Making a saline water solution by dissolving table salt (NaCl) in water This article is about chemical solutions. For other uses, see Solution (disambiguation). In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. A common example is a solid, such as salt or sugar, dissolved in water, a liquid. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can combine with other gases to form mixtures, rather than solutions.[1] Solutions are characterized by interactions between the solvent phase and solute molecules or ions that result in a net decrease in free energy. Under such a definition, gases typically cannot function as solvents, since in the gas phase interactions between molecules are minimal due to the large distances between the moieties. This lack of interaction is the reason that gases can expand freely and the presence of these interactions is the reason that liquids do not expand. Examples of solid solutions are alloys, certain minerals and polymers containing plasticizers. The ability of one compound to dissolve in another compound is called solubility. The physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, molality, and parts per million (ppm). Solutions should be distinguished from non-homogeneous mixtures such as colloids and suspensions.