Why does water take time to boil on the stove, but immediately stop once taken off the stove?


Why does water take time to boil on the stove, but immediately stop once taken off the stove?

In: Physics

To boil water you need to rise the temperature to the boiling point and that is what take time.

You then need to add energy all the time to convert the water to steam and when you remove the power souse it will stop boiling immediately. If you then put it back it will start to boil immediately again. More exact it is almost immediately because the temperature might have dropped a bit when it was removed.

Applying a heat source causes the water molecules to begin vibrating. The longer the water is exposed to the heat source, the intensity of the vibrations increase. As the water approaches the “boiling point” the water molecules reach a point where the vibrations are intense enough to create what we call boiling water.

Once the heat source responsible for the vibrations is removed, the water molecules rapidly revert to a less excited state.

Consider a microwave oven causing water to boil. My understanding is that the magnetron causes the water molecules to vibrate, the vibration causing friction, the friction causes and increase in the ambient temperature eventually causing the water to boil in the same was as when a heat source is applied.

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.

It takes a good bit of energy to heat water to it’s boiling point, under normal atmospheric conditions. (It takes less heat to boil at lower pressures) When you are heating up water, heat transfer from the stove to the water by conduction and convection. When water vaporizes into water vapor, it takes a good bit of the heat energy with it (out of the water and into the vapor). This is called latent heat of vaporization, which is a much faster process then conduction and convection. As a result, it requires constant heating to stay boiling. If the heat source is removed, that latent heat of vaporization quickly takes enough energy from the water to lower it’s temperature to just below boiling. At that point, heat is lost only though conduction and convection, so the temperature will only gradually decrease. If your pot is made of a thicker material that has a higher heat capacity, like an iron pot, there will be enough remaining heat in the pot itself to cause boiling for a short time after it’s removed from the stove. Aluminum pots are thin and don’t retain heat for long, so you basically see boiling stop almost immediately after removing from the stove.

There are two parts to boiling.

Raising the temperature to boiling point, and actually converting the water from liquid to gas once it’s at that temperature. Both parts require energy (from heat)

When you heat water up to that boiling point, it hasn’t actually boiled yet. You need to keep heating it, adding energy, in order to convert water to steam.

As soon as you take the water off the stove, that process of water turning into steam stops because energy is no longer being added.