A solution is a homogeneous (i.e. evenly mixed) mixture of two or more substances. Solutions are composed of two parts: the solute and the solvent. The solvent is the substance that retains its phase after mixing or, in the case where the substances are in the same phase at the beginning of the mixing, the substance present in the greater amount. The solute is then the substance that is added to the solvent. When water is the solvent, the process of dissolving a solute in the solvent is known as hydration, and the resulting solution is an aqueous solution. However, not every solute dissolves in a given solvent. Solvents dissolve solutes that have similar polarity. An ionic solute will be dissolved by a polar solvent like water. To dissolve a non-polar species such as napthalene, a non-polar solvent such as carbon tetrachloride would have to be used. Generally, "like dissolves like."
When an ionically bonded solute is dissolved in an appropriate solvent, its component ions separate in the solvent. Such a solution with charged particles is capable of carrying electrical charge and is called an electrolyte. The ions resulting from the dissolution of the solute are known as cations (positively charged ions) and anions (negatively charged ions). Some commonly occurring polyatomic ions are listed below.
