The pot makes people slow

Why does water boil faster in a saucepan with a lid?

(hp) “Put the lid on the pot, then the water will boil faster!” - this is a truism among kitchen tips. And everyone thinks they know why that is. But you thought wrong! Because it is not because the lid keeps the heat in the pot that the water boils faster.


In order to bring water to a boil in a saucepan, you have to supply energy - i.e. heat - via the hotplate. A good part of this heat is lost again before the water boils.
For example, some of it is emitted into the room as heat radiation via the outer surface of the pot. Another part is lost as heat radiation via the water surface in the pot itself. In order to reduce at least this part of the energy loss via the water surface, one might come up with the idea of ​​putting a lid on the pot. In principle, this is a good idea - but the lid still does not hold back the heat in the pot: After a short time it also becomes very hot and then releases the energy into the room via its outer surface. And of course it doesn't matter whether this radiation is emitted from the surface of the water or the outside of the lid.

The secret of why a lid on the pot still allows the water to boil faster lies in the part of the energy supplied that is needed to convert part of the water into steam.
Even before the water boils in the pot, you can see how steam rises from the surface. But it takes a lot of energy to convert water into water vapor. This energy is not lost in the process of evaporation, but is "stored" in the water vapor. Because exactly the same amount of energy that was necessary to convert the water into steam is released again when the steam "condenses" - that is, when it is reflected on a cool surface again.

The energy that has to be added to the water for evaporation is called in physics the "heat of evaporation". The energy that is released again during condensation is called "condensation heat".

If you don't put a lid on the pot, the steam rises unhindered and is reflected, for example, on the cool kitchen tiles, on which the first drops will soon be pouring down. This means that the heat of evaporation supplied to the water in the pot is transferred as heat of condensation to the kitchen tiles and the air in the room - and is lost to the pot.

But if you put a lid on, the steam can no longer escape and has to condense on the inside of the lid. The steam releases the previously absorbed heat of evaporation as heat of condensation inside the pot. The water drips back, the cycle is closed. No water vapor has left the pot and therefore the heat of evaporation and condensation has remained trapped inside the pot.
It is this heat that is retained in the pot that no longer has to be added to the water until it boils - which is why water boils faster in a pot with a lid.