(a) Define a double displacement reaction. (b) Write the chemical equation of a double displacement reaction which is also a (i) Neutralization reaction and (ii) Precipitation reaction. Give justification for your answer.

(a) Double Displacement Reaction: A chemical reaction in which there is an exchange of ions between the reactants to form new compounds is called a double displacement reaction.

(b) (i) Neutralization Reaction:

• Chemical Equation:

$$NaOH(aq) + HCl(aq) ightarrow NaCl(aq) + H_2O(l)$$

• Justification: In this reaction, sodium hydroxide (a base) reacts with hydrochloric acid (an acid) to form sodium chloride (salt) and water. There is an exchange of ions between the reactants: the sodium ion ($Na^+$) combines with the chloride ion ($Cl^-$), and the hydrogen ion ($H^+$) combines with the hydroxide ion ($OH^-$). Since it involves an exchange of ions, it is a double displacement reaction. Because an acid and a base react to neutralize each other and form salt and water, it is also a neutralization reaction.

(ii) Precipitation Reaction:

• Chemical Equation:

$$BaCl_2(aq) + Na_2SO_4(aq) ightarrow BaSO_4(s) + 2NaCl(aq)$$

• Justification: In this reaction, aqueous solutions of barium chloride and sodium sulphate react to form a white insoluble solid, barium sulphate ($BaSO_4$), which precipitates out of the solution, along with soluble sodium chloride. There is an exchange of ions: barium ions ($Ba^{2+}$) combine with sulphate ions ($SO_4^{2-}$), and sodium ions ($Na^+$) combine with chloride ions ($Cl^-$). Since there is an exchange of ions, it is a double displacement reaction. Because an insoluble precipitate is formed, it is also a precipitation reaction.