Chemistry

1.
When a substance exists as a liquid, the molecules can only move touching each other. When a substance exists as a gas, the molecules can move rapidly and freely. Therefore, the molecules in gas have much higher kinetic energy than the molecules in liquid. When liquid of a substance is changed to gas, a large amount of energy is absorbed in order to increase the kinetic energy of the molecules.


2.
At a higher pressure, water molecules in vapour have more chance to hit the liquid surface and thus the rate of condensation increases. To maintain the liquid-gas phase equilibrium, temperature has to be raises in order to increase the rate of evaporation. Therefore, the boiling point of water increases as the increase in pressure.


3.
As boiling point decreases as the decrease in pressure. Therefore, when temperature is high and pressure is low, the state of water is gas (provided that the temperature is not high enough to break down water molecules.)