Solids, liquids, and gases are the three states of matter commonly found on Earth. A solid is rigid and possesses a definite shape. A liquid flows and takes the shape of its container, except it forms a flat or slightly curved upper surface when acted upon by gravity. Both liquid and solid samples have volumes nearly independent of pressure. A gas takes both the shape and volume of its container.

Scientists have discovered a fourth state of matter, plasma, that occurs naturally in the interiors of stars. A plasma is a gaseous state of matter that contains appreciable numbers of electrically charged particles. The presence of these charged particles imparts unique properties to plasmas that justify their classification as a state of matter distinct from gases. In addition to stars, plasmas are found in some other high-temperature environments (both natural and man-made), such as lightning strikes, certain television screens, and specialized analytical instruments used to detect trace amounts of metals.

Some samples of matter appear to have one or more properties of solids, liquids, and gases simultaneously. This can occur when the sample is composed of many small pieces. For example, we can pour sand as if it were a liquid because it is composed of many small grains of solid sand. Matter can also have properties of more than one state when it is a mixture, such as with clouds. Clouds appear to behave somewhat like gases, but they are mixtures of air (gas) and tiny water particles (liquid or solid).

The differences in the properties of a solid, liquid, or gas reflect the strengths of the attractive forces between the atoms, molecules, or ions that make up each phase. The phase in which a substance exists depends on the relative extents of its intermolecular forces (IMFs) and the kinetic energies (KE) of the molecules. Particles in a solid are tightly packed together and often arranged in a regular pattern. In a liquid, they are close together with no regular arrangement. In a gas, they are far apart with no regular arrangement. Particles in a solid vibrate about fixed positions and do not generally move in relation to one another. In a liquid, they move past each other but remain in constant contact. In a gas, they move independently of one another except when they collide.

This text is adapted from Openstax, Chemistry 2e, Section 1.2: Phases and Classification of Matter and Openstax, Chemistry 2e, Section 10.1: Intermolecular Forces