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Chemical reactions in aqueous solutions fall into three main categories: precipitation, acid-base, and oxidation-reduction. In precipitation reactions, dissolved ions combine to form insoluble compounds that settle out. Acid-base reactions involve neutralization where protons and hydroxide ions form water. Oxidation-reduction reactions involve electron transfer between reactants, where one substance loses electrons and another gains them.

Water-soluble ionic compounds dissociate into separated ions that disperse throughout the solution, while water-insoluble ionic compounds remain as solid precipitates. Molecular solids dissolve as intact molecules without breaking apart. This distinction is crucial when writing equations for aqueous reactions because it determines how substances are represented in chemical equations.

A molecular equation shows complete neutral formulas of compounds as if they existed as whole units, providing stoichiometric information for calculations. A complete ionic equation explicitly represents all dissolved ions as free species in solution. The complete ionic equation reveals spectator ions—ions appearing identically on both sides that don't participate in the actual reaction.

Spectator ions are ions that appear in identical forms on both sides of a complete ionic equation. They are neither chemically nor physically changed during the reaction and are required only to maintain charge neutrality. Removing spectator ions produces the net ionic equation, which shows only the ions and molecules that actually participate in the reaction.

In a precipitation reaction, dissolved cations and anions from separate ionic compounds exchange partners to form an insoluble ionic compound. For example, aqueous potassium chloride and aqueous silver nitrate react to produce insoluble silver chloride that precipitates from solution. The net ionic equation shows only the ions that combine to form the solid precipitate.

In an acid-base reaction, a proton (H+ ion) from an acid reacts with a hydroxide ion (OH−) from a base to form water. The counterions from the acid and base combine to form a salt. For example, hydrogen bromide reacts with potassium hydroxide to produce water and potassium bromide, with the acid and base neutralizing each other.

In oxidation-reduction reactions, electrons transfer from one reactant to another. The reactant that loses electrons is oxidized, while the reactant that gains electrons is reduced. For example, when potassium and chlorine combine to form potassium chloride, a potassium atom loses one electron to a chlorine atom, resulting in electron transfer between the two elements.