Re: Why does pure ice melt slower in salt water than fresh water?

Actually, when salt is used to melt ice (like on roads) it isn't so much a 
matter of speeding up the melting as it is allowing the ice to melt at 
all.  What you want in that situation is for the liquid to be stable; pure 
liquid water will change to solid ice if the temperature is below 0 
degrees Celsius, and at those tempertures ice won't melt at all.  Adding 
salt lowers the freezing point of the water, which allows the liquid to be 
stable at lower temperatures and allows the ice to melt.  Scientifically, 
that would be called shifting the "equilibrium" and doesn't have anything 
to do with rates.

If, as in your experiment, the water is above 0 degrees C anyway, the 
equilibrium state will be for the ice to melt, and the presence of salt in 
the water won't change that one way or the other.  The *rate* of melting 
in such a situation will mainly be governed by how fast heat from the 
warmer water is able to flow to the ice cube.  Among other things, this 
will depend on the temperature difference between the liquid and the ice 
and the thermal conductivity of the liquid.  Salt solutions usually have a 
thermal conductivity a little bit smaller than pure water, so that could 
be the source of the effect you are seeing as it does not conduct heat to 
the ice cube as well.  I would not expect that effect to be very big, but 
it could make maybe 10% difference or so.

It's also important to do such an experiment several times to make sure 
you didn't just happen to choose a slightly larger ice cube for the salt 
water -- try 5 or 10 pairs of cubes and see if the results are 
consistent.  It would also be potentially important to make sure the water 
temperatures were the same (sounds like you did that) and that the amount 
of water in each beaker was the same and the beakers were the same shape.  
Measuring the water temperature (after a bit of stirring to get it evened 
out) after both cubes had melted might also give you some hints as to what 
might be different.

Keep experimenting, it's a good way to learn!

Allan Harvey, aharvey@boulder.nist.gov
"Don't blame the government for what I say, or vice-versa."