Steam

[johnlambert]

Is steam a gas or water vapour?,

When I was at school i remember my chemistry teacher saying that steam is a gas and when it cools down it becomes water vapour, thats the part that people can see and think is steam.

Please help settle an arguement (discussion really) with a work colleague

PS if anyone says that Americans produce 13 million tons of it, I will hunt you down and eat your babies

[Loosehead]

Vapour is another word for gas.

[dynamicduo]

Trust your teacher

[jake-the-peg]

It's a problem of word definitions.

dictionary.com has a definition of vapour as "the gaseous phase of a substance that is normally liquid or solid"

Now I would imagine your chemistry teacher was considering a definition of something like a suspension of liquid particles in a gas and if you equate steam to mean gaseous water he's perfectly correct.

But the word steam is likely to have been around long before such fine distinctions were made and doubtlessly originally referred to visible water vapour.

So it seems to me that your Chemistry teacher was in effect complaining that the use of the word steam is in use with a meaning that he disagrees with and that in his opinion it should have a more modern scientific definition.

Perhaps that's why he taught Chemistry rather than English! :c)

[johnlambert]

Thanks all.

Your babies are safe

[Confab]

Water vapour is a gas. Boil a kettle and you'll see an invisible bit just above the spout when the waters boiling. This is water vapour. When the vapour (gas) cools it becomes steam.

[Gef]

Not quite contab. Both steam and water vapour are invisible and they are the same thing being composed of individual, free moving water molecules. The term 'steam' is used if the temperature is 100C or above. Water vapour is the term used at lower temperatures.

[brachiopod]

As several were hinting at - steam is the invisible part above a boiling kettle spout, and is the gaseous phase of a substance above its boiling point.

What is commonly referred to as 'steam' is the 'cloudy stuff' from a kettle. This is in fact true steam that has fallen below its boiling point and condensed as liquid water. The fact that it forms tiny suspended droplets causes the cloud effect - but, like clouds in the sky and fog, it is not a gas, but suspended droplets of liquid.

Vapour is usually meant to imply the gaseous phase of a volatile liquid below its boiling point. (Note that 'volatile' means it evaporates easily, and NOT that it is explosive etc!)

[elginsmarble]

Vapours and gases have one distinction: vapours may condense to a liquid if you increase the pressure but a gas will not.

Consult Andrews' Isothermal experiment on carbon dioxide if you have further doubts.

[elginsmarble]

P.S. Steam is a gas.