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#### WootBot

quality posts: 15 Private Messages

Staff

T. S. Eliot wrote that “April is the cruelest month,” but January brings the Northern Hemisphere its cruelest temperatures of the year. We’ve asked ex-Jeopardy! champ Ken Jennings to come in from the cold and put a chill on some of the most persistent cold-weather myths he could think of. You shall know the truth and the truth shall set you freeze (yes, we stole that from Arnold Schwarzenegger’s dialogue in Batman and Robin.)

Icy Myth #4: Hot Water Freezes Faster Than Cold Water.

It’s a common bit of scientific trivia that, for some mysterious reason, hot water freezes faster than cold water. In general, that’s not true—and would violate the laws of thermodynamics if it were. Think about it this way: for a container of hot water to freeze before a cold one does, it would have to be losing its heat at a faster rate. Why would it do so? And even if it did lose enough heat to “catch up” to the cold sample, the two containers would then be identical, right? Why would the initially-hotter water “remember” its past and continue to cool off faster?

However, repeated trials have found that, under certain limited conditions, it’s possible to observe warmer water freezing before cooler water does. This effect has been described since Aristotle, but it wasn’t widely believed by modern scientists until a Tanzanian high schooler named Erasto Mpemba noticed it in 1963 while making ice cream in a home ec class. The phenomenon is still called the Mpemba Effect in his honor, but it’s not completely understood to this day.

The problem is that there are so many initial variables that can affect the way water cools that it’s difficult to isolate the one (or more) that may best explain the Mpemba effect. At least eight possible mechanisms have been proposed, most arising from the realization that the average water temperature of the two containers may not give a full picture of what’s really going on in there. These explanations range from the elementary (hot water will evaporate faster as it cools, leaving a smaller volume of water to freeze) to the more exotic (the cold water may be more likely to “supercool,” meaning that its temperature can fall below the freezing point without ice forming). Convection currents in the water, dissolved gases, the way the water conducts heat to the surface it’s sitting on—all these have been suggested. In the meantime, hundreds of grade-school science fair projects will continue to confirm what everyone already knows: cold water freezes faster than hot water. (Except when it doesn’t.)

Quick Quiz: What fictitious mineral is used to “freeze” Han Solo at the end of The Empire Strikes Back?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.

Photo by Flickr member Kevin Saff. Used under a Creative Commons License.

#### pastorbill83

quality posts: 2 Private Messages

I actually did the "does hot water freeze faster than cold" experiment for ninth grade science--what does happen is that hot water cools at a faster rate than cold. But as Jennings points out in general, it cannot pass the cold. (It also showed something about statistics--just because something happens at a faster rate doesn't mean that that rate will not slow down in the future. Hence why one should doubt any future projections of the debt.)

#### DrGaellon

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Water which has been BOILED THEN COOLED will freeze faster than water from the tap at the same temperature, because the dissolved gases have been driven out of it. It will also freeze clearer.

#### sjkrishna

quality posts: 1 Private Messages

Now, hold on. This post is smartly written and a good explanation but I'm a physical chemist and Mr. Jennings has no justiifcation for the "would violate the laws of thermodynamics" crap -- that's just trying to put a deep scientific authority where it doesn't belong. Yes, if the system is in some thermal equilibrium all the time (there's ways to formalize that but for now let's leave it at that), then sure: the state of the system is uniquely determined by its physical properties (temperature, pressure, what-have-you) and so it has no "memory" -- the bowl of water, once it's at, oh, five degrees Celsius, doesn't remember whether it started at twenty degrees or at ninety. But the story here is about a process that's happening faster than that coming-to-equilibrium-with-the-surroundings process: Mr. Jennings invokes the "laws of thermodynamics" wrongly.

#### theslt

quality posts: 1 Private Messages

I am not a scientist. I am not a chemist. I am not an engineer. I do not have any specialized scientific background.

But I can say with certainty, it was carbonite. Han Solo was frozen in carbonite.

And, for the record, Han DID shoot Dear Princess Celestia…

EDIT: 'First', I wrote 'first' there. What kind of alien auto correct is this?

#### tjamil

quality posts: 26 Private Messages

Ans: Carbonite. No i'm sorry, the correct answer is who gives a beep!

Source (ff to 9m if link doesn't)

#### jcolag

quality posts: 8 Private Messages
...for a container of hot water to freeze before a cold one does, it would have to be losing its heat at a faster rate. Why would it do so?

Newton's Law of Cooling would be a pretty good reason. Change in temperature is proportional to the difference between the temperature of the object and the temperature of the environment.

That doesn't disrupt the conclusion, because the cooling quickly is an Achilles and the Tortoise situation, but hot water definitely cools faster than cold water, if they're in the same ambient temperature.

#### whitleyca

quality posts: 0 Private Messages

The best twist on this subject is that in a home freezer warm water will freeze faster and you will your ice cubes quicker because the warm water will force the compressor cycle to start and the normal overshoot will lower the freezer temperature enough to compensate for the increased cooling needed. An obvious alternative would be to put some warm water near the thermostat and some cold water near the cold air inlet.

