Explain why:
$(a)$ the colour of copper sulphate crystals changes on heating?
$(b)$ baking soda acts as an antacid?
$(c)$ an acid should be added to water while diluting?

(N/A) Copper sulphate pentahydrate $(CuSO_4 \cdot 5H_2O)$ crystals are blue due to the presence of $5$ molecules of water of crystallisation. On heating, they lose these water molecules and become anhydrous copper sulphate $(CuSO_4)$, which is white in colour.
$(b)$ Baking soda $(NaHCO_3)$ is mildly alkaline in nature. When taken, it reacts with the excess hydrochloric acid $(HCl)$ produced in the stomach, neutralising it and providing relief from acidity.
$(c)$ The process of diluting a concentrated acid is highly exothermic. If water is added to a concentrated acid, the large amount of heat generated can cause the mixture to splash out, potentially causing severe acid burns. Therefore, acid is always added to water slowly with constant stirring.