals attractions, dipole forces if the molecules are polar, hydrogen bonds, and electrostatic attractions between ions of a molten salt.

We will be concerned mainly with molecular liquids such as water, in which hydrogen bonds and van der Waals and dipole forces are the most important factors.

Not all molecules in a liquid move with the same speed. In general, the higher the temperature, the faster they move; but the molecules in a liquid have a range of speeds rather than one uniform speed.

As molecules collide with one another they gain and lose energy, but the liquid as a whole maintains a velocity distribution of the type shown below. Increasing the temperature simply shifts the distribution maximum to higher speeds.

Go on to the next page to see this animated.