How Does Hot Water Freeze Faster Than Cold Water?

This controversy has been raging ever since the early seventeenth century, when Sir Francis Bacon became a charter member of the Betcha-the-hot-water-freezes-first camp.

The only appropriate answer to this puzzle is, “It depends.” It depends on precisely how the freezing is being carried out. Freezing water may sound like the simplest of happenings, but there are many factors that can affect the result. How hot do you call hot? How cold do you call cold? How much water are we talking about? What kind of container is it in? How much surface area does the water have? How is it being cooled? And exactly what do we mean by “freezes first”, a skin of ice on the surface or a solid block?

Let’s listen to some of the yea-sayers and nay-sayers.

Naysayer: It’s impossible! Water has to be cooled down to 32 degrees Fahrenheit (0 degrees Celsius) before it can freeze. Hot water simply has further to go, so it can’t possibly win the race.

Yeasayer: Yes, but the rate at which heat is conducted away from an object is greater when the temperature difference between the object and the surroundings is greater. The hotter an object is, then, the faster it will cool off in degrees per minute. Therefore, heat will be leaving the hot water faster and it will be cooling faster.

Naysayer: Maybe. But who says the heat is leaving by being conducted away? There’s also convection and radiation.

Anyway, that would mean only that the hot water might catch up with the cold water in the race toward 32 degrees. But it could never pass it. Even if the hot water gets to be the same temperature as the cold water, they will thereafter continue to cool at the same rate.

At best, they’ll freeze at the same time.

Yeasayer: Oh, yeah?

Naysayer: Yeah!

Having reached the point of diminishing rationality, we may mediate the discussion by stating that thus far, the nays have it. Clearly, under absolutely identical, controlled conditions, hot water could never freeze faster than cold water. The problem is that hot water and cold water are inherently not operating under identical conditions. Even if we had two identical, open containers being cooled in exactly the same way, there are several factors that could possibly bring about a victory for hot water. Here are some of them:

• Hot water evaporates faster than cold water. If we start with exactly equal amounts of water (which is essential, of course), there will be less water remaining in the hot-water container when it gets down to rug-cuttin’ time at 32degrees. Less water, naturally, can freeze in less time.

If you think that evaporation can have only a trivial effect, consider this: At the typical temperatures of hot and cold household tap water (140 degrees and 75 degrees Fahrenheit), the hot water is evaporating almost seven times as fast as the cold. Over a period of an hour or two, a container of hot water can be substantially diminished by this rapid evaporation. Of course, as the hot water cools down, its rate of evaporation will gradually decrease. Nevertheless, on the way down it may well have lost a substantial amount of water.

• Water is a very unusual liquid in many ways. One of these ways is that it takes quite a lot of heat, relatively speaking, to raise its temperature by each degree. (Techspeak: Water has a high heat capacity.) Conversely, quite a lot of cooling is required to lower its temperature by each degree. If there is only slightly less water in a container, then, it may require substantially less cooling to get it down to freezing temperature. Thus, if the originally hot container has lost even a little bit of its water by evaporation, it may reach the freezing temperature quite a bit sooner than the water in the other container. That is, it could actually overtake the cold water and get to the finish line first. Moreover, once it is at the freezing point, water must have a great deal of extra heat removed from it to make it actually freeze into ice: eighty calories for every gram. So again, a little less water can mean that a lot less cooling is necessary to freeze it.

• Evaporation is a cooling process. The faster-evaporating hot water will therefore be adding some extra evaporative cooling to whatever cooling process is operating on both containers. Faster cooling can mean faster freezing.

• Hot water contains less dissolved air than cold water does, Anything, including a gas, that is dissolved in water makes the water freeze at a lower temperature. The more air (or anything else) that is dissolved in water, the lower the temperature to which it must be cooled, in order to freeze. Having less air in it, the hot water doesn’t have to be cooled to as low a temperature as the cold water does, and can freeze sooner.

However, this last, often-quoted argument doesn’t hold water, so to speak. The lowering of the freezing point due to dissolved air amounts to only a couple of thousandths of a degree. Nevertheless (there’s always a nevertheless), many people claim that when the water pipes freeze in an unheated house in the winter, it is usually the hot water pipes-containing previously heated water, that freeze first.

All things considered, then, it is quite possible that under some circumstances a bucket of hot water, left outside in the winter, will freeze faster than a bucket of cold. The claims of Canadians that they have seen it happen many times can be believed, even by scientists who “know better” and by other skeptics. The biggest and most probable effect in this case is the loss of water by evaporation. Extensive research, however, has not yet revealed why Canadians leave buckets of water outside in the winter.

But there are still a couple of hefty monkey wrenches in the works. First of all, a container of water doesn’t cool uniformly throughout, until it all gets down to 32 degrees and suddenly freezes. It cools irregularly, depending on the shape and thickness of the container, what it is made of, the prevailing air currents, and several other variable factors. The first skin of ice to form at the surface of the water, therefore, may be a bit of a fluke, and may not be signaling that the rest of the water is ready to freeze. (The first ice to form will invariably be at the surface of the water.)

Second of all, believe it or not, water can be chilled well below 32 degrees Fahrenheit without freezing. It can be supercooled, yet it will not crystallize into ice unless some outside influence stimulates it to do so. The molecules may be all ready to snap into their rigid ice-crystal formation, but they need some final encouragement, perhaps in the form of a speck of dust that they can gather around, or maybe an irregularity on the wall of their container.

In view of these uncertainties, precisely when can we say that a given container of water has “frozen”? Our two buckets of water are racing without a clearly defined finish line.

All things considered, the best we can say is, “Hot water can freeze faster than cold water. Sometimes.”

If you are tempted to run right into the kitchen to fill two ice-cube trays, one with hot water and one with cold, and put them into your freezer to see which one freezes first, don’t bother. There are just too many uncontrolled variables, things you can’t control. You can get one result one time and the other result the next.

That’s the problem with people who say, “I know it works; I tried it,” referring to anything from freezing water to curing warts. You just have to examine everything that could possibly affect the outcome. And there can be dozens of unsuspected angles, even to an apparently simple experiment like making ice cubes.