A precipitation reaction is a type of double displacement (metathesis) reaction that occurs when two soluble aqueous solutions are mixed and their ions recombine to form a new compound that is insoluble, or only sparingly soluble, in water. This insoluble product, called the precipitate, separates out of the solution as a solid, often appearing as a cloudy suspension or gradually settling to the bottom of the container.

Whether a given pair of ions will form a precipitate is generally predicted using solubility rules — broad guidelines about which classes of ionic compounds tend to be soluble in water — and, more rigorously, by comparing the ion product of the dissolved species with the compound's solubility product constant (Ksp); when the ion product exceeds Ksp, the excess solid separates out of solution.

A classic example is:

AgNO₃(aq) + NaCl(aq) → AgCl↓ (white precipitate) + NaNO₃(aq)

Here, silver ions from silver nitrate combine with chloride ions from sodium chloride to form silver chloride, which has very low solubility in water and therefore separates out as a solid precipitate, while the sodium and nitrate ions remain dissolved in solution as spectator ions, taking no further part in the visible reaction.

The remaining options describe entirely different categories of chemical change, not precipitation:

• Formation of a gas during a reaction is characteristic of processes such as an acid reacting with a carbonate (releasing CO₂) or a reactive metal reacting with an acid (releasing H₂), not precipitation.
• Formation of a single product from two reactants describes a combination (synthesis) reaction, such as 2H₂ + O₂ → 2H₂O.
• Breaking down of a compound into simpler substances describes a decomposition reaction, such as CaCO₃ → CaO + CO₂ upon heating.