(a) Sometimes the pH of our mouth gets lower than 5·5. Why? (b) A basic salt 'X' is obtained by heating baking soda followed by crystallisation. Identify 'X' and state its two industrial uses. (c) Why do copper sulphate crystals turn white on heating?

(a) Lowering of pH in the Mouth

The pH of our mouth falls below $5.5$ due to the action of bacteria present in the mouth. These bacteria degrade sugar and food particles remaining in the mouth after eating, producing acids. When the pH drops below $5.5$, the acid begins to attack the tooth enamel (calcium phosphate), leading to tooth decay.

(b) Identification and Uses of Basic Salt 'X'

• Identification of 'X': The basic salt 'X' is Washing Soda (Sodium carbonate decahydrate, $\mathrm{Na_2CO_3 \cdot 10H_2O}$).
• Preparation Reaction:

Heating baking soda yields sodium carbonate:

$$2\mathrm{NaHCO_3(s)} \xrightarrow{\Delta} \mathrm{Na_2CO_3(s)} + \mathrm{H_2O(l)} + \mathrm{CO_2(g)}$$

Recrystallisation of sodium carbonate gives washing soda:

$$\mathrm{Na_2CO_3} + 10\mathrm{H_2O} \rightarrow \mathrm{Na_2CO_3 \cdot 10H_2O}$$

• Two Industrial Uses of Washing Soda:

1. It is used in glass, soap, and paper industries.
2. It is used for removing the permanent hardness of water.

(c) Dehydration of Copper Sulphate Crystals

Copper sulphate crystals ($\mathrm{CuSO_4 \cdot 5H_2O}$) contain $5$ molecules of water of crystallisation per formula unit, which gives them their characteristic blue colour. On heating, these crystals lose their water molecules and turn into anhydrous copper sulphate ($\mathrm{CuSO_4}$), which is white:

$$\mathrm{CuSO_4 \cdot 5H_2O(s)} \xrightarrow{\Delta} \mathrm{CuSO_4(s)} + 5\mathrm{H_2O(g)}$$