In chemistry, a solution is one or more substance (the solute) dissolved in another substance (the solvent) forming a homogenous mixture. A common example would be a solid dissolving into a liquid, like salt or sugar dissolving in water (or even gold into mercury, forming an amalgam); but also gases may dissolve into liquids, like carbon dioxide or oxygen in water, and liquids and gases into themselves.
The solvent is defined as the substance that exists in a greater quantity than the solute(s) in the solution. If both solute and solvent exist in equal quantities (such as in a 50% ethanol 50% water solution), the substance that is more often used as a solvent is designated a solvent (in this case, water).
Solvents can be broadly classified into polar and non-polar solvents. Common polar solvents include water and ethanol. Generally polar or ionic compounds will only dissolve in polar solvents. An excellent test for the polarity of a liquid solvent is to rub a plastic rod, to induce static electricity. Then hold this charged rod close to a running stream of the solvent. If the path of the solvent deviates when the rod is held close to it, it is a polar solvent.
When a solute is dissolved into a solvent, especially polar solvents, a structure forms around it (a process called solvation), which allows the solute-solvent interaction to remain stable.
When no more of a solute can be dissolved into a solvent, the solution is said to be saturated. However the point at which a solution can become saturated changes significantly with different environmental factors, such as temperature, pressure, and contamination. Raising the solubility (such as by increasing the temperature) to dissolve more solute, and then lowering the solubility causes a solution to become supersaturated.
In general the greater the temperature of a solvent, the more of a given solute it can dissolve. However, some compounds exhibit reverse solubility, which means that as a solvent gets warmer, less solute can be dissolved. Some surfactants exhibit this behaviour.