Steam

[Toleman]

Steam is evaporated water, right?

If water evaporates at 100degrees C then how come steam comes from non boiling water?

How come its also more noticable if the hot liquid is in a cold enviroment

eg. a cup of tea outside in the winter.

Chris

[Teddio]

When water vapour or steam condenses in cold air you see a cloud of tiny water droplets. This is more noticeable in cold air, especially when the humidity is high.
Water only boils at 100 Celsius when the atmospheric pressure is 101kPa (1 atmosphere) At lower temperatures, at the same pressure, water will evaporate.

[gen2]

Quote:
"Steam is evaporated water, right?"

Not quite.
Steam is the name given to the gaseous phase of water at 100�C or higher
Steam is invisible (colourless gas)

Water molecules in the gaseous phase below 100�C is generally called water vapour.
Water vapour is also invisible.

If you think you can see either of these, then you are too late - it has already condensed back into tiny droplets of liquid water (eg clouds).

[gen2]

Quote:
"If water evaporates at 100degrees C then how come steam comes from non boiling water?
"

It doesn't - water vapour does.
If you can see it, it has already condensed back into water.


Quote:
"How come its also more noticable if the hot liquid is in a cold enviroment "

What is "it"?
If "it" is noticable, it is no longer a gas - it is tiny water droplets that have condensed.


Consider this analogy.
Generally speaking, the hotter a liquid is, the more solvent it can dissolve.
Similarly, the hotter the air, the more water vapour it can hold.

If you cool down a saturated solution, the excess solute will crystalise out as visible crystals.
If you cool down a saturated gas, the excess water vapour will condense out as visible droplets.

Above a cup of hot tea, the air will become warmed and hold a relatively high level of invisible water vapour. This hot moist air is less dense than surrounding air and will rise. As it moves into cooler air above, the excess water vapour will condense out as visible droplets. This is a small-scale imitation of how clouds form.

[sddsddean]

As said, steam is a colourless gas, water vapour is that 'white stuff'. To see the difference, boil a kettle. When it is boiling, look at the spout and you will see that the white stuff (water vapour) does not start until about an inch or so away from the spout. The colourless 'inch or so' is steam at 100C or above and the 'white stuff' is water vapour that has cooled and thus condensed as it has cooled to below 100C.

[gen2]

Not quite sddsddean

If you can see it, it is NOT water vapour - it has already condensed back into water. Water vapour is invisible.

Consier the water cycle in the atmosphere which takes place well below 100�C.
Water evaporates from the sea (or moist ground) and rises as vapour in the air until it becomes chilled below its dew-point, at which point it condenses into the liquid phase as water droplets known as clouds.
Between the sea (or land surface) and the cloudbase you see nothing (like Manuel in Fawlty Towers). That is where the water vapour is.

Be aware though, that as the water vapour condeses into droplets, there is still some water vapour remaining in the air between these droplets. You do not lose all the water vapour from the air until it cools below minus 40�C.

That is similar to crystals crystalising out from solution as it is cooled, but some still remains in solution.

[EdwardB]

To summarise. Steam is invisible. Water vapour is invisible. The white stuff that is seen consists of tiny droplets of liquid water which have resulted from the condensation of steam or water vapour.

[gen2]

Well Toleman, Has any of that helped?

You seem to have lost interest since posting this question.

[Toleman]

Im sorry, just haven't had a chance to come on here. Cheers for all your answers. you are some very smart people.

Yes thanks Gen2 - All been answered

All the best Chris.