#### walvord

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I never had the inclination to test this myself, but had always assumed that hot water heats faster due to the increased surface area of the hot water. More surface area would imply that it would cool faster, until it reaches the point where it contracts of course and that slight advantage is lost.

#### walvord

quality posts: 0 Private Messages
pastorbill83 wrote:I actually did the "does hot water freeze faster than cold" experiment for ninth grade science--what does happen is that hot water cools at a faster rate than cold. But as Jennings points out in general, it cannot pass the cold. (It also showed something about statistics--just because something happens at a faster rate doesn't mean that that rate will not slow down in the future. Hence why one should doubt any future projections of the debt.)

Cool - that backs up my theory and now I don't have to try the experiment. The hot water cools quickly until it reaches the temperature of the cold water due to the increased surface area. Then the two continue to freeze at the same rate.

#### jsh139

quality posts: 13 Private Messages

Whoa whoa whoa whoa whoa whoa whoa.... whoa.... whoa. whoa.

Carbonite isn't real?

Insert signature here.

#### johnnyicemaker

quality posts: 1 Private Messages
jsh139 wrote:Whoa whoa whoa whoa whoa whoa whoa.... whoa.... whoa. whoa.

Carbonite isn't real?

I thought Carbonite could be a plausible compound of Carbon and Liquid Nitrogen (which is really cold). Who really knows what unknown/undiscovered compounds can be found in distant galaxies "far far away". Fictitious mineral statement debunked.

#### Jertyrael

quality posts: 21 Private Messages
tjamil wrote:Ans: Carbonite. No i'm sorry, the correct answer is who gives a beep!

Source (ff to 9m if link doesn't)

Classic

"Woot and all our various sites will continue to be an independently operated company full of horrible, useless products and an untalented jerkface writing staff, same as it ever was." -- Matt Rutledge

#### Shinespark

quality posts: 33 Private Messages
jsh139 wrote:Whoa whoa whoa whoa whoa whoa whoa.... whoa.... whoa. whoa.

Carbonite isn't real?

Sure it's real, it's just a polyatomic ion of carbon and oxygen and not all that interesting.

#### glassglass

quality posts: 1 Private Messages

I once asked several plumbers if this was actually true that hot water froze before cold water and frequently they reported that they more often observed "freeze-ups" in the water pipes and did not know why. It is my speculation that this phenomena, at least in pipes, is related to the unique nature of what happens to water when it freezes. Water is like most things in the world that as it cools it contracts. But the funny thing about water is that it then expands at 32* Fahrenheit to form ice, which is less dense. If stagnant water inside a hot water pipe is contracting as it gets colder, that contracting could aid in the later need for the water to expand in order to form ice by producing ever so slight of a "vacuum" within the pipe. This "vacuum" would not exist in a similarly stabilized but cold water pipe. This is what I believe is the probable answer to why hot water pipes might freeze before cold water pipes.

#### wootfertexas

quality posts: 2 Private Messages
Shinespark wrote:Sure it's real, it's just a polyatomic ion of carbon and oxygen and not all that interesting.

Not at all interesting till it's boiled. After that, just add people and viola, icy pop.

#### jessbry

quality posts: 0 Private Messages
glassglass wrote:I once asked several plumbers if this was actually true that hot water froze before cold water and frequently they reported that they more often observed "freeze-ups" in the water pipes and did not know why. It is my speculation that this phenomena, at least in pipes, is related to the unique nature of what happens to water when it freezes. Water is like most things in the world that as it cools it contracts. But the funny thing about water is that it then expands at 32* Fahrenheit to form ice, which is less dense. If stagnant water inside a hot water pipe is contracting as it gets colder, that contracting could aid in the later need for the water to expand in order to form ice by producing ever so slight of a "vacuum" within the pipe. This "vacuum" would not exist in a similarly stabilized but cold water pipe. This is what I believe is the probable answer to why hot water pipes might freeze before cold water pipes.

Had a teacher explain this in high school. The hot-water-pipe freeze up is due to 2 scientific phenomena.

The first is solubility of a gas, in that cold liquids can hold more dissolved gas than hot. Think of whether a soda stays fizzy longer whether it is hot or cold. You'd rather have a cold soda so it doesn't go flat.

The second is freezing-point depression when water has stuff dissolved in it... Salting roads makes the ice take a lower freezing temperature, and thus not be frozen at the normal 32 C. This is true when other things (like gasses) are dissolved in water, not just salt.

Put this together and the water that has been heated has had the gasses (like the oxygen, which we know is dissolved in water because it lets fish breathe) that would normally be dissolved in it leave the solution. Having less dissolved gas lets the hot water pipes freeze at a higher temperature than cold water pipes whose water has more of the dissolved gas.

This was always my understanding of why you should let the hot-water drip if it's supposed to freeze.

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