Boiling water requires putting heat energy into a vessel of water and thus raising the temperature to the point where the water boils. At its boiling point, given as 100C or 212F (see section below), the temperature stops getting hotter. Even though more heat energy is going in the temperature remains constant. This additional heat energy is used to transfer the waters state from liquid to steam
Does Water always boil at 100C (212F)
If you were to carry out a careful experiment to check when water actually begins to boil you might be surprised to find that 100 degrees centigrade is rarely found.
Lots of variables come into play. The outside air pressure, the material the water is being boiled in. Even the type of water (hard or soft) and whether the water has any dissolved air within it.
You can raise or lower the boiling point of water. So 100 degrees Centigrade (or 212 degrees Fahrenheit) is at best a guide an optimal value but necessarily a true value.
Find out more about the experiments here
Hard and Soft
A lot of water supplied to the home is what’s called hard water. This term means that the water contains mineral salts dissolved within it. Soft water on the other hand is pure H2O and contains no impurities.
As water falls as rain, it falls as soft water. But as it goes through the ground, over rocks, through ravines. It dissolves any minerals on passing and this is what makes the water hard.
There are various degrees of hardness. But the important thing is that these salts and minerals can come back out of the water due to various physical processes. And one of these processes is heat.
Water boiling is an extreme process and as the volume of water becomes less, due to water leaving as steam. So the capacity to hold dissolved minerals is reduced.
The result is that boiling hard water results in a hard white substance called limescale, which is left behind in the heating vessel.