At the triple point of water the number of phases in equilibrium are:
A. zero
B. one
C. two
D. three

Answer Verified Verified
Hint: Triple point of water is a point that represents the co-existence of solid, liquid and gas at particular temperature.

Complete step by step answer:
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (solid, liquid and gas) of that substance co-exist in thermodynamic equilibrium. So, at the triple point of water, the number of phases exist are three. At this temperature, it is possible to change all the substances to ice, water, or vapour by making arbitrarily small changes in pressure and temperature. Gas to liquid and liquid to solid, triple point of water corresponds to minimum pressure at which liquid water can exist. At pressure below the triple point, solid ice when heated at constant pressure is converted directly into water vapour in a process known as sublimation. Above the triple point, solid ice when heated at constant pressure first melts to form liquid water and then evaporates or boils to form vapour at a higher temperature.

Therefore, the correct option is (D).

It is important to note that the triple point of water correlates with pressure necessary for liquid water to exist because ice is less dense than liquid water, ice frozen at pressure below the triple point will sublime directly into water vapour.