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latent heat and phase shift

 
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gregor



Joined: 04 Feb 2011
Posts: 2
Location: Glasgow

PostPosted: Sat Feb 05, 2011 12:14 am    Post subject: latent heat and phase shift Reply with quote

Hello.
Could someone explain latent heat energy, please? I kind of get the idea, but still not sure.
A kettle boiling water- temp reaches 100 C, temp stays at 100 C for a while until water becomes steam. During this 'phase' the water reaches 100 C , then gets it's atoms all shoogled up so it can then change state into steam.
That right?
What about steam burns being worse than water burns?
After boiling point does the heat skip through the boiling water, straight into the steam above the kettle?
What about microwaving water- what about that extra heat?
(Sorry, is that too many questions)?
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hepcj
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Joined: 23 Jun 2007
Posts: 125

PostPosted: Sat Feb 05, 2011 2:15 am    Post subject: Reply with quote

You are right. Latent heat is the energy required to be absorbed or released for a substance to undergo a change of state.

As the temperature does not change during the change of state, the additional energy comes from the potential energy of the bonds between molecules.

Steam burns are worse than water burns because there is more energy in the steam. It is at 100 deg C but it also has the energy of the latent heat of vapourisation which was required for it to turn from water to steam.

Quote:
After boiling point does the heat skip through the boiling water, straight into the steam above the kettle?

Not sure what you mean by this?

With a microwave, you can superheat the water if the water has no dissolved gas in and is undisturbed. Surface tension of the can prevent the forming of bubbles of gas and so energy still goes into heating the water but the boiling process cannot start because there is nowhere for the nucleation of gas bubbles to occur. At the surface the water can escape as a vapour however. In this situation the water is in a metastable state. If the water is disturbed by introducing an asperity where boiling initiate, then it can violently turn to steam.

A good example of superheating can be seen in this video:
http://www.youtube.com/watch?v=pgUWQgJ1TbY
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gregor



Joined: 04 Feb 2011
Posts: 2
Location: Glasgow

PostPosted: Sun Mar 06, 2011 2:55 pm    Post subject: latent heat Reply with quote

Thanks for that!
I follow the reasoning, but I am still having trouble actually visualising what is happening. For example, what is the latent energy that makes the steam burn worse, if it has the same temperature?
I will stick with it, though!
Thanks so much for your prompt reply as well, G.
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hepcj
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Joined: 23 Jun 2007
Posts: 125

PostPosted: Mon Mar 07, 2011 1:40 am    Post subject: Reply with quote

One way in which you can understand the severity of steam burns is to remember that the steam has a greater energy because of the latent heat of vapourisation required to change from water to gas. It works both ways, though.
Your hand or some other part of your body is at a much lower temperature than the steam and so the steam will have to release its latent heat of vapourisation as it condenses and changes its state from gas to liquid. This will be absorbed by your body. After the change of state the temperature will still be at 100 deg. So you can see that the steam burn will put more energy into into the body than a scald from boiling water.
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