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Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a substance yields ions in solution, that is, if the compound undergoes 100% dissociation, then the substance is a strong electrolyte. Complete dissociation is indicated by a single forward arrow. For example, water-soluble ionic compounds like sodium chloride dissociate into sodium cations and chloride anions in aqueous solution.

Other examples of strong electrolytes are hydroxides of group 1 and group 2 metals, and strong acids like HCl and HNO₃.

If only a tiny fraction of the substance dissociates into ions, that is, if the compound undergoes partial dissociation in solution, it is called a weak electrolyte. Partial dissociation is indicated by a reversible arrow. For example, weak acids like acetic acid partially ionize in water to give acetate ions and hydronium ions.

Other examples of weak electrolytes are poorly soluble salts like AgCl and PbCl₂, and weak bases like ammonia.        

The electrostatic attraction between an ion and a molecule with a dipole – called the ion-dipole attraction – plays an important role in the dissolution of ionic compounds in water. When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them.

Solutions of covalent compounds conduct electricity because the solute molecules react chemically with the solvent to produce ions. For example, pure hydrogen chloride is a gas consisting of covalent HCl molecules. This gas contains no ions. However, an aqueous solution of HCl is a very good conductor, indicating that an appreciable concentration of ions exists within the solution.

Because HCl is an acid, its molecules react with water, transferring H+ ions to form hydronium ions (H₃O⁺) and chloride ions (Cl⁻):

This reaction is essentially 100% complete for HCl (a strong electrolyte). Likewise, weak acids and bases that only react partially generate relatively low concentrations of ions when dissolved in water and are classified as weak electrolytes.     

Substances that do not yield ions when dissolved in water are called nonelectrolytes. Such substances dissolve as neutral molecules in solution, each surrounded by water molecules. For example, a molecular compound like sucrose dissolves in water as intact molecules.

In summary, substances may be identified as strong, weak, or nonelectrolytes by measuring the electrical conductance of an aqueous solution containing the substance. To conduct electricity, a substance must contain freely mobile, charged species. Most familiar is the conduction of electricity through metallic wires, in which case the mobile, charged entities are electrons. Solutions may also conduct electricity if they contain dissolved ions, with conductivity increasing as ion concentration increases. Applying a voltage to electrodes immersed in a solution permits assessment of the relative concentration of dissolved ions, either quantitatively, by measuring the electrical current flow, or qualitatively, by observing the brightness of a light bulb included in the circuit. 

This text is adapted from Openstax, Chemistry 2e, Section 11.2: Electrolytes.