Why does taking a near-boiling saucepan of water off the hob immediately cause a lot of steam to be released from the water?
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Chris Evans Earby, Lancashire, UK
The questioner is confusing steam with a mist of water droplets. Steam, a gas, is invisible. What is happening is that the heated pan is producing the invisible steam; taking it off the hob allows cooling, so the visible water droplets condense out.
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Hazel Russman London, UK
With a boiling kettle, where you have water vapour coming out of a fairly narrow spout, steam often first appears at some distance from the edge of the spout, due to the cooling off of the vapour.
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David Muir Edinburgh, UK
The amount of invisible water vapour that air can hold depends on temperature. The lower the air temperature, the less water vapour can remain in the gas phase. If the air temperature falls, there will come a point, the dew point, when the air is fully saturated with water vapour. When the temperature drops further, water vapour condenses into the tiny droplets we observe as mist/cloud/steam, depending on where it forms.
If you have a gas hob, you can switch the gas off and on every few seconds and watch the steam appear then disappear. This can be confirmed by holding a thermometer several centimetres over heated water and removing the pot from the hob or switching the gas off. A drop of 5°C is almost immediate with the appearance of steam, released from the air, not from the water as suggested by the questioner.
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Tim Jackson Haslingden, Lancashire, UK
In my experience, the steam cloud seen when moving a pan off the hob, or turning off the gas, isn’t an increase in steam released, but a disturbance of the convection plume over the pan making the steam more visible.
While the pan is in a steady state, the plume flowing from it is fairly smooth, with regular layers, and the water vapour remains hot and transparent until it condenses on a cool surface or gets drawn into an extractor.
When the flow is disturbed, it becomes turbulent and mixes with cold air causing a cloud of droplets to condense out. Likewise, if you lift the lid off a simmering pan, the pan quickly fills with a steam cloud.
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Sam Edge Ringwood, Hampshire, UK
I suspect that the act of moving the pan causes turbulence in super-critical zones – where the water is still liquid, but slightly above the boiling point temperature.
This provides enough extra energy to change the water’s phase from liquid to gas. It may also move the water from relatively smooth parts of the pan’s surface to ones with microscopic flaws that provide seed points for the growth of bubbles.
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