If water boils at 100°C, and boiling is the process of turning liquid into gas, why are bathrooms full of steam when showering at only 40°C?
In: Chemistry
First of all there is the concept of partial pressures in a mixture of gases. If I have two gases mixed together, with a pressure of 1 bar, and I have 25% of gas A and 75% of gas B, then gas A will behave the same way pure gas A at 0.25 bar will behave, and gas B will behave the same way pure gas B behaves at 0.75 bar.
Water boils at 100°C at 1 bar pressure. At lower pressure, it boils at a lower temperature, and at higher pressure it boils at a higher temperature. At 40°C, water boils at about 0.073 bar. That means, if I have a mixture of water vapour and air at 40°C, I can only have 7.3% water vapour in that mixture. If there is more water vapour, it will start to condense and form droplets.
If I take a hot shower, some of the water will evaporate. It will continue to evaporate until the air is saturated. At 40°C, that happens when 7.3% of the air-water vapour mix is water vapour. When I turn off the shower, the hot water is no longer making the air hot, and because the rest of the house is colder, the air will start to cool down. As it cools down, the fraction of water vapour that can be contained in the air goes down too. That causes the excess water vapour to condense as clouds of water droplets (commonly mis-named “steam”), or to form as water drops on surfaces like walls, mirrors, windows etc.
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