Question

Which of these salts on strong dry heating evolves a mixture of colourless gases?

• A. ${\mathrm{CaCO}}_3$
• B. $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2$
• C. ${\mathrm{FeSO}}_4$
• D. ${\mathrm{KClO}}_3$

Answer 1

The correct option is C

${\mathrm{FeSO}}_4$

1. Among the above salts, ${\mathrm{FeSO}}_4$, (Ferrous sulphate) crystals are green in color.
2. It is present in hydrated form and on heating it loses water molecule and becomes anhydrous Ferrous sulphate which is white in color and on further heating it forms Ferric oxide (${\mathrm{Fe}}_2{\mathrm{O}}_3$) which is reddish brown in color along with this a mixture of colorless gases i.e., Sulphur dioxide(${\mathrm{SO}}_2$) and Sulphur trioxide (${\mathrm{SO}}_3$) is evolved.
3. $$\begin{gathered} 2{\mathrm{FeSO}}_4\left(\mathrm{s}\right)\stackrel{∆}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{SO}}_3\left(\mathrm{g}\right) \\ (\mathrm{Anhydrous}\mathrm{ferrous}\mathrm{sulphate})(\mathrm{Ferric}\mathrm{oxide})(\mathrm{Sulphur}\mathrm{dioxide})(\mathrm{Sulphur}\mathrm{trioxide}) \end{gathered}$$.
4. When ${\mathrm{CaCO}}_3$ (Calcium carbonate) is heated, $\mathrm{CaO}$ (Calcium oxide) and ${\mathrm{CO}}_2$(Carbon dioxide) is formed but it doesn't evolve mixture of gases, so this is incorrect option.
5. $$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ (\mathrm{Calcium}\mathrm{carbonate})(\mathrm{Calcium}\mathrm{oxide})(\mathrm{Carbon}\mathrm{dioxide}) \end{gathered}$$.
6. When Lead nitrate is heated, the reaction is given as $$\begin{gathered} \mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }\mathrm{PbO}\left(\mathrm{g}\right)+4{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ (\mathrm{Lead}\mathrm{nitrate})(\mathrm{Lead}\mathrm{oxide})(\mathrm{Nitrogen}\mathrm{dioxide})\left(\mathrm{Oxygen}\right) \end{gathered}$$ and here only Oxygen gas which is colourless is formed.
7. Here, in last option, $$\begin{gathered} 2{\mathrm{KClO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }2\mathrm{KCl}\left(\mathrm{s}\right)+3{\mathrm{O}}_2 \\ (\mathrm{Potassium}\mathrm{chlorate})(\mathrm{Potassium}\mathrm{chloride})\left(\mathrm{Oxygen}\right)\left(g\right) \end{gathered}$$, only one gas is evolved, so this option is also incorrect.

Hence, the correct option is (c). i.e., ${\mathrm{FeSO}}_4$ salts on strong dry heating evolves a mixture of colorless gases.