Why does water take time to boil on the stove, but immediately stop once taken off the stove?
In: Physics
Steam takes a *lot* of energy away with it.
What’s happening when water boils is that water molecules are shifting from a liquid to a gas *en masse*. That shift happens because the water molecules like to stick to each other a bit, so it takes energy to make them stop sticking to each other and zip around like a gas. When the *average* energy is right at that line between not enough and just enough, you get those bubbles of hot water vapor forming an expanding and rising to the top.
Temperature is a measure of the *average* kinetic energy of the molecules in the thing you’re measuring. For water to boil, the *average* temperature has to be 100C, but that’s just the average. There will always be a lot of water molecules with far less energy, and some with far more. There will always *always* be a tiny percentage of water molecules that bang into each other just the right way that one will end up with enough energy to get shot off as a gas, assuming it’s at the surface. If not, it’ll almost immediately bang into more water molecules and shed that energy.
And likewise, if there is water vapor in the air, occasionally a molecule will smash into the surface of the liquid water and won’t have enough energy to leave again, and will become a part of the liquid. These two things always happen and oppose each other. Depending on the temperatures of the gas and the liquid and pressure and whatnot, one will happen more than the other until they reach an equilibrium where molecules are “boiling” off the liquid as a gas at exactly the same rate that vapor is getting stuck and becoming liquid.
When you’re boiling a pot, you’re pushing that equilibrium hard towards making the water a gas. Importantly, though, the pot of water is still at that temperature line where some doesn’t have enough energy to be a gas, and some does. And very importantly to answer your question, the water that leaves as a gas *takes its energy with it*. If you are measuring just the temperature of the water, every time a molecules leaves as a gas, the temperature of the water goes down by the amount of energy it took to send off that molecule. Whenever the vapor hits the liquid, it will add its energy to the liquid and raise the temperature, but when you’re boiling a pot that steam is spreading out through the room, colliding with much colder air molecules, the wall, you, the ceiling, etc. so the vast majority of that energy will never end up back in the pot of liquid.
As long as you have the heat on, that energy is being replaced, so the water keeps boiling. As soon as it leaves the heat, though, all of the water that can be vapor is still leaving, and nothing is replacing the energy they take with them. So the temperature of your pot of water immediately goes right back down to *just under* 100C. When you first put the pot onto the heat, you’re trying to get the water from ~13C all the way up to 100C, and that takes a while. Once it gets to 100C, the water itself can’t go any higher or it becomes steam. Without heat, the steam takes all it’s *more than 100C* energy with it, and your water remains at 99.99…C and doesn’t boil.
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