Single and double replacement

A single replacement or single displacement reaction is a reaction in which one substance replaces another.

A + BC → AC + B

The replacement of an ion in solution by a metal higher in the activity series is a special example of this reaction type.

The relative positions of the elements in the activity series provides the driving force for single displacement reactions.

A double replacement reaction is one in which there is an actual exchange of partners between reacting species. This reaction is more common between ionic substances;

AB + CD → AC + BD

Such reactions are usually driven by;

formation of precipitation formation of water and a gaseous product