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Name: Alli
Status: student
Grade: 6-8
Location: CA
Country: USA
Date: Spring 2010


Question:
Does dry ice sublimate faster in water or air. Why?



Replies:
This is not such an easy experiment to do. First, I assume you are talking about liquid water, not wet air. Second, I assume you are also talking about "bone dry" air, i.e. containing no water vapor. It is easy to talk about these conditions but actually a challenge in practice.

Given those caveats the sublimation from water is significantly SLOWER for several reasons. 1. The solid CO2 freezes water forming a layer of solid water ice on the dry ice. This reduces the heat conduction -- i.e. the ice layer insulates the sample being tested. 2. Without the ice "blanket", convection of dry air due to the temperature difference provides an additional mechanism for removing CO2 vapor from the block of "dry ice".

It is true that the jacket of water ice will crack open from the internal pressure, but the low temperature, t << 0 C., causes the fracture to reseal quickly. These are but a few mechanisms for the retarding sublimation in the presence of water.

Vince Calder





Hi Alli, It sublimates faster in water. This is because heat is transferred more quickly and efficiently by water than by air, all things being otherwise equal.

Best, Dr. Topper


Dr. Topper--

Would it not be slower due to the formation of a jacket of water ice that would form around the dry ice? I am not sure. Please help me understand.

Nathan A. Unterman
NEWTON BBS Co-sysop.


Hi Nate,

Not in my experience. I give a lot of demonstrations to kids and students in which I show them a chunk of dry ice which only gently steams, and then I drop it into a very large beaker of room-temperature water and very rapid sublimation occurs.

I think you might be thinking of what would happen if the dry ice were dropped into a small container of water (especially cool water). Then the sublimation would draw enough heat from the water to freeze the water around the surface of the dry ice. In my mind I was comparing the effect of warm water to the effect of warm air (that was the "all things being equal" caveat).

Does that make sense?

best, Robert Topper


Yes, indeed, I was thinking of a large sample in relatively small amounts of water. Thanks for your clarification. It makes sense in the context you describe.

Nathan A. Unterman


Hi Alli,

Dry ice should sublime faster in water. While I do not have experimental data to back up my hypothesis, I think this answer should hold true. The fundamental properties that governs the rate of phase change is heat capacity and heat transfer.

Simply put, heat capacity is the amount of heat that a material can absorb from another medium. Take a frying pan for example: if the pan is made from steel or iron, it will stay much hotter for longer than aluminum because aluminum has a lower heat capacity. Said another way, aluminum can not hold as much heat as steel or iron.

The other property at work here is heat transfer. There are only three possible methods of heat transfer: convection, conduction and radiation. I will not spend time talking about all of these methods, but as quick examples of heat transfer here they are: convection-->heat transfer via liquid-liquid, liquid-gas or gas-gas, diffusion; conduction-->heat transfer via solid-liquid or solid-gas; radiation-->transfer of energy via a wave of electromagnetic radiation (photons), microwave.

Believe it or not, water has the highest heat capacity of any compound, so it has a higher capacity to transfer stored heat to the dry ice compared to air. Conduction between a liquid and solid is quicker than conduction between a gas and solid, so water also has this going for it too.

Between these two issues, dry ice should sublime quicker in water.

Matt Voss


Hi Alli,

The speed of dry ice sublimation will depend on the amount of heat that is transferred from the water or air to the dry ice.

Heat (energy) is transferred between two objects that have different temperatures either by conduction, convection, or radiation. Conduction is the kind of heat transfer that happens when two objects that have different temperatures come into contact - so we will focus on that.

There are several factors that control the rate of heat transfer: the temperature difference between the two objects, the amount of contact, the specific heat, the mass of the water or air available, etc.

If we assume that the water and the air are at the same temperatures, then the amount of contact becomes very important because every point of contact is a chance for heat transfer. Since there would be a lot of water molecules coming into contact with the dry ice (then if it were air) then a lot of heat would transfer and sublimation will be faster. Think of how you can stand in air that is 70 degF and not really feel cold, but being dunked in water that is 70 degF feels very cold - this is because a lot of your heat is taken away so much faster by cold water, than by cold air.

One more thing, if the water is only a spoonful worth, it is possible that the dry ice will remove heat from the water very fast, cause the water temperature to drop really fast, and so -because the amount of heat transfer depends on the difference in temperature and since the water temperature drop really low- the amount of heat transfer decreases and so less sublimation will happen. This does not happen with air, because there will always be a lot of air moving around to come in contact with the dry ice (compared to a spoonful of water).

This is why I said that the temperatures have to be the same to make a good comparison. However, since the specific heat (the amount of heat that has to be removed in order for the temperature of the substance to change) of water is very high compared to air, quite a bit of heat will be removed from water before the temperature falls, so this effect is not so strong - - but it can still be a factor depending on how much water there is.

Greg (Roberto Gregorius)
Canisius College


Hi Alli,

I would expect that the answer would depend on the experimental conditions.

If we put a small piece of dry ice in a large bucket of warm water, so that the temperature of the water would not drop much due to the presence of the cold dry ice, I would expect it to sublimate faster in water. Water is really efficient at transferring heat away from hot objects and into cold objects compared to air.

However, if we put a small piece of dry ice into a small beaker of initially warm water, the water will cool off rapidly and you may even see a frozen layer of ordinary ice form around the piece of dry ice. That would slow down the sublimation rate drastically, making it slower than the sublimation rate in warm air.

I hope this helps!

Best, Dr. Topper



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