Which gas is released when acid reacts with metal?

Which of the following is(are) true when $HCl(g)$ is passed through water?
$(i)$ It does not ionise in the solution as it is a covalent compound.
$(ii)$ It ionises in the solution.
$(iii)$ It gives both hydrogen and hydroxyl ion in the solution.
$(iv)$ It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule.

$500 \ ml$ aqueous solution is prepared by dissolving $2$ moles of $HCl$ in water. What will be the molarity of this solution (in $M$)?

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?

Identify the compound $X$ on the basis of the reactions given below. Also,write the name and chemical formulae of $A$,$B$ and $C$.

Which of the following is a mineral acid?