Question

What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube? What type of reaction is this?

Answer 1

Hint:Potassium iodide (chemical formula-$KI$) is not soluble in water while Lead nitrate (chemical formula-$Pb{(N{O_3})_2}$). When these two react together a white precipitate compound is formed which is also insoluble in water.

Complete step by step answer:
Lead nitrate on reaction with potassium iodide yields precipitates of lead iodide. When a solution containing potassium iodide is mixed with a solution containing lead nitrate in a test tube, a precipitate of a yellowish solid is observed. The yellowish solid formed is lead iodide. Potassium nitrate is formed as a by-product with lead iodide. The chemical reaction for this is;
\[KI(aq.) + Pb{(N{O_3})_2}(aq.) \to 2KN{O_3}(aq.) + Pb{I_2}(s)\]
The reaction between $KI$and $Pb{(N{O_3})_2}$ is an example of a double displacement reaction. When two compounds react with each other by an exchange of ions and form two new compounds, such reactions are called double displacement reactions. In these reactions, the exchange of positive ions is done by negative ion partners. A double replacement reaction is represented by the general equation.
$AB\, + \,CD \to AC + BD$
In a double displacement reaction, atoms from two different compounds exchange their places. Reactants are two different compounds and the products are two different compounds. Double displacement reactions generally occur between ionic compounds.
Potassium iodide and Lead nitrate both are ionic compounds having their individual cations and anions which when reacts together exchange the cations and anions of the other atom, thus making the reaction a double displacement reaction.

Note:
Lead iodide is the yellow precipitate which is prepared by the double displacement reaction between $KI$and $Pb{(N{O_3})_2}$. Lead iodide is used as a high energy photon detector for the X-rays and gamma rays.