Question

when the vapour is cooled at a pressure less than its triple point

Answer 1

Triple Point is temperature and pressure at which the vapor, liquid, and solid phases of a substance are in equilibrium.

At pressures lower than the triple point, water cannot exist as a liquid, regardless of the temperature.

Freeze-dried foods, like this ice cream, are dehydrated by sublimation at pressures below the triple point for water.

Answer 2

When it is in equilibrium state.....