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

$A$ metal $M$ forms an oxide with the formula $M_{2}O_{3}$. It dissolves in both dilute sulphuric acid and dilute sodium hydroxide solution. Identify the metal and write the chemical equations for the reactions involved.

Which enzyme becomes active in the stomach during a low $pH$ value?

Acids do not show acidic behaviour in the absence of water but do so in water. Justify this statement with the help of a chemical equation.

Crystals of copper sulphate are heated in a test tube for some time. What is the source of liquid droplets seen on the inner upper side of the test tube during the heating process?

How is an aqueous solution with $pH = 2$ compared to an aqueous solution with $pH = 4$?