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Name: Olivia
Status: other
Grade: 9-12
Location: AK
Country: N/A
Date: 11/16/2005


Question:
I am doing a science fair project on the difference in melting rates between snow and ice. Could you tell me if the density of snow effects the rate of melting? Also, what factors effect the rate of melting? Do they also have to same effect on ice?


Replies:
Since they are both ice, I think we need three distinct names to think clearly:

1) snow, and 2) "block-ice", are both examples of 3) ice. A common ice-cube would be a piece of "block-ice".

Snow is merely ice in fine particles. So a kilogram of snow and a kilogram of block-ice take the same number of calories or Joules or BTU's or kilowatt-hours to melt. In that sense there is _no_ difference in the melting rate. That rate could be phrased as: kilograms of melt-water per Joule of energy.

But perhaps you have in mind a different definition of melting rate. the there can be differences. What differences depend a lot on your definitions of the rate, and on the details of your execution.

To melt block-ice or snow the heat must first get into the bulk of the ice, often from some outside warm object. either block-ice or snow might be slower, depending on the way in which the warm object is near the ice. A hard, clear 1kg cube of block-ice 1 meter from an infrared heater might melt slower than a big white fluffy 1kg cube of snow, just because it is smaller so it catches fewer of the rays emitted by the heater.

Likewise block-ice might melt slower than snow in a microwave oven, especially if adjacent to it is a "dummy load" such as a cup of water to absorb any microwaves not immediately absorbed by the ice-container. I suspect that frozen ice absorbs microwaves less readily than liquid water. Is that correct? If so, all the melt-water stuck between grains of snow will catch waves, make heat, and melt the snow faster. Just a guess.

If you use visible sunlight, whose wavelengths are mostly too short to be absorbed quickly by water, (water looks transparent, after all) then snow might well melt slower because it is white instead of clear. Most of the energy-carrying rays are reflected away from white stuff. Clear stuff allows rays to go deeper and deeper in, until something (perhaps the dark ground underneath, perhaps dirtiness of the water making a slightly darkish color) absorbs them and makes heat and melts ice. If you flew over a snow-filed and an ice-pond, which would look whiter, and which darker? Thin ice would usually be darker, so a higher percentage of sunlight would be absorbed there. Thick ice would be darker too, if it was very clear and smooth and non-crystallized, and a little contaminated with dirt.

The case of warm air: One would think the fluffiness of snow, the many surfaces and the many tiny air gaps or pockets, would give it a thermally insulating property, such as is beneficial in igloos and Polystyrene foam. Fluffy air-filled insulators have a low heat transport rate for a given temperature difference. But, because you are in the process of melting the ice-or-snow, it is all at about the same temperature: 0 degrees C. There is no temperature difference, there is only your "insulator" melting and draining away right on the outside where warmer air touches it. Same process, same rate, for snow and block-ice. Igloos only offer insulation from sub-zero cold outside, and only if you are happy with 0 degrees C inside. If your block-ice is fresh from the -20 C freezer into warm air, and is colder than 0 C inside, then

a) I tend to think that is cheating, or,
b) if you do this for block-ice, you must do the same for your body of snow.

Even if you choose (b), I think your snow will melt at the same mass/time rate as block-ice, for a given surface area and air temperature. Snow being less dense, a given mass will have more surface area and melt faster. The detailed surface area due to fine particles probably does not matter, only the common-sense surface area of the overall body shape matters.

An object embedded in the ice-or-snow would eliminate worries about this variable; all the heat it emits in any way (except as visible light) would be caught by ice and used to melt ice. A heater-resistor powered with 1 kilowatt of electricity would generate a very predictable trickle of melt-water, the same for snow or block-ice.

Similarly if you pile snow or ice on a hot-plate with a fixed wattage, then turn it on, (careful the melt-water cannot drain into any electrical parts inside) a similar equality of melt-water production rate should result, I think. Gravity helps us, by draining the melt-water away and nicely pressing whatever ice remains against the hot-plate. Of course, since the snow is less dense, one would melt a larger volume per unit time of snow, than of block-ice.

Which kind of test, which kind of rate, was on your mind?

Jim Swenson


Since you are doing this for a science fair, the idea is for you to experiment and figure it out on your own, however, I can give you some ideas. First, you need to measure the density of the snow and ice. Try weighing each filling up the same size container. Make sure you melt both with the same heat level and time each. How much water is left over in both afterwards? Would quantity effect it? Does rain effect it? Or salt? How much evaporated? How does wind effect it? These are all things you need to think about and take into consideration while doing your experiment. Good luck!

Grace Fields



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