What is meant by (a) displacement reaction, and (b) double displacement reaction? Explain with the help of one example each.

(a) Displacement Reaction: The reactions in which one element displaces another element in a compound, are known as displacement reactions.

When Iron nail is placed in a copper sulphate solution,  iron sulphate and copper are obtained. In this reaction, Iron displaces copper from copper sulphate compound. The blue colour of copper sulphate solution turns green due to the formation of zinc sulphate.

(b) Double displacement reactions: The reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions.