Ice Melting in Water and Oil
Most recent answer: 10/22/2007
Q:
Why does ice melt faster in water than oil when both substances are at the same temperature?
- Angie (age 25)
- Angie (age 25)
A:
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
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)
Follow-Up #1: oil and ice
Q:
When oil companies remove oil from under the polar ice caps and fill the void with sea water, doesn't the removal of oil and filling of sea water cause the ice caps and ice bergs to melt at a faster rate? Isn't safe to assume that the oil has kept the ice insulated? Don't the oil companies know about this? Why haven't I heard about this in the media? Have I asked an ORIGINAL question?
- Mary Thurlow (age 38)
Fallbrook, CA 92028
- Mary Thurlow (age 38)
Fallbrook, CA 92028
A:
That is definitely an original question, and I'm not 100% sure of the answer. I hadn't even heard of the process you are describing. I believe that the oil removal would have no important direct effect on the rate of melting of the ice caps, which is controlled mainly by the surface heating. However, as soon as the oil is burned it contributes CO2 to the atmosphere, and that greenhouse gas is the biggest culprit in changing the radiation input/output balance, creating that surface warming.
Mike W.
Mike W.
(published on 02/23/2008)
Follow-Up #2: oil on ice
Q:
A much messier question. If there is a crude oil spill on an ice cap, will it affect the rate of melting? A thin layer of oil may act to absorb heat (darker than the ice) and float on the water produce when the ice melts. A small amount of oil could cover a large area of ice...
- Rick (age 40+)
TX
- Rick (age 40+)
TX
A:
Certainly to the extent that the oil absorbs more light than (very reflective) ice, it will increase the melting rate. However, I don't think this is a great danger, because crude oil becomes very viscous when it's cold. It would probably not spread out all that much. I hope.
Mike W.
Mike W.
(published on 12/09/2009)