The ability of a substance to conduct electrical currents depends on how easily charges can move through it. Metallic solids, as we have seen, conduct electricity because the structure of the electronic orbitals allows electrons to migrate freely. In the case of aqueous solutions, electrical conductivity is governed by the presence and concentration of ions in solution. The movement of these ions in response to an electric field is what makes up a current. Therefore, pure water does not conduct an electrical current well since the concentrations of hydronium and hydroxide ions are very small. Solutes whose solutions are conductive are called electrolytes. A solute is considered a strong electrolyte if it dissociates completely into its constituent ions. Examples of strong electrolytes include ionic compounds, such as NaCl and KI, and molecular compounds with highly polar covalent bonds that dissociate into ions when dissolved, such as HCl in water. A weak electrolyte, on the other hand, ionizes or hydrolyzes incompletely in aqueous solution and only some of the solute is present in ionic form. Examples include acetic acid and other weak acids, ammonia and other weak bases, and HgCl2. Many compounds do not ionize at all in aqueous solution, retaining their molecular structure in solution. These compounds are called nonelectrolytes and include many nonpolar gases and organic compounds, such as oxygen and sugar.