Solution, a mixture of two or more substances in which the particles of each substance are of molecular or atomic size. A solution is always a homogeneous mixture - that is, every part of the mixture is exactly like every other part. Solutions are formed by dissolving one substance, called the solute, in another substance, known as the solvent. One of the world's most abundant and common solutions is seawater, which consists primarily of salt (the solute) dissolved in water (the solvent).
Solutions can occur in any of the three states of matter: solid, liquid, and gaseous. The most common solutions are liquid, composed of a solid, liquid, or gas dissolved in a liquid solvent. Other solutions result from the dissolving of one gas in another, a liquid in a gas, or even one solid in another. (Solid solutions are formed while the materials—copper and zinc for brass—are heated to the liquid state.) Water is the most abundant and widely used solvent. Its solutions are usually called aqueous solutions.
The fluids found in plants and animals are solutions of great complexity. Many drugs, cleaning compounds, beverages, and cosmetics are supplied in solution form. In industry, hundreds of products are made with the help of chemical reactions that occur in solutions.
In a solution, the solute breaks down into tiny particles that become uniformly distributed among particles of the solvent. In some solutions, such as sugar in water, the solute particles are molecules. In solutions of acids, bases, or salts in water, the solute breaks down into electrically charged particles known as ions. Ionic solutions, called electrolytes, conduct electricity and are very active chemically. Because of their extremely small size, solute particles (such as the sugar in a solution of sugar in water) will not settle out, nor can they be filtered out, as is possible with other mixtures such as colloids and suspensions.
Depending on their nature, the solute and solvent may or may not undergo a chemical change when a solution is formed. When sugar is dissolved in water, the change is only physical, since when the water is evaporated away, the sugar remains. When hydrochloric acid dissolves a metal such as zinc, there is a chemical change. If the solution is evaporated, it is zinc chloride, not the original zinc, that remains.