Question

using balanced chemical equation the difference between a displacement reaction and a double displacement reaction plz........ answer fast it's really vry important plz answer fast

Answer 1

A Displacement reaction is a chemical reaction wherein a highly reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions. 
ex: Fe + CuSO4⇒ FeSO4 + Cu    [Fe is more reactive than Cu and displaces it]

A Double Displacement reaction is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.
ex: Na2SO4 + CaCl2 ⇒ NaCl + CaSO4 
[cations Na and Ca switch places; anions sulphate and chloride switch places]

Answer 2

In displacement reaction, one element from its salt is displaced by a more reactive element e.g. in following example, Cu is displaced by Zn from CuSO4 because Zn is more reactive.

CuSO4 + Zn —> ZnSO4 + Cu  

In double displacement reaction, exchange of ions takes place between two reactants to form new products.

Na2SO4 + BaCl2 —> BaSO4 + 2NaCl