Solutions d'électrolyte et substances non-électrolytes

Water dissolves ionic compounds because it is a polar solvent with partial negative charge at oxygen and partial positive charge at hydrogen. When ionic solutes are added, water molecules interact with the ions. If solute-solvent attractive forces overcome solute-solute interactions, the ions separate and become hydrated by surrounding water molecules, dispersing uniformly throughout the solution.

Electrolytes are substances that produce electrically conductive solutions when dissolved in polar solvents because they yield ions. Nonelectrolytes do not conduct electricity because they dissolve as neutral molecules. For example, sodium chloride is an electrolyte that dissociates into ions, while sucrose is a nonelectrolyte that remains intact in solution.

Strong electrolytes dissociate nearly completely into ions, making their solutions good conductors of electricity. Weak electrolytes only partially dissociate, so their solutions conduct electricity to a small degree. For example, hydrochloric acid is a strong electrolyte while hydrofluoric acid is a weak electrolyte, resulting in different levels of electrical conductivity in their aqueous solutions.

Ion-dipole attraction is the electrostatic attraction between an ion and a molecule with a dipole, such as water. When ionic compounds dissolve, water molecules surround and solvate the ions through ion-dipole interactions, reducing the strong electrostatic forces between them. This allows the ions to separate and disperse uniformly throughout the solution.

Pure hydrogen chloride gas contains covalent HCl molecules with no ions and does not conduct electricity. However, when dissolved in water, HCl molecules react chemically with water, transferring H+ ions to form hydronium ions and chloride ions. This reaction is nearly 100% complete, producing appreciable ion concentration that makes the aqueous solution a very good conductor.

You can measure electrical conductance by applying voltage to electrodes immersed in an aqueous solution. Quantitatively, measure electrical current flow; qualitatively, observe light bulb brightness in the circuit. Conductivity increases with ion concentration, allowing you to classify substances as strong electrolytes (high conductivity), weak electrolytes (low conductivity), or nonelectrolytes (no conductivity).

When a weak acid like acetic acid dissolves in water, only some molecules dissociate into ions through a reversible reaction. This partial dissociation produces relatively low concentrations of ions, such as acetate ions and hydronium ions. Because ion concentration remains low, weak acids are classified as weak electrolytes and conduct electricity only to a limited degree.