$A$ solution turns red litmus blue,its $pH$ is likely to be

$A$ solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains:

$A$ milkman adds a very small amount of baking soda to fresh milk.
$(a)$ Why does he shift the $pH$ of the fresh milk from $6$ to slightly alkaline?
$(b)$ Why does this milk take a long time to set as curd?

Which one of the following types of medicines is used for treating indigestion?

Write word equations and then balanced chemical equations for the reactions taking place when:
$(a)$ Dilute sulphuric acid reacts with zinc granules.
$(b)$ Dilute hydrochloric acid reacts with magnesium ribbon.

Plaster of Paris should be stored in a moisture-proof container. Explain why?