I have observed this while cooking, but if you boil something at high flame and then turn the flame low, the liquid stops boiling. E.g. Water boils at 100C. On high gas, it starts boiling. That means the water temp is now 100C. Then if I lower the flame, the water stops boiling even though the flame temperature is greater than 100C at all times. At high flame, the water has reached 100C, so if I lower the flame the water temp should still remain 100C.
In: Physics
To start with, heat equals energy. There are several areas in which energy is lost between the end of the flame, and the water to be boiled in the pot. The flame heats the air around it, as well as the pot. The pot in turn heats the air around it that wasn’t heated by the flame, as well as the water inside the pot. So, not all of the energy in the flame makes it to the water in the pot.
Assuming these losses have been overcome, the reason turning the flame down causes the water to stop boiling is due to a property known as heat of vaporization. This is the energy needed to turn water at 100C to steam at 100C. For water, this is 586 calories. This means that it takes 586 times as much energy to make steam as it does to raise the temperature of the water from 99C to 100C.
The pot of water is losing heat by several methods. The big ones are evaporation and conduction through the sides of the pot to the colder air around it. If the flame isn’t putting in enough heat to overcome the losses to the air, the temperature of the water will drop until it’s at a new lower temperature where those losses match the input from the flame.
While the flame is over 100 degrees, the air surrounding the pot is much lower and constantly trying to cool the water. The heat you’re adding with the fire has to be more than the cooling effect of room temperature air, otherwise the water temperature will decrease.
The water can’t go beyond 100C (because then it turns to gas) all the water is not at 100C or else it would all instantly turn to gas.
Whenever a molecule turns to gas it takes that energy out of the pot. So the water will turn. Because it can’t go beyond 100 the moment the heat source stops the temp will reduce to 99.9999 or whatever and stop boiling.
It takes a lot of energy to convert liquid water into steam, aka boiling, which requires a high flame. A lower flame simply heats the water instead
As you apply more heat to 100C water, the temperature of the water actually does not increase beyond 100C. The excess energy is released as steam.
This also explains why removing the heat source causes the water to stop boiling nearly immediately – there is no more energy being added, so no more energy needs to come out.
When your pot of water reaches 100 C it doesn’t turn to steam all at once. It take additional energy to convert the water from liquid to gas. This is called the latent heat of vaporization. It takes a lot more energy to turn water into a gas than it does to heat it from room temp to a boil.
>but is still more than the boiling temperature
That is the key mistake here – the water is no longer at a boiling temperature. Since the temperature of the water is constantly being cooled by the air and even the pot holding it, the lower fire means that the water is now being cooled faster than it is being heated.
Water will boil to the boiling temperature and not higher.
You can’t increase temperature above the boiling point. In normal pressure thats is.
As soon as you remove it from the heat source it will stop boiling but stay just below it boiling temperature for a while.
To create bubbles at the bottom of the pot requires a lot of energy. When a gas bubble forms it has to displace the liquid around it and is under pressure from the water above it. So with the high flame adding a ton of energy there’s enough energy to create a high enough pressure bubble to rise to the surface (boiling). When you reduce the flame, the temperature may stay 100c but there’s not enough energy to create bubbles to boil. It’s why if you watch water start to boil you can see the bubbles form and collapse.
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