Betsy asks on The Twitters:
A while back I did an article on simmering vs. boiling which covered the basics of the differences between two states of water which, according to high school physics, really should be the same thing. After all, the boiling point of water is 100°C, so how is there something called simmering and something called boiling that both happen at this 100°C. So I gave part of the explanation in the other article, but it wasn’t complete enough. So, first read that old article, then let’s learn some more, shall we?
Water is a social molecule, enjoying time with just about anything it comes across (short of oil and its ilk). This gives water the nickname “the universal solvent”, and is why water causes so much damage to everything it goes past: it will envelop other molecules, carrying them away from their sources. It’ll act as an enabler for acids, which don’t really do much dry but are very active when wet. Water will carry oxygen to fish that need to breathe it or to iron which it will rust away.
Aside from liking other molecules, water also really likes water. It’ll stick together given the opportunity, even while it’s folding other molecules in-between its own. When you heat water in order to boil it, you are trying to get those molecules to break away from each other and fly off into the atmosphere, and that takes a lot of energy.
To take 1g of water from 0°C to 100°C takes 100 calories of heat. To get that same gram of water, already at 100°C (the boiling point) to actually boil, it takes an additional 539 calories of heat. So it’s almost five and a half times the amount of energy to get it to boil than it was just to raise the water to the boiling point.
As we discussed in the earlier article, when you heat something on the stove, the heat is not distributed evenly. Parts near the element or the flame are going to heat up sooner than the rest of the liquid above it, and flame especially can be uneven because there are several individual points of contact rather than something distributed along a larger surface. Of course the pot itself will help to distribute the heat, but even with the best pots, you are going to concentrate all of that heat on the bottom.
While the bottom of the pot is warmer, the water near the bottom will absorb more heat more quickly than the rest of the pot. So the water near the bottom will get the extra heat necessary to change its phase from water to steam sooner than the rest of the pot. While there is an imbalance, you have simmering.
You can balance the simmering relatively easy by getting it to the point of simmering and only putting in roughly as much heat into the system as you are losing from evaporation. There’s no math involved here, of course, you just turn down the burner until the bubbling seems steady. And because you have that really wide margin between reaching the boiling point and actually boiling, you can be quite a bit off on this balance without causing much trouble.
However, let’s say you’ve brought everything to a complete boil. This means that you have added enough heat to the pot to make all of the water have that additional 539 cal/g of energy it needs in order to boil. If you let the heat drop from there, the top is going to cool faster than everything below it, but you add head from the bottom. So eventually everything will drop below that 539 calories/g that it needed to boil, but it’ll be even cooler at the very top and mostly still right next to boiling from just below that top layer down. So when you add any heat at all to the system from the bottom, it will very quickly hit the boil again, and it won’t simmer. There’s just too much energy in the system already.
If you let the whole system cool down for a while, or even drop that 1°C below the boiling point, then you will be able to simmer again easily. Even if you don’t drop the degree, giving it time will give you that buffer that you need to be able to maintain the simmer easily.
Also! You’ve almost certainly noticed that when you’re simmering, stirring the pot will stop all bubbling, and stopping the stirring starts the bubbling back up. This is why. All that heat that was concentrated to allow bubbling gets distributed throughout the whole pot when you stir, so you don’t have that small layer of water absorbing the amount of heat it needs in order to change state. This also means that when you stir continuously while you heat the liquid, when it finally does bubble you will be very close to a full boil.
The distribution of heat when you’re stirring helps to see why a watched pot never boils. After all, it’s easy to ignore a pot that’s trying to boil on its own, but when you have to stir it, you are, by necessity, attached to it and watching it regularly. A stirred pot will eventually boil, but not until well after a pot that is completely ignored will start to bubble.