Single replacement reactions occur if the lone element can replace a similar element in a compound. Metals can replace less active metals. Some metals can replace hydrogen in acids. Very active metals, such as sodium and potassium, can replace hydrogen in water. Halogens can replace other halogens. An activity series of the elements is used to make definitive decisions about the validity of single replacement reactions.

Double replacement reactions happen if they produce any of these three substances:

The formation of a precipitate is predicted with a solubility table. Substances that are insoluble or partially soluble precipitate. Soluble substances do not precipitate.