# Slow down evaporation when cooking

Does water evaporate faster when rolling boil with a lid or simmer without a lid? If I use a certain stove plate with a certain sauce pan set to its lowest setting it will cause the water to roll boil with lid on and just simmer without a lid.

My question is, will the lid compensate for that water evaporates faster (I assume?) when it is rolling boiling compared to when it just simmer?

The context is cooking tea. I have found a new tea that should be cooked for as long as possible, an hour is not too much, and I want to leave it unattended. Hence I want a to avoid too much evaporation.

With evaporating or boiling water, the evaporation of the water takes away an enormous amount of heat. It takes one small calorie (which is 1/1000th of a food calorie) to raise the temperature of one gram of water one degree C, but it then takes 575 calories to boil that water into vapor. In other words, if you add heat to almost-freezing water until it starts boiling, it will take almost six times as much heat energy again to completely boil the water away.

With your lid off, it becomes easier for the water to evaporate away, which extracts a large amount of heat energy from the water, keeping your example pot at a simmer. Put the lid on, and you make it harder for the vapor to escape, so less heat is removed, so your pot heats up further to a rolling boil.

But, since you're adding the same amount of heat to both pots, in both cases the pot will heat up until the heat lost per time equals the heat added. Although the majority of the heat loss in both cases will be the heat of vaporizing water, having a lid lets you have proportionally more heat loss through conduction to the surrounding air (which is why, with a glass lid, you'll see water condensing on the underside; that transfers heat to the lid, which then transfers heat to the air). So, you'll lose somewhat less water with a lid, but I'll guess there wouldn't be a large difference.

In your case, my guess is that you'll get more reliable boiling with a lid, especially a glass one where you can see the contents. And, the somewhat reduced water loss will be a bonus.

• A lid that drips the condensation back into the pan would reduce net evaporation compared to one that vents the steam (and get hotter). Most lids are somewhere in between, but some of mine can be rotated to align vents with pouring spouts on the pan, or for a closer fit. – Chris H Dec 17 '18 at 7:47

Covering (using a lid) will greatly reduce evaporation, whether at the full boil or just simmering...or sitting on the countertop.

• If the lid more or less does its work this is the correct answer – Alchimista Dec 17 '18 at 14:14

You could look at reducing the heat coming from the burner/ring, so you could leave the lid on & also reduce the boil to a simmer.

One of these flat metal simmer rings that spread the heat [\$£€ 2.50 on eBay] is a cheap & easy way to reduce the heat arriving at the pan from the burner.