Question

Which of the following information about H₂O is correct -

• A. Its triple point increases with the increase in pressure
• B. Its boiling point increases with increase in pressure
• C. Its freezing point increases with increase in pressure
• D. It can never be directly converted into vapour form from solid form.

Answer 1

Answer: (B)

B

Answer 2

The properties of water's phase transitions depend on pressure and temperature, as depicted in its phase diagram.

(A) The triple point is a unique fixed point (temperature and pressure) where all three phases coexist; it does not change with pressure.

(B) The boiling point is the temperature where liquid and gas are in equilibrium. The phase boundary between liquid and gas has a positive slope for water, meaning boiling point increases with pressure.

(C) The freezing point is the temperature where solid and liquid are in equilibrium. The phase boundary between solid and liquid has a negative slope for water (because ice is less dense than liquid water), meaning freezing point decreases with pressure.

(D) Sublimation (solid to gas) occurs at pressures below the triple point pressure. Water ice sublimes at pressures below 611.7 Pa.