The hot water that falls from the shower does not need to reach 100ºC to transform into a gaseous state. If water only turned to vapor at 100ºC, there would never be rain, as water from rivers, lakes and seas would never evaporate. The secret is that, in addition to boiling, which requires reaching 100º Celsius, at sea level, for water to change from a liquid to a gaseous state, there are two other ways. The first is heating, a change from liquid to vapor that can even occur immediately, with the water being at room temperature, depending on specific conditions. It occurs when you throw water on a hot plate, for example. The second is evaporation. To evaporate, water needs to be just a little warmer than the air. This is where the evaporation of water from rivers, seas and lakes comes in, which occurs on the surface, as water molecules gain enough energy to transform into vapor, at room temperature, depending on atmospheric pressure and other factors.
The hot water that falls from the shower also undergoes evaporation: The electric shower or heater can generate this difference in temperature to make the water turn into steam. Droplets that fall at a temperature at least 20ºC higher than the ambient temperature have a good chance of evaporating. Just to give you an example, in a very hot bath, with an average temperature of around 45ºC, around 10% of the water is lost in the form of steam. In a common electric shower, this means that around 1.5 liters pass into a gaseous state during a quick shower lasting just five minutes.
Steam is formed by water molecules that have managed to transform into a gaseous state. What defines the state of water — solid, liquid or gaseous — is the agitation of its molecules. The more you stir, the higher the temperature of the liquid.
In the water that flows from the shower at an average of 40ºC, for example, there are some droplets with molecules in agitation equivalent to 50ºC. For them, it is easier to break away from the water column and become vapor.
In that same bath at 40ºC, there are droplets in which the molecules have a temperature of 30ºC. As its molecules are less agitated, they have less energy and cannot pass into the gaseous state. They remain in a liquid state and guarantee our bath.
Source: Text adapted from an article in Revista Superinteressante, produced by Fernando Tió Neto and updated on July 4, 2018.