[Physics] isothermal compression of water and definition of temperature


During isothermal compression of water vapor (below critical temperature), the pressure increases initially, and then remains constant up to certain point, and then steeply increases with small decrease in volume. This means that initially water is in the form of vapor, and finally it becomes liquid. But temperature is the measure of kinetic energy of the molecules. But liquids have less kinetic energy compared to gases. Then how can both liquid and solid phases exist at the same temperature?

Best Answer

Why do you think that liquids have less kinetic energy compared to gasses?

Equipartition theorem (https://en.wikipedia.org/wiki/Equipartition_theorem) states that average kinetic energy is the same per degree of freedom and is 1/2 * k * T. The motion of a molecule of water inside a liquid is jittery, but still the molecule has 6 degrees of freedom so the kinetic energy should be the same.

This may look counter-intuitive. We heat water, water evaporates, energy consumed => molecules in gas should move faster. Isn't it? Actually the energy goes to breaking the attractive forces between molecules in liquid.