What happens when metal oxide reacts with water?

During the preparation of hydrogen chloride gas on a humid day,the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

Which of the following statements is not correct?

$(a)$ $A$ salt is produced by the reaction between an acid and a base. Identify the acid and the base from which the following salts have been formed:
$(i)$ $Na_{2}SO_{4}$
$(ii)$ $NH_{4}Cl$
$(iii)$ $KNO_{3}$
$(iv)$ $NaCl$

When soap is scrubbed on a stain of curry on a white cloth,why does it become reddish-brown,and why does it turn yellow again when the cloth is washed with plenty of water?

Crystals of copper sulphate are heated in a test tube for some time.
$(a)$ What is the colour of copper sulphate crystals:
$(i)$ before heating, and $(ii)$ after heating?