An electrolyte is a solution that is able to conduct electricity. To meet this criteria, a solution must contain ions. Commonly, electrolyte solutions contain acids, bases, or salts as their ionic solute. Electrolyte solutions are normally formed when a salt is placed into a solvent such as water. The individual components dissociate due to the thermodynamic interactions between the solvent and solute molecules in a process called solvation. For example, when table salt, NaCl, is placed in water, the salt (a solid) dissolves into its component ions, according to the dissociation reaction:
NaCl(s) → Na+(aq) + Cl−(aq)
It is also possible for substances to react with water to produce ions. For example, carbon dioxide gas dissolves in water to produce a solution which contains hydronium, carbonate, and hydrogen carbonate ions. To determine the concentration of a dissolved species in a solution, one must consider the number of moles of ions formed per mole of solute added. For example, one mole of sodium chloride dissolved in one liter of water produces a solution that is two molar in ions, as it contains one mole of sodium ions and one mole of chloride ions. For a species such as magnesium bromide, MgBr2, one mole of solute will produce three moles of dissolved ions.
When solutes dissociate (or if a molecule ionizes), the solution can conduct electricity. Compounds that readily form ions—making them good conductors—are known as strong electrolytes. If only a small amount of ions are formed, electricity is poorly conducted, and the compound is known as a weak electrolyte. A strong electrolyte will dissolve completely and all ions will dissociate. A weak electrolyte, on the other hand, will partially dissociate, but some ions will remain bonded together. An example of a weak electrolyte is acetic acid, which only forms acetate anions and hydronium cations to a small extent.
Nonelectrolytes are solutions which cannot readily conduct electricity. The solute molecules are typically held together by covalent rather than ionic bonds. This leads to an inability to dissociate when dissolved. These solutions exhibit the normal effects of boiling-point elevation and freezing-point depression. For purposes of determining concentration, one mole of solute remains one mole after being dissolved in the solvent.