Dry Ice in Water and Air
Date: Spring 2010
Does dry ice sublimate faster in water or air. Why?
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.
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
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
Nathan A. Unterman
NEWTON BBS Co-sysop.
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
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.
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)
I would expect that the answer would depend on the experimental
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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Update: June 2012