Stopping the heat walking out
Why Does a Lid Make Water Boil Faster?
Most of the heat leaving an open pan goes out as steam, and a lid sends it back.
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Step 1 of 5
Evaporation carries away a lot of heat.
Every molecule that leaves as vapour takes energy with it. Well before boiling, an open pan is losing heat this way faster than any other.
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Step 2 of 5
A lid sends the vapour back.
Vapour hits the cool lid, condenses and drips back in, giving up the energy it took with it. The heat never leaves the pan.
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Step 3 of 5
It also stops the draught.
Air moving across an open pan takes warm air away and brings cold air in. The lid ends that completely.
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Step 4 of 5
So the pan reaches boiling much sooner.
With the two big losses closed off, nearly all the burner's heat goes into raising the temperature instead of replacing what escaped.
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Step 5 of 5
Once boiling, the lid changes what you cook.
A lid keeps a stew from reducing and can boil it over. Cooks take the lid off precisely when they want water to leave.
It saves real energy too — a lidded pan uses roughly a third less gas or electricity to reach the boil.
The short version
A lid returns escaping steam and stops draughts, so almost all the heat goes into the water instead of leaving with it.
Try it yourself
Time two identical pans of water to the boil, one lidded. The difference is usually several minutes.
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