The legend
You should always cover a pot of water you want to boil, so that it boils faster.In other words, if you're heating water (or anything else), boiling is achieved more quickly if the pan is covered than if it's not.
How to check?
Boil 1 liter of water in a covered pan, timing the time, then 1 liter of water in an uncovered pan, again timing the time.
Comparing the times will show whether you really need to cover the pot to go faster.
Let's check:
We measure 1 liter of water and place it in an uncovered saucepan on a hotplate, into which we dip a thermometer.
Room temperature and initial water temperature: 19°C (70°F).
Turn on the stove and start the timer, reaching 100°C (210°F) in 9 minutes and 30 seconds.
The stove is left to cool down for about an hour, returning to room temperature.
Another 1 liter of water is measured and placed in a
covered saucepan on the stove, still using a thermometer.
Turn on the stove and start the stopwatch, reaching 100°C (210°F) in 9 minutes and 27 seconds.
The result is even more obvious if you compare the temperature curve, with the uncovered pan in red and the covered pan in green.
Conclusion
The time difference is too small to be significant: covered or uncovered, it's practically the same thing.
Some people have pointed out to me that if we were using a (much) larger volume of water, it wouldn't be the same; the lid would be effective, but in home cooking, we only use small volumes.
To sum up:
"You should always cover a pot of water you want to boil, so that it boils faster", is not true.
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