Why is it?

[Avatar]

when heating a tin of, say soup, on the hob, when its hot and starts to bubble, if you take the pan away from the heat you get a load of steam given off the soup, return to heat steam stops, take off again steam returns etc.
Most odd.

[heathfield]

I suspect the hot air rising round the pan from the hob is evaporating any steam from the soup. When you move the pan away from the hob into cooler air, the steam doesn't evaporate and is visible. Think of it like your breath on a cold day. In the warmth of the house, you don't see it, but step outside and you can.

[Avatar]

Aaaah!

Thx

[chakka35]

The thing is that steam is an invisible gas. In and around the heat of the stove, as heathfield says, it remains as a gas so you can't see it. In cooler conditions it condenses into tiny droplets of water, a visible mist.

We were always taught at school to look closely at the vapour coming out of the spout of a kettle. The first half-inch or so was true steam and therefore not visible.

My Chambers Dictionary confuses the issue by defining steam both as the gas and as the water droplets. Can't say I agree. That would mean that the fine spray of water droplets coming from, say, spraying bottle should be called steam.

[mibn2cweus]

Molecules vibrate as they absorb and release quanta (discrete packets) of energy. There is a positive correlation between the molecular activity (vibration and motion) in a volume of air and its temperature (kinetic energy) . . . the warmer the air the more energetically and violently the molecules suspended in it collide with each other. Depending on the water content of the air this constant activity and collision of molecules keeps water from forming particles of sufficient size to be seen as mist, or fog until it cools to the dew point.

http://weathersavvy.c...Science_Dewpoint.html

[chakka35]

That's true, mib, but it omits mention of the fact that water in its gaseous state is, well, a gas and an invisible one. Water droplets, however fine. do not constitute a gas.

[mibn2cweus]

Get a work, chakka . . .

http://en.wikipedia.org/wiki/Evaporation

;o)