# Is the relative humidity of pure water vapour 100%?

If there is pure water vapour in the gas phase, can the partial pressure be taken to be equal to the vapour pressure of water at that particular temperature? edit: How is relative humidity calculated for pure water vapour?

The nature of your post seems to indicate that you've conflated some different things. Let me just sort them out and I think you answer will be clear.

Vapor pressure of water

Just the partial pressure of the water vapor, which may or may not be at equilibrium.

Equilibrium vapor pressure of water

For a closed isothermal system, with a gas phase and a liquid phase and between 0 C and 100 C, there is a particular vapor pressure of water that exists for the system at equilibrium. In general the higher the temperature in that range the higher the vapor pressure of water at equilibrium.

The vapor pressure of the water vapor does indeed depend on the purity of the water. So at a given temperature the vapor pressure of salt water would be less than that of pure water. Also if there is a solvent mixture of water the vapor pressure water will be less than that for pure water.

Relative humidity

For a given temperature there is a maximum amount of water (vapor pressure) that can exist in the vapor phase. Any excess will condense. So the relative humidity for a given temperature is expressed as the percentage of measured vapor pressure as compared to the maximum vapor pressure over pure water.

• Thanks a lot. So is it right to say that even if a gas phase consists only of pure water vapour, the relative humidity can vary between some small value to 100%?
– math
Nov 1, 2018 at 17:17
• @Monisha The concept of relative humidity is irrelevant for pure water vapour. It is a measure of how much water vapour is in a mixture. If you have pure water vapour everything you need to know will be given by the temperature and pressure (and the major issue will be whether there is any liquid present at equilibrium). Nov 2, 2018 at 14:03
• For a temperature, T, 100% humidity is the equilibrium partial pressure of pure water at temperature T. Relative humidity at temperature T has to vary from 0% to 100%.
– MaxW
Nov 2, 2018 at 14:22
• @matt_black Initially I was of the view that the concept of RH can only be applied to air-water mixtures. But a paper in a chemistry journal mentioned conducting their metal oxidation experiments with pure water vapour and 100% RH.
– math
Nov 2, 2018 at 18:35