Question

When Anupreeta heated ferrous sulfate crystals in a test tube, she observed

(i) green color changes to yellow

(ii) green color changes to reddish-brown

(iii) blue color changes to brownish color

(iv) smell of burning sulfur is obtained

• A. (i) and (iv)
• B. (ii) and (iv)
• C. (ii) and (iii)
• D. (i) and (iii)

Answer 1

The correct option is B

(ii) and (iv)

Step 1: Decomposition reaction

Decomposition is a chemical reaction where a single substance breaks into two or more simple substances when heated.
General representation of the decomposition reaction is: AB →C + D + …...
For example, the decomposition of water $\left({\mathrm{H}}_2\mathrm{O}\right)$ to hydrogen $\left({\mathrm{H}}_2\right)$ and oxygen $\left({\mathrm{O}}_2\right)$ occurs by the absorption of electricity and is called the electrolysis of water.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\mathrm{electricity}}{\to }{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \left(\mathrm{Water}\right)\left(\mathrm{Hydrogen}\right)\left(\mathrm{Oxygen}\right) \end{gathered}$$

Step 2: Explanation for correct option

When ferrous sulphate is heated, the water molecules get removed in the following way:

$$\begin{gathered} {\mathrm{FeSO}}_4.7{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Heat}}{\to }{\mathrm{FeSO}}_4\left(\mathrm{s}\right)+7{\mathrm{H}}_2\mathrm{O} \\ \left(\mathrm{green}\mathrm{colour}\right)\left(\mathrm{white}\mathrm{colour}\right) \end{gathered}$$

When anhydrous ferrous sulphate $\left({\mathrm{FeSO}}_4\right)$ is heated, it decomposes to give sulfur dioxide $\left({\mathrm{SO}}_2\right)$, sulfur trioxide $\left({\mathrm{SO}}_3\right)$, and ferric oxide $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$, which is reddish-brown in colour. A pungent, burning sulfur-like odor is also observed due to sulfur dioxide. This is an example of a decomposition reaction.

The decomposition reaction is given as:

$$\begin{gathered} {\mathrm{FeSO}}_4\left(\mathrm{s}\right)\to {\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{SO}}_3\left(\mathrm{g}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right) \\ \left(\mathrm{reddish}\mathrm{brown}\right) \end{gathered}$$

Hence, statements (ii) and (iv) are correct i.e., option B is correct option.