Hi Angie,
There are two components that are at work here -- convection, and
the specific heats of water and oil. Also the density of the oil
compared to that of the ice plays a role, as described below.
Some oils are less dense than others. A quick look at the CRC
Handbook of Chemistry and Physics seems to indicate that the density of
ice is similar to that of many oils. So depending on what kind of oil
you have, and whether the ice cube has air bubbles in it, the ice cube
may float or sink in the oil.
If the ice cube sinks to the bottom of the oil, you can expect it
to melt very slowly. The water melted from the ice cube will not mix
with the oil, but will stay very close to the unmelted ice. This
creates a blanket of unmoving fluid with a slowly-varying temperature,
and the rate of heat transfer is reduced when the temperature varies
slowly in space. Keep cold water next to the ice and the ice won't melt
as fast. Run warmer water past the ice, and the ice will melt more
quickly.
This second situation, of flowing warmer water past the ice,
happens in the case where ice is floating on top of warm liquid water.
Ice is less dense so it floats, but warm water is also less dense than
cold water, so the warm water rises to where the ice is. As the warm
water cools off near the ice, it gets more dense and some of it falls
directly under the ice cube. Warmer water flows in from the sides,
coming freshly into contact with the ice cube, as warmer water flows up
the side of the container. This flow of water is called convection and
is responsible for much of the thermal transfer. Because water has a
high specific heat, each little bit of water flowing past can give lots
of thermal energy to the ice cube. Oil has a smaller specific heat, and
so more oil has to flow past to give the same amount of heat energy to
the ice cube.
If the ice cube floats on the oil, a sort of messy mixed
convection may occur. Melted water (more dense than most oils) will
form small bubbles which will fall down from the melting ice cube and
will collect on the bottom of the glass. The oil will convect as the
water does (if the ice cube sinks to the bottom, the convection won't
work because the cold oil will sink to the bottom and stay there).
Another effect reducing the effect of convection in the oil is the
oil's viscosity. Most oils are more viscous than water and resist flow,
slowing the transfer of heat energy.
Even if convection is assisted (say, by stirring the fluids), the
oil will still melt the ice more slowly because of its lower specific
heat. After a certain quantity of ice has melted, the oil will be
colder than the water because to give up the same thermal energy, the
oil has to change its temperature more. Colder oil will melt ice less
rapidly than warmer oil or water. You may even find that (if the
containers are perfectly insulated and there isn't much much more oil
than ice), that you can melt the ice cube with the water but the oil
cools all the way down to 32 degrees F and fails to melt all of the
ice.
Tom
(published on 10/22/2007)
