Why does dry $HCl$ gas not change the colour of the dry litmus paper?

How is the concentration of hydroxide ions $(OH^-)$ affected when excess base is dissolved in a solution of sodium hydroxide?

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

You have been provided with three test tubes. One of them contains distilled water,and the other two contain an acidic solution and a basic solution,respectively. If you are given only red litmus paper,how will you identify the contents of each test tube?

What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.

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