Gas
Gas, one of the three ordinary states of matter. The other two ordinary states of matter are solid and liquid. Both solids and liquids are made up of particles that touch one another. The attraction between the particles of a solid is so strong that the particles hold rigidly together. This rigidity gives solids a definite shape and volume. The attraction between particles in a liquid is great enough to hold the particles near each other but too weak to prevent the particles from sliding around. Liquids have a definite volume but take the shape of their container. The particles that make up a gas, however, are completely separated from one another. Empty space accounts for more than 99 percent of the total volume of air, for example. Because gas particles are separated, the attractive forces between them are extremely small and are insufficient to hold gases in a definite shape or volume. Gases expand freely to fill their containers. PROPERTIES OF GASES The characteristics or properti