One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is

Crystals of a substance changed their colour on heating in a closed test tube but regained it after sometime when they were allowed to cool down. Name the substance and write its formula and explain the phenomenon involved.

State what happens when:
$(a)$ Hydrated copper sulphate is heated.
Also,write the chemical equation for the reaction.

$Na_{2}CO_{3} \cdot 10H_{2}O$ and $Na_{2}CO_{3}$ are two chemical compounds. Write their chemical names. Explain the significance of $10H_{2}O$ and mention the term used for water molecules attached to a salt.

Name the acid and base from which the following salts have been formed:
$(i)$ Sodium acetate.
$(ii)$ Ammonium chloride.

Which of the following substances is hygroscopic?