Question

Identify the gas from the following observations:

A colourless gas, which changes to steamy vapours on cooling. It forms tiny colourless droplets of a liquid, which turn anhydrous copper sulphate blue and anhydrous cobalt chloride salt pink.

Answer 1

A colourless gas, which changes to steamy vapours on cooling is water vapours.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{∆}{\to }{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right) \\ \left(\mathrm{liquid}\right)\left(\mathrm{vapours}\right) \end{gathered}$$

They turn anhydrous copper sulphate blue and anhydrous cobalt chloride salt pink as shown below.

Two tests for the identification of water vapours are as follows:

• 1. In the presence of water vapours, the anhydrous white Copper sulphate (CuSO₄) powder turns into blue crystals of Copper sulphate pentahydrate (CuSO4.5H₂O).

$$\begin{gathered} {\mathrm{CuSO}}_4\left(\mathrm{s}\right)\stackrel{\mathrm{water}\mathrm{vapours}}{\to }{\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right) \\ (\mathrm{Copper}\mathrm{sulphate})(\mathrm{Copper}\mathrm{sulphate}) \\ \left(\mathrm{anhydrous}\right)\left(\mathrm{hydrated}\right) \end{gathered}$$

• In the presence of water vapours, the blue Cobalt chloride (CoCl₂) powder turns pink due to the formation of hydrated salt Cobalt chloride hexahydrate (CoCl₂.6H₂O).

When dry Cobalt chloride (blue) comes in contact with water vapours, it turns pink indicating the presence of water vapours.