Hot water doesn’t freeze faster than cold water because, duh, it’s hotter. BUT, for the most part, if you run hot water out of your faucet, it will freeze faster than cold water. Here’s why. The hot water pulls apart tiny pieces of dirt, rust, lime, and whatever else is inside the pipes in your house. So, the hot water that you freeze has all these tiny particles floating around in it, and therefore, the ice has something to grip on to as it’s forming. Ice usually starts forming from the edges of the container, but with hot water, it can start to form throughout the entire volume of the water. Hence, hot water will freeze faster than cold water. Well, sort of…
Really? And cold water doesn’t do this, too? I would think hot water has less of these contaminants than cold water anyway. After all, hot water may sit in in the heater tank for extendend periods, thus giving the particulates a chance to settle out.
Cold water really doesn’t pick up the deposites in the pipes as much as hot water does. If I pre-wash my dishes with cold water, all the food particles on the plate just stay there, and in fact they’re condensing because cold things condense. But if I wash my dishes with hot water, the food flys right off the plate, and presumably, this is what’s happening inside the pipes. True, I’m sure some deposits settle to the bottom of the water heater, but when that scorching water travels up from the heater to your sink, that’s where the particles come off the pipes. Don’t beleive me? Try it out. 
[Moderator Hat: ON]
This is a comment on one of Cecil’s columns, Which freezes faster, hot water or cold water?
So that’s where I’m moving it.
David B, SDMB Great Debates Moderator
[Moderator Hat: OFF]
This is my very first attempt at proving my ignorance to the SD gang. My thought on this subject goes back before I ever read Cecil’s column on the subject when a friend said: " Hot water freezes faster than cold water". I put it into the Urban Myth categorie. Why anyone would think this is that if they lost their heat or just had a terribly cold cellar, the first pipe to stop flowing and/or burst would be the hot water pipe.And it does. But mind you, you have a stagnant hot water pipe and a stagnant cold water pipe at 2:00 in the morning,(most logical time for your furnace to quit,in January) Both pipes at that time are at ambient temperature.Since the hot water pipe is full of water that has been through the water heater and has precipitated solids and now has less dissolved gasses(oxygen/air)it freezes first.
Now have at me.
Well, the food on your plate isn’t quite the same as a copper pipe, is it? The food “flys off the plate” when using hot water because hot water helps ot melt the fats and greases in your dinner. Cold water does not. But, of course, there’s no fat content in copper.
Yes, but there’s more in your pipes than just copper.
Am I the only one that notices that the cold water comes out clear, but hot comes out slightly cloudy? Also, why do the drano instructions say to pour hot water in after you put the Drano in?
And, I sure hope that the amount of stuff in the pipes that settles to the bottom of the water heater isn’t that great, or else eventually You’ll have a water heater that only holds half of what you paid for.
Here is my second attempt.
Anyway,want to see some crud come off the pipes. Shut the water valve off where it comes into your house,simulating a real problem. To make this good,open up several valves in sinks or whatever to make sure household pressure is at zero. Wait a while and re-energize the system. Depending on your water source and type of pipe, watch the stuff come out. It can plug up your aerators on your faucets quick.More than likely,if your on a municipal supply and only have plastic pipes,you will only notice a minimal effect on your hot water lines. If you have a well and copper or galvanized steel lines,it could be enertaining/annoying.
Now as for crud building up in your hot water tank, it can. You will notice this first when it reaches the level of your lower heating element and shorts it out, causing a dramatic loss in hot water volume when taking a shower. That’s why one should “strip” the tank once a year or more often,by opening the drain valve to flush any sediment. This was in the literature sent with your HW tank, which we probably didn’t read.
Hot water can be cloudy because the air dissolved in tapwater comes out of solution at high temperature and low pressure. For the same reason, releasing the pressure on a soda pop bottle when it is warm will release a great many bubbles.
The hot pipe might freeze first because hot water is used less, so it is liklier to have sat idle longer. A leaky toilet will have the cold water replaced frequently in the pipe.
I would tend to believe that the effect of additional nucleation sites within the ice cubes is of minor importance. The important point when freezing water is to get rid of (some of) it’s thermal energy. There are several means of doing this, e.g. radiation, sublimation, convection, and plain old conduction.
Let’s now separate the process in three different parts:
[ul]
[li]interface water-air: sublimation is a very powerful way to dissipate heat. However, the air close to the ice tray will quickly get saturated, and modern fridges (at least mine) are built in order to diminish air circulation within the cooled volume. This means that the water vapour will have to condense somewhere nearby, and thus release its energy again. This is a relatively efficient way to transport heat, but we have to make sure that the heat really leaves the compartment, prefferably by condensing on the cooling elements, although this build-up of ice will decrease the efficiency of the freezer… This would be a worthy topic of a longer discussion, but for now I’ll leave it alone. Summa summarum hot water might be better.[/li]
[li]interface water-ice-tray-freezer: I believe that most of the heat is transported by direct conduction via the ice-tray, to the surface of the freezer compartment. This is where I’d like to point out something that apparently has escaped even Cecil:[/li]If the surface under the ice-tray is covered with ice and
if the water is hot
then the hot water might heat the ice tray, melting the underlying ice and thereby create a much smoother surface / better thermal conductivity. I believe that this effect is much more important than the other effects mentioned.
[li]within the ice cubes: In order to get the heat out to the edges (from where it can be removed by the processes described above). If we succed in having a considerable temperature gradient the convection will be more pronounced, and therefore hot water might freeze faster. This efect alone is probably not enough to counteract the fact that there is more heat to dissipate though.[/li][/ul]
Conclusion: If you have an older type freezer, where the ice tray may sit on ice covered cooling elements, by all means use hot water. And try to find a metal ice tray! In other case, it probably doesn’t matter all that much, but using hot water might be better, if you don’t mind the rest of your freezer covered in ice.
There are several uncertainties in the reasoning above, and I would love to be able to explore this wonderfull subject in more detail, including some mouthwatering experiments with different types of freezers, but unfortunatelly my funding is on par with my speling…
There is another issue that ought to be mentioned: If you use water that has been recently boiled, it will have much less dissolved gasses, and thus create a much clearer and less cloudy ice!