Liquid
. Liquid, one of the three physical states in which matter can exist. The other two states are solid and gaseous. The molecules of liquids are arranged less tightly than those of solids but more closely than those of gases . Liquids and gases take the shape of their container, unlike solids, which keep their own shape. Liquids and solids maintain a definite volume, or size, while gases will expand to fill a container. For example, a liter of liquid will not expand to fill a two-liter container, but a liter of gas will. Most substances can exist in the liquid state at the right temperature and pressure. Water, for example, exists as a liquid at room temperature (20° C/68° F) and normal atmospheric pressure (the pressure of the atmosphere at sea level). Helium, on the other hand, is a gas under these conditions. It must be cooled to a very low temperature or compressed to a very high pressure to become a liquid. Iron is a solid at room temperature and normal pressure and must be heated