The three states of matter

In principle, all substances can exist in three states: solid, liquid, and gas. As shown in the figure below, gases differ from liquids and solids in the average distance between molecules. In a solid, molecules are held together in an orderly manner, with limited freedom of movement. The molecules in a liquid are close to each other, without being held in a rigid position, allowing them to move. In a gas, molecules are separated from each other by large distances compared to the size of the molecules themselves.

Conversions between the three states of matter are possible without changing the composition of the substance. When you heat a solid (for example, ice), it melts and transforms into a liquid (water). (The temperature at which this transition occurs is called the melting point.) Further heating converts the liquid into a gas. (This conversion occurs at the boiling point of the liquid.) On the other hand, cooling a gas causes it to condense into the form of a liquid. Further cooling of this liquid causes it to freeze into its solid form.