$A$ reversible reaction is one which

Amongst the following chemical reactions,the irreversible reaction is:

Which of the following processes are reversible?
$I$. Vaporization of a liquid at its boiling point.
$II$. Expansion of gas into vacuum.
$III$. Transformation of a solid substance into liquid at its melting point.
$IV$. Neutralization of an acid by a base.

In summer,for cooling water,ice is placed in water. Which type of equilibrium is this?

What is the difference between physical and chemical equilibrium?

The reaction $3Fe_{(s)} + 4H_2O_{(g)} \rightleftharpoons Fe_3O_{4(s)} + 4H_{2(g)}$ becomes reversible when it is performed in ................