Double-displacement

Meaning: In a double-displacement (metathesis) reaction, the cations and anions of two ionic compounds in solution exchange partners. No electrons are transferred, so every oxidation number stays the same. The reaction goes appreciably forward when a product removes ions from solution: an insoluble solid (precipitation), a gas such as CO₂, or a weak electrolyte such as water (acid-base neutralization). If all four ions stay dissolved, nothing net happens.

Equation: AB + CD → AD + CB, where A and C are cations, B and D are anions, and the coefficients are mole ratios (mol) chosen to balance atoms and charge. Example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).

On the MCAT: A passage mixes two solutions. You use solubility rules (nitrates, Group 1 salts and ammonium salts are soluble; most chlorides are soluble except those of Ag⁺ and Pb²⁺) to find the precipitate. You then write the net ionic equation by cancelling spectator ions, or use stoichiometry and the limiting reagent to find the mass of solid formed. Neutralization of an acid by a base is also a double displacement.

Common confusion: Students mix it up with single displacement, where an element replaces an ion in a compound (Zn + Cu²⁺ → Zn²⁺ + Cu). Single displacement is redox, because oxidation numbers change. Check them: if none change, it is double displacement. Also, swapping partners on paper does not guarantee a reaction; one product must be a solid, gas, or weak electrolyte.