Why do things thaw more quickly in water than they do in air if the temperature of the water and the air are the same?
I used to work in a blood bank and we would thaw out frozen plasma either by storing it in a refrigerator at 4 degrees celsus or in a water bath that was also 4 degrees celsius. If thawed in the refrigerator, the process would take about 24 hours compared to about 3 in the water bath.
If the temperatures of the water and the air are the same, why would the plasma thaw out 8 times faster in water?
It’s largely to do with the specific heat capacity of the medium, which is related to its mass; simply put, even though the volume of air may be the same as that of water, there’s less matter in the air into which the heat can be transferred.
Conductivity, viscosity and convection also enter into it, but not as significant as specific heat capacity.
First, everything Mangetout said is completely correct, so this post is rather superfluous :). However, to help get a feel for just how big a factor the specific heat capacity is, imagine this little experiment. First, stick your hand in a 200 °F oven. You could probably hold it there for quite a long time without discomfort. Even if the oven was at 400 °F, it wouldn’t bother you much for a short time. Now imagine sticking your hand into 200 °F water (please don’t really do this!). You would be immediately scalded very badly. That’s because even though the temperature is the same, the amount of heat in the water is immensely greater than the heat in the air in the oven. Hope this helps.
Actually, in the world of thermodynamics, I learned that conduction is the largest element.
You can thaw a frozen breast of chicken under cool running water in less than an hour. I bet you could get those blood packs down to thirty minutes if you put them under running water. But then, you’d waste a lot of water.
But yes, for all other things being equal, water has a much higher thermal density, and basically can either “absorb” more temperature, or, in the case of thawing, “give” more temperature. The air around the blood pack can only give so much heat until new air is needed, and without convection, that new air will take a while to surrond the blood.
Water is also a better conducter, so in addition to being able to give/take more, it just plain does it faster.
And that’s why forced air cooling, or heating is superior to convetion. It increases the volume of air that is in contact with the item to be cooled or heated in any given period of time.
Another way to look at it is that water is warmer than air because it is literally hotter than air, even at the same temperature. This is another of those situations where the popular confusion of heat and temperature leads to even more confusion.
Water can quite simply hold more energy than air because it has more mass, it weighs more. That energy is able to be absorbed by the blood to warm it up. Air has less energy within it and so it warms things up much slower.
Conduction is important, but if the thermal density is low conduction will also be low. You can blow air across a block of ice at 10 times the flow rate of water and it still won’t thaw as fast.
Probably not on both counts. All the water baths I’ve seen for defrosting/cooling have been agitated to prevent the water stratifying. IOW the water is essentially flowing over the objects anyway, with no loss of water.
Water has an abnormally high heat capacity, which basically means that it can absorb an anomalous amount of heat in any given volume without experiencing any great change in temperature. As a result it can actually absorb more heat from something immersed in it than density alone would predict.
Of course mercury is so much more dense that I suspect it would thaw faster, but there are certainly liquids denser than water won’t.
Water has specific heat of 1 calorie per degree centigrade per gram which is higher than anything else I could find. Air has about 1/4 that so even with equal masses of air and water there is four times as much heat in water.
Note that mercury has a specific heat of 0.03 and a density of 13.6. Water is by definition 1 for both those factors so a given volume of mercury has only about 4/10 the heat capacity as the same volume of fresh water.