21. At a particular temperature and atmospheric pressure, the solid and liquid phases of a pure substance can exist in equilibrium. Which of the following term defines this temperature?

(a) Normal melting point

(b) Equilibrium temperature

(c) Boiling point

(d) Freezing point

(a, d)
At a particular temperature and atmospheric pressure, the solid and liquid phases of a pure substance exist as solid$⇌$liquid.
They exists at normal melting point or normal freezing point.