Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: siewwen168 on 17/12/2004 09:40:47

Title: melting point
Post by: siewwen168 on 17/12/2004 09:40:47
If a thin wire with heavy weights at each end is placed over a block of ice, the wire rapidly cuts through the block of ice. I understand that an increase in pressure under the wire will lowers the melting point of ice, but why the wire will cut through? If the ice is replaced with dry ice, wire does not cut through the dry ice, is this also something to do with melting point?[?][:)][:)]

another question regarding to melting point also.Given that the melting point of water at 218 atm is 1.61 Celcius degree,what is the melting point of water at 0.5 atm?can anyone show the calculation step by step?[:)][:)]

I do know that my question is quite long, hope anyone that wishes to solve won't mind bout it,y our help will be most appreciated.[:p]


Title: Re: melting point
Post by: chris on 18/12/2004 11:34:56
The wire cuts through the ice because, as you correctly point out, the wire compresses the water molecules.

Water is unusual in that it expands as it freezes, so applying pressure to it reverses the equilibrium, lowers the melting point, and favours the formation of water again.

The wire then sinks into the layer of water and compresses the ice molecules below, leaving the water above to refreeze (if you've got the experiment set up in a sufficiently cold area).

However, this is not the whole story because repeating the experiment with a non-heat-conducting material of similar dimensions to wire - e.g. cotton - does not produce the same results, indicating that aprt of the effect is down to the wire conducting heat into the ice from the surroundings. Predictably, the effect tails off as you lower the temperature further.

For dry ice the argument would be that, unlike water, it doesn't expand as it freezes and hence compressing it does not promote melting. It would also require much more heat to flow through the wire from the environment to sustain the melting process - hence it does not work rapidly as it does with water.


"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
Title: Re: melting point
Post by: gsmollin on 18/12/2004 23:21:03
Here's a link to a phase diagram for water that shows the melting temperature at 1/2 atmosphere. You can Google 800,999 more.
Title: Re: melting point
Post by: Les the Scientist on 20/12/2004 00:59:43
Dry ice sublimes straight to a gas so you won't see any melting as such.  Comparing chalk and cheese a bit really.
Title: Re: melting point
Post by: siewwen168 on 20/12/2004 02:15:04
thanks alot to you all.[:)]
i think it is quite useful to me,thanks again for solving my problem.[:D][:D]