Solutions are everywhere. The tears in your eyes are a solution of water and salt, and the nectar in flowers is a solution of water and sugar. In chemistry and biology, a solution consists of a solvent and a solute and, by definition, the solvent is the component with the higher concentration. A solution is usually a liquid, but it doesn't have to be. Metal alloys are examples of solid solutions; to make stainless steel, for example, manufacturers add molten chromium to molten steel and let the mixture cool. In the case of stainless steel, the concentration of steel is higher, so it's the solvent and chromium is the solute.
The Solute Dissolves in the Solvent
To qualify as a solution, a solvent must contain a dissolved solute. Dissolution is an electrostatic process whereby the solvent molecules surround those of the solute and force them to break apart. A solution is not a suspension or emulsion, which is a liquid that contains undissolved particles. Another word for that type of mixture is a colloid. Because the particles are large and undissolved, they give the mixture a cloudy or milky appearance. Speaking of milky, milk is a classic example of a colloidal mixture.
Polar and Non-Polar Solvents
Water is one of the most familiar and best solvents in the world, and the reason is because of the high polarity of the water molecule. The mechanism by which it dissolves solutes applies to all similarly polar solvents, such as methanol. The geometry of the molecule gives it distinct positive and negative ends and the ability to interact electrostatically with the molecules of polar solutes. Water molecules are attracted to electrically charged solute molecules. If the attraction is strong enough to break the solute molecules apart and distribute them evenly, the solute dissolves. Non-polar solutes, such as fats, oils and greases, won't dissolve in water. At best, they'll create an emulsion.
Non-polar solvents, such as carbon tetrachloride and benzene, also dissolve solutes by electrostatic attraction. The solvent electrons tend to group on one side of the molecule and attract similarly large, non-polar solute molecules. This is how greases, fats and oils, which won't dissolve in water, dissolve in non-polar solvents.
Organic and Inorganic Solvents
Besides polarity, chemists categorize solvents by their chemical composition. Inorganic solvents, of which water and ammonia are examples, do not contain carbon. Organic solvents (those containing carbon) can be oxygenated, which means they contain oxygen. Examples are alcohols, ketones and glycol ethers. Hydrocarbon solvents contain only carbon and hydrogen; gasoline, benzene, toluene and hexane are some examples. Finally, halogenated solvents contain one of the halogens: chlorine (Cl), fluorine (F), bromine (Br) or iodine (I). Carbon tetrachloride, chloroform and chlorofluorocarbons (CFCs) are some examples of halogenated solvents.
The word "solvent" gets tossed around rather carelessly in the world of paint technology. Technically, all paints contain a solvent – it's a key ingredient. However, when paint technologists call a paint "solvent-based," they're talking about one that doesn't contain water. It may contain turpentine or any one of a number of other organic solvents, including toluene, xylene or mineral spirits. According to this imprecise language, the opposite of a solvent-based paint is a water-based paint, even though water is probably the best solvent in the world. Go figure.