How Do We Know a Thing Is Solid, Liquid or Gas?
The relations of the molecules in any substance to each other is what determines whether a substance is a solid, a liquid or a gas. A gas is a substance in which the molecules are constantly moving rapidly about among each other, but always in straight lines. A liquid substance is one in which the molecules are also constantly moving about but which do not move in straight lines. Solids are substances in which the molecules stick together in one position by the power of cohesion which they have. Cohesion means the power of sticking together.