A double replacement reaction is a type of chemical reaction in which two compounds exchange parts of their ions to form two new compounds. In simple words, the positive ion from one compound swaps places with the positive ion from another compound.
These reactions usually happen in solutions where the compounds are dissolved in water. A double replacement reaction often results in the formation of a precipitate (an insoluble solid), a gas, or water. That’s why they are sometimes called precipitation reactions, neutralization reactions, or gas-forming reactions, depending on the products.
For example, when an acid reacts with a base, it is a double replacement reaction because the hydrogen ion from the acid swaps with the metal ion from the base, forming water and a salt.
This kind of reaction is important in daily life and industry, such as in making soaps, neutralizing stomach acid with antacids, and purifying water.