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So I've noticed this many times over the years, and in my head come to different conclusions as to what is happening, and then it struck me that I could ask here and maybe finally be sure.

When cooking something on the hob, noodles being a common example, but also it happens with soup, stir frys, or anything with a medium/large moisture content - I've noticed that the moment you remove it from the heat, or turn the heat down, there's a appearance of steam which wasn't previously visible.

Now I presume the steam is always there, and you just don't 'see' it until you remove the heat source - but why?

I've come to the conclusion that the steam that I see is because the water molecules are forming into larger water droplets once it's removed from the heat, and while it's still on the heat this doesn't happen as the steam has enough energy to move away while still too small to see - but maybe I'm wrong, can anyone settle this for me?

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2 Answers 2

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While your pan is on the heat, a lot of hot air surrounds the pan. The water vapor that escapes is diluted by the warm air before it reaches a point where it can condense (reaches saturated vapor pressure because there is a lot of water in cold air).

The "blanket of warm air" gives the water vapor a chance to diffuse more - concentration becomes lower, so by the time it reaches cold air it is not dense enough to condense.

When you turn off the burner, this blanket of warm air disappears - and so the steam can form.

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  • $\begingroup$ I would put it a little differently, especially for a gas stove : There is a very large volume of warm air (gasses) from the burner, these dilute and carry away the steam so the steam is not as pronounced. The same affect will happen by just turning off the gas burner and not moving the pan. $\endgroup$
    – blacksmith37
    Commented Oct 9, 2017 at 20:48
  • $\begingroup$ @blacksmith37 we are trying to say the same thing... $\endgroup$
    – Floris
    Commented Oct 9, 2017 at 21:10
  • $\begingroup$ Thank you, I figured this was the sort of thing that was happening, but was wondering if I'd come to some wrong conclusion some how. $\endgroup$
    – djsmiley2kStaysInside
    Commented Oct 10, 2017 at 9:21
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Steam will be released with any kind of agitation that causes the bubbles on the bottom or sides to be bumped upward.

They float to the top and the pop into the air.

Also, uneven heating of the contents can cause this, because any agitation will cause the molecules with higher heat to mix with molecules that have not yet been heated as much.

So, whether stirring or by moving the pan, the cause is simple agitation of the water.

Turning down the heat without agitating the pan should not cause this, so I believe you are somehow bumping the pan as you turn down the heat - such as if it is on a hot plate. Or, if on a stove, the turning down of heat changes the convection such that the molecules with different amounts of heat mix differently.

This is very similar to the question of why coffee can "explode" when you remove the cup out of the microwave oven:

Can coffee explode in the microwave oven?

EXCERPT:

Sometimes just picking up the container can have an explosive effect as the superheated liquid comes into contact with air bubbles on the periphery.

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  • $\begingroup$ The phenomenon described will occur without touching the pan - it is sufficient to turn the flame off. Your explanation is wrong. $\endgroup$
    – Floris
    Commented Oct 9, 2017 at 19:53
  • $\begingroup$ The proper answer is a mix of the previous answerS and comments. $\endgroup$
    – Alchimista
    Commented Oct 9, 2017 at 21:04

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