When hydrogen chloride gas is dissolved in water,what happens?

Match the chemical substances given in Column $(A)$ with their appropriate application given in Column $(B)$.

Column $(A)$	Column $(B)$
$(A)$ Bleaching powder	$(i)$ Preparation of glass
$(B)$ Baking soda	$(ii)$ Production of $H_2$ and $Cl_2$
$(C)$ Washing soda	$(iii)$ Decolourisation
$(D)$ Sodium chloride	$(iv)$ Antacid

Match the acids given in Column $(A)$ with their correct source given in Column $(B)$.

Column $(A)$	Column $(B)$
$(a)$ Lactic acid	$(i)$ Tomato
$(b)$ Acetic acid	$(ii)$ Lemon
$(c)$ Citric acid	$(iii)$ Vinegar
$(d)$ Oxalic acid	$(iv)$ Curd

Dry hydrogen chloride gas does not turn blue litmus red whereas hydrochloric acid does. Give one reason.

Tooth enamel is the hardest substance in our body. Name the compound of which it is made up. At what $pH$ of the mouth does it get corroded? State the role of bacteria present in the mouth. Suggest a method to prevent tooth decay.

$A$ sulphate salt of a Group $2$ element of the Periodic Table is a white,soft substance,which can be moulded into different shapes by making its dough. When this compound is left in the open for some time,it becomes a hard solid mass and cannot be used for moulding purposes. Identify the sulphate salt and explain why it shows such behavior. Provide the chemical reaction involved.