Double displacement
Consider the fourth reaction of activity 16.2. Here barium chloride and sodium sulphate react and exchange parts.
barium chloride + sodium sulphate → barium sulphate + sodium chloride
BaCl2 + Na2SO4 → BaSO4 + 2NaCl
Double displacement reaction
A reaction in which an element or an ionic group in one compound exchanges with an element or an ionic group in another compound is called a double displacement reaction.
AB + CD → AD + CB
Two more examples
ferrous sulphate + sodium hydroxide → ferrous hydroxide + sodium sulphate
FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4
calcium chloride + sodium carbonate → calcium carbonate + sodium chloride
CaCl2 + Na2CO3 → CaCO3 + 2NaCl
How to spot one
Both sides hold compounds only — there is no lone element anywhere. Instead the two compounds have swapped their parts with each other.
Single or double?
These two are often confused. If a lone element is involved it is a single displacement; if both sides are compounds it is a double displacement.
In all three examples one of the products is insoluble in water and so appears as a precipitate — which is itself the evidence that the reaction happened.