When a compound or element exists in different phases (states), a physical equilibrium exists between them.
(a) $\underset{\text{(solid state of } H_2O)}{\text{Ice}} \ce{<=>} \underset{\text{(liquid state of} H_2O)}{\text{Water}}$ (physical equilibrium)
(b) $I _{2}(s) \rightleftharpoons I _{2}(g)$ (physical equilibrium)
(c) $S (l) \rightleftharpoons S (g)$ (physical equilibrium)
(d) $3 O _{2}(g) \rightleftharpoons 2 O _{3}(g)$
$O _{2}$ and $O _{3}$ are the allotropes of oxygen, ie, have different composition, so the equilibrium exist between them is considered as chemical equilibrium.