Question

Name the type of the reaction in which a yellow precipitate is formed when two colourless solutions are mixed.

Answer 1

Step 1: Chemical reactions

A chemical reaction is a process of conversion of one or more substances (reactants) into new substances with different chemical compositions (products) by the formation and breaking of chemical bonds. Chemical reactions are divided into different types based on the nature of the products formed nature of the process.

Step 2: Formation of yellow precipitate when two aqueous solutions are mixed

Reactions in which a solid insoluble precipitate is formed when aqueous solutions of two soluble ionic salts are mixed are called precipitation reactions. Therefore, the formation of a yellow precipitate is an example of a precipitation reaction.

Ionic salts remain in ionic form in an aqueous solution. When aqueous solutions of two soluble salts are mixed, the ions are exchanged between the metal ions forming two new products. These reactions are called double displacement reactions. Precipitation reactions occur by double displacement.

Therefore, the given reaction is a precipitation reaction that occurs by double displacement.

Step 3: Example of the given type of reaction

When colourless aqueous solutions of lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$ and potassium iodide $\left(\mathrm{KI}\right)$ are mixed, a yellow precipitate of lead iodide $\left({\mathrm{PbI}}_2\right)$ is formed. Potassium nitrate $\left({\mathrm{KNO}}_3\right)$ is also formed in the reaction which remains in the solution. The chemical equation for the reaction is:

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)+2\mathrm{KI}\left(\mathrm{aq}\right)\to {\mathrm{PbI}}_2\left(\mathrm{s}\right)+2{\mathrm{KNO}}_3\left(\mathrm{aq}\right) \\ \left(\mathrm{colourless}\right)\left(\mathrm{colourless}\right)\left(\mathrm{yellow}\mathrm{ppt}\right) \end{gathered}$$

In this reaction, two colourless solutions give a yellow precipitate. The iodide ions and nitrate ions are exchanged between the metal ions to form the products. Therefore, it is a precipitation reaction that occurs by double displacement.