A solvent is a substance in which some other substance, called a solute, can dissolve to form a solution. Both the solvent and the solute can be solid, liquid, or gas, but liquid solvents and liquid or solid solutes are the most common and useful. Such substances are commonly used in industrial chemical processes, in a variety of experiments and processes in chemistry, and in some household chemical products. Solvents are not universal — different types of substances must be used to dissolve different solutes. The amount of solute that can dissolve in a given substance depends heavily on the temperature, volume or mass ratio, and various chemical properties of the substances involved.
The solubility, or tendency of a substances to dissolve in a given other substance, depends heavily on polarity, which is determined primarily by the distribution of electrons across a molecule. Chemists tend to follow a basic rule that is generally formulated as "like dissolves like." This means that a polar solute is likely to dissolve in a polar solvent while a non-polar solute is likely to dissolve in a non-polar solvent. Other properties, such as volume and temperature of the substances involved, are also important determinants of solubility, but polarity is usually the most important factor.
In both science and industry, it is important to determine the best conditions in which to dissolve a solute. Solutes and solvents can be quite expensive, particularly when used in large quantities, so choosing the best substance and temperature conditions for dissolving a solute can save a great deal of money. In some cases, this involves using an excess of solvent in order to ensure that all of the solute is dissolved, as undissolved solute is often wasted. In some cases, solvents can be reused after chemical processes, while in others, they are unusable and must be disposed.
Uses for solvents are quite numerous, even outside of industrial and scientific settings — many chemicals for cleaning or personal use, for instance, contain solvents. They are used in detergents, soaps, paint thinners, and a variety of other household chemicals, particularly those used for cleaning purposes. Some personal care products, such as nail polish remover, are also chemically based in solvents. Most of the solvents aside from water that are used in homes, industry, and laboratories are organic, meaning that they contain carbon and, in most cases, hydrogen. Some inorganic ones are also used, but these are almost exclusively used for research in chemistry.