# Can water boil at 100% absolute humidity?

The definition of absolute humidity is the amount of water that can exist in solution with air as a gas. My question is, in a closed vessel that contains a flask and a burner or heat source below it and the air inside the vessel is at 100% absolute humidity, would the water in the flask boil if the heat source remais stable and the temperature does not change?

The thought of this seemed a bit far fetched but it would make sense for the water not to boil relying on what I have gathered. I am not a scientist or a student, so please don't ask if this is for a homework assignment.

• I think you mean relative, not absolute humidity. Absolute humidity is a net amount of moisture per volume, while relative measures the amount relative to the saturation amount (which is where % can come in).
– JMac
Apr 5, 2018 at 16:25

Reading "100% absolute humidity" to mean

"all the gas present is vapor and the vapor is in equilibrium with the condensed state",

I would suggest that you consider the case of a triple-point cell designed for calibrating thermometers to absolute scale. By allowing heat to enter or leave the cell you can cause the ratio of solid/liquid/vapor to change.

A little less exotic situation comes from enclosing pure liquid water into a very strong container and heating it until you achieve a mixture of liquid and vapor. Again, by allowing heat to enter or leave the cell you can affect the fraction of material that is vaporous.

BTW—I mention the more complex triple-point cell first because I know you can order one commercial off-the-shelf. MY search, turned up a product from Fluke, for instance.

You’re asking what happens when you add heat to a “saturated” system. The answer is “it depends”.

• If the volume can expand at constant pressure, e.g. to expand into a large atmosphere, the volume taken up by steam will increase as water converts to steam. In this case, the temperature stays constant.

• If the volume is fixed, both the pressure and temperature will go up.

In either case, liquid water will become steam as heat energy is added. And, if that’s done fast enough, you’ll see something that looks like our usual idea of boiling.