(a) The reaction in which one element takes place of another element is called displacement reactions.

Ex- CuSO₄ (aq.) + Zn (s) → ZnSO₄ + Cu

When zinc is added to copper sulphate solution, then zinc displaces copper from its aqueous solution forming Zinc sulphate(aq.) and Copper metal. Here, copper is displaced by Zinc from its compound.

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

Ex- AgNO₃ (aq.) + NaCl (aq.) → AgCl (s)+ NaNO₃ (aq.)

Here, Silver and sodium displaces each other from their aqueous solution to form Silver chloride(solid) and Sodium nitrate (aqueous).