Ice Evaporation

Water does indeed evaporate at temperatures at which it is a solid. The process of changing from a liquid to a solid requires evaporation, which is why hot water freezes faster than cold water. The "preferred" material state of water is gaseous, and therefore, evaporation occurs whether the water is currently in a liquid or a solid state. As my study of chemistry dates back more than thirty years, i suspect someone more knowledgeable will come along with a better explanation.

Evaporation is what happens when a liquid changes to a gas. Sublimation is what happens when a solid changes directly (no intervening liquid state)to a gas.

Hot water doesn't freeze faster than cold water. Its temperature drops at a faster rate, but it doesn't freeze sooner.

In fact, it does. I've done the experiment of pouring measured amounts of hot and cold water into identical containers which were purchsed nested. I did this after seeing a David Suzuki television program in which he alleged as much.

i concur with 'sublimation', but disagree with the 'statement' on freezing.

water freezes at 0 degrees celcius, but at temperatures below 'freezing' hot water will drop to 0 degrees faster than cold water (since it is expanded and the molecules are more 'excited' due to the temperature, it gives off heat faster) when exposed to the same environment, thus 'freezing' faster, or at least reaching the point of freezing sooner; water freezes at the same speed, whether it was previously hot, or cold.

[some logic still applies here, if we compare water @ 1 degree celcius to boiling water, the cold water may freeze first]

I have not tried the hot water freezes quicker than cold water experiment. My guess is cold water freezes quicker than hot for small or large temperature differences, even if the humidity is very low and/or volitiles such as air are disolved to a greater extent in the cold water.
I think ice and some other substances sulimate because the vapor presure of ice (or whatever) exceeds that of normal air pressure. If the air pressuere was much higher or the temperature below -40 c = -40 f or thereabouts ice would sublinate much less, or not at all. Do most solids sublinatete a few parts per trillion per hour in a high vacuum at 20 degrees c? Neil

Don't know, neil, but if you can smell a solid, I would bet it is subliminating, to some degree or other. Of course, some things can be smelled at much lower concentrations than others.

Roger-

If I can smell a solid that might mean that molecules of it are dropping off, moving through the air and into my nostrils. I don't think that sublimation (the change of state of a solid directly to a gas) is involved.

The rate of change of temperature of an object is a function of the difference between the temperature of the object and the ambient temperature. The hotter object will cool at a faster rate, but will not reach a specific temperature before the colder object. If what you (Setanta) are suggesting is true, then the curves (temperature vs. time) of the two containers of water must cross. This makes no sense. At best, the originally hotter container of water would "track" the colder container when their temperature curves intersect.

We sometimes say there are only three states of matter, but there are at least 5 counting vapor and plasma. as I picture the vapor state, each molecule of water (or whatever) are separated on all sides by molecules of air from other water molecules that are nearby. When two or more of these touch we have a micro droplet of water which looks like fog, if there are many micro droplets. Water can exist as a gas only above 100 degrees c = 212 degrees f (at standard pressure) or at very low air pressure above about 80 degrees f. Neil

Agreeing with markr's last post--
hot water cannot possibly freeze faster than cold water. Suppose you have two equal volumes of water, one at 80 oC and one at 50 oC. If the volume at 50 oC takes x seconds to reach the freezing point, the volume at 80 oC will take x seconds plus the amount of time it takes to cool from 80 oC to 50 oC. If you get different results experimentally, then your experiment may not have been controlled properly.
Regarding the loss of volume in the ice cube trays, sublimation to any significant extent isn't likely at ambient pressure. If you're using plastic trays, I'd check for hairline cracks. Otherwise, are the trays level in the freezer? Is it possible the water is flowing toward one end of the tray before it freezes, leaving smaller cubes at the elevated end?

The behavior of any substance changing between solid, liquid and vapor phases can be explained by the Phase Diagram. Here's a good "in general" one:

For water, you can go back and forth between solid (ice) and liquid (water) by melting and freezing, but only between certain temperatures and pressures (sorry - this chart doesn't have the pressure and temperature values shown).

You can go back and forth between liquid (water) and vapor (steam) by boiling and condensing, but once again, only within certain pressures and temperatures.

Finally, you can go between solid (ice) and vapor (steam) by sublimation and deposition, but once again, only within certain pressures and temperatures.

As for the cooling and freezing of water. Water freezes at 32 F at most pressures you are likely to run across. Let's say you have one pound of water at 40 F, and a second pound of water at 100 F. At these temperatures, the heat capacity (the amount of thermal energy required to change the temperature of one pound of water by one degree F) of water is relatively constant at 1 BTU/lb F. To get the water at 40 F to 32 F, you have to remove 40-32=8 BTU. To get the water at 100 F to 32 F you have to remove 100-32=68 BTU. This has to be done for both cups of water before you can start to talk about freezing them. Because the gradient (the temperature difference) between the 100 F water and your freezer temperature is greater than the gradient for the 40 F water and your freezer, the rate of heat transfer out of the 100 F water will be greater than out of the 40 F water. But it is not physically possible that the 100 F water will reach 32 F before the 40 F water reaches 32 F, in the same freezer.

Once water falls to 32 F it does not automatically freeze, any more than water automatically boils once it reaches 212 F. To get the water at 212 F to boil, you have to add an additional 1000 BTU/lb of thermal energy. I don't remember the exact value of heat you have to remove from water at 32 F to get it to freeze (not something I've ever used since school), but the value 472 BTU/lb sounds familiar.

Do frost-free freezers periodically raise their temperatures to defrost? Perhaps there is some surface melting/evaporating during a defrost cycle.

Check this out:
http://www.discovery.com/area/skinnyon/skinnyon970509/skinny1.html

The triple point of water is 273.16 K (~0 oC) at 611.2 Pa (0.006 atm). Atmospheric pressure is defined as 1 atm. Unless your freezer is under vacuum (pressure<0.006 atm), the ice cannot sublime--it can only melt. It's possible, though, in light of the explanation given on the link you provided, that there is a film of liquid water on the surface of the ice cubes that gradually evaporates and is replenished by melting of the ice at the surface layer. This could explain the shrinking of the ice cubes.

hot water vs. cold water freezing
I tried the test myself. In containers of equal sizes, I put boiling water in one container, hot tap water in another, and cold tap water in the third. I put them all in the same freezer at the same time. The cold tap water froze first. The hot tap water froze second. The boiling hot water froze last. That seems pretty straight forward, but contradicts a popular notion. I don't know whether or not the boiling water loses temperature at a faster rate than room temperature water, but that would seem logical.

UgLee

Hot water can freeze faster than cold water

http://www.sciam.com/askexpert_question.cfm?articleID=0008EB6B-6C7E-1C71-9EB7809EC588F2D7&pageNumber=1&catID=3

While the above responses are interesting I have a simple answer for you. You probably have a self defrosting freezer, almost all now are. If you don't use them for several weeks at a time the ice cubes are simply being defrosted away.