What Is the Definition of "solid?"

Solids are defined as substances that retain their shape, even when they are not confined. Substances usually assume the solid state of being at lower temperatures than they do when they are liquids, gases or plasmas.

For example, a piece of ice retains its form even when not in a container. However, if the temperature rises, the ice will begin to melt and turn into a liquid. Liquids do not retain their shape when outside of a vessel, so the water will tend to leak away from the ice. If the temperature warms even more, the water will take the form of a gas. Gases have even less shape than liquids, and the gas may even rise in the air.

Many elements naturally occur in the solid state, including gold, carbon, sodium and silver. However, the term “solid” applies to both pure chemicals as well as compounds and mixtures. For example, water occurs as the three primary forms of matter at different places in the world. The solid form of water, also known as ice, occurs in places where the temperature is cold enough that the molecules move very closely together and lock into place.

Some solids exhibit a crystalline structure at the molecular level. This gives some of these materials, such as diamonds, remarkable strength. However, many crystalline solids are prone to fracturing along a single plane or surface.