Why is the reason for "Mpemba effect" (warm water can freeze faster than cold) still a mystery?

Sorry, upon re-read of the OP you were asking why it’s unsolved, not why lay-people don’t get it.

If leading physicists can’t solve it, they will certainly not be asking me for answers.

Water will not freeze straight away at 0 C. It will usually supercool before solidifying- the amount of supercooling will depend on things like the amount of dissolved gas - water that has been heated will contain less dissolved gas - this may make a key difference. If the rate of supercooled ice formation is slower than the rate of cooling then this may be the answer to the problem.

E.g it might take 5 minutes for a 40 C water sample to cool to -4 in the freezer and 10 minutes for a 90 C water sample. But if ice formation took 30 minutes due to supercooling, then that effect wins out.

Do you have a cite for this, because I’m not buying it.

I don’t think we need to start revising basic physics, yet. As the wiki article states, there are a lot of possible explanations that are perfectly compatible with physics. I think evaporation and convection are most likely.

That’d also be true if I meant 5 Kelvin, and if I meant a 5 degree angle, it wouldn’t make sense at all. :slight_smile:

Why would you assume I was talking about degrees Fahrenheit?

My physics book says that heat flows from hot to cold. The greater the temperature difference, the faster the heat flows. The sample of hot water placed in a freezer looses its heat faster than the cold water. The temperatures rapidly equalize and the two samples freeze at the same time.

Another mystery to ponder is if the question stumped Aristotle, Roger Bacon, Francis Bacon, Descartes and all the best brains for the last 2,300 years, why is it named after a school child who asked about it in 1968?

I’m still waiting to hear about a mystery. The only way hot water freezes faster than cold water is when the conditions are different. So hot pure water freezes faster than cold salt water, hot water that evaporates rapidly so that a much smaller volume of water is left to freeze will freeze faster than cold water at the original volume, hot water in a metal container will freeze faster than cold water in an insulated container. There’s no mystery there.

I am offering my own 1000 pound prize to anyone who can show that hot water freezes faster than cold water using the same amount of water by weight in sealed containers of the same type in the same freezer conditions. Wait… what’s 1000 pounds in real money? … let’s see … 30 days hath september … carry the one … oh yeah, I can afford that. Not like I’ll have to. But to keep things reasonable, the volume of water has to be a least 1 cubic inch, and the freezer kept at no lower than 0F, and the container made out of common materials.

Will boiling water melt a ziploc bag?

Because its not a physics problem. