MadSci Network: Physics

Re: Does ice melt faster in alcohol than water?

Date: Wed Aug 11 13:35:51 1999
Posted By: Allan Harvey, Staff,National Institute of Standards and Technology
Area of science: Physics
ID: 931524047.Ph

That's a good question, and I'm not sure there is a simple answer.  So I 
suggest you try it and see -- just make sure the alcohol and the water are 
at the same temperature and in identical containers.  You also need to try 
to make sure the two ice cubes are the same size; of course you can 
average out size variations by repeating the experiment several times.  It 
could also make a difference *which* alcohol -- "alcohol" is a class of 
chemical compounds and you would need to specify methanol or ethanol or 
whatever.  Probably the easiest to get for experimentation would be 
rubbing alcohol, which if I remember correctly is isopropyl alcohol with 
some water in it.

WARNING: Isopropyl alcohol is flammable and slightly poisonous, so you 
would need to be careful and (you don't say how old you are) check with 
your parents and your teacher as appropriate.

I can think of three different things that would make a difference here.  
Since these factors work in opposite directions, it is hard to say what 
the overall difference would be.

1) One factor in melting rate is the driving force for the water melted 
from the ice to diffuse out into the liquid.  That is related to a 
concentration gradient -- the less water there is the faster the melted 
water diffuses out.  So this aspect would make it melt faster in alcohol.

2) Another factor is how fast heat can be conducted from the liquid to the 
ice cube in order to provide the energy necessary for melting.  Water has 
a thermal conductivity several times bigger than alcohols and other 
organic liquids, so this factor would make it melt faster in water.

3) Since alcohols are 20% or so less dense than water, an ice cube will 
actually sink to the bottom of alcohol rather than floating like on 
water.  Whether this will make the melting slower or faster will depend on 
factors like the temperature of the air that the floating cube would be 
exposed to and the thermal conductivity of the bottom of the container.

So in summary, there are several different factors at work here, so 
without knowing which is more important (which may vary from case to case) 
I can't give you a general answer.  Try it and see!

Allan Harvey, Physical & Chemical Properties Division, NIST
"Don't blame the government for what I say, or vice-versa."

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