When we take a hot shower it is pretty common to see a white vapour around the bathroom. But the water I am using is below 100º C.
I know the boiling point of the water can be lower than this with a different pressure, but anyway I think I'm at one atmosphere of pressure approximately.
What's the reason for this vapour?
Answer
Water and other liquids have a partial vapour pressure, even at temperatures well below their boiling point. At boiling point that vapour pressure becomes equal to the atmospheric pressure and the evaporation manifests itself as bubbles of vapour leaving the bulk of the liquid, a phenomenon known as boiling.
Even at room temperature a saucer of water will evaporate, given some time, due to the relatively low vapour pressure at that temperature. Shower water, say roughly at $50\:\mathrm{C}$, already has a much higher vapour pressure, so the shower generates quite a bit of water vapour, which then condenses in the colder surrounding air, forming a white mist.
Even below $0\:\mathrm{C}$ water has some, limited vapour pressure: see the industrially exploited process of freeze drying.
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