In winter,in the presence of fog,which type of equilibrium is observed?

All reactions involving chemical dissociation are:

Amongst the following chemical reactions,the irreversible reaction is:

When a chemical reaction is progressing:

$A$ reversible reaction is one that....

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.