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Author Topic: What happens to the temperature of water while it is boiling?  (Read 15801 times)

Offline trigger

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Hi all,

We all know that water in a kettle is heating untill it boils,
So apart from the obvious, What happens to its temperature while its boiling.
« Last Edit: 08/12/2008 20:41:43 by chris »


 

lyner

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It stays the same if the water is pure. (The temperature may go up a bit if there are significant quantities of dissolved salts which may elevate the boiling temperature as the concentration gets higher).
This is the 'latent' heat of vaporaistion (energy needed to form steam), as opposed to the 'sensible' heat of warming the water up.
btw, what is "the obvious"?
 

Offline Chemistry4me

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But steam from pure water can be more than 100 degrees, if more heat is added, right?
 

Offline Bikerman

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Err...only if you contain the steam and therefore increase the pressure - then you can superheat it.
 

Online Bored chemist

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No, if you run steam through a red hot pipe it comes out hotter than 100C whatever the pressure.
 

Offline Bikerman

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But surely temperature is simply a measure of the kinetic energy of the steam. Increase the temp and you increase the KE - and therefore the pressure....
or have I got it wrong?
 

Offline LeeE

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You can't actually see steam because steam is water vapour, at 100C or higher, and just like the water vapour in the air that constitutes it's humidity, is invisible.  The stuff that you can see is actually water droplets, which will be at 100C or lower.
 

Online Bored chemist

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Lets talk about air rather than water.
I can get some liquid air and heat it up; it will boil at roughly -200C.
I can then that the air that comes off it and heat it to essentially any temperature I like at any pressure I like provided that I don't compress it into a liquid or get into the range of supercritical fluids.

I can do the same with water.

If I seal some air in an aeroplane and set it flying the air has kinetic energy- but it's at exactly the same pressure as it was before.

If I compress some air into a cylinder it has potential energy.
 

lyner

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But surely temperature is simply a measure of the kinetic energy of the steam. Increase the temp and you increase the KE - and therefore the pressure....
or have I got it wrong?
You are perfectly correct in your definition of temperature.
The energy you need to put into a given mass of (ideal) gas to raise its temperature is its heat capacity. If you keep the volume the same, the pressure will increase because there will be more, faster, collisions against the walls of your container.
If you let the gas expand, keeping the pressure constant, you need to supply more energy because of the work done (PΔV) in expanding the gas. If you let the gas expand on its own, the work done corresponds to thermal energy lost so the gas will cool.
So there may be more factors in a general situation.
 

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