The term state can be defined as a set of conditions that describe a person or thing at a given time. It is in this sense of the word that scientists divide matter into the three states shown in the figure below. The three states of matter have characteristic properties.
Solids have a distinct shape.
When they melt, the resulting liquid conforms to the shape of its container. Gases expand to fill their containers. There are two reasons for studying gases before liquids or solids. First, the behavior of gases is easier to describe because most of the properties of gases do not depend on the identity of the gas. Second, a relatively simple, yet powerful, model known as the kinetic molecular theory is available, which explains most of the behavior of gases.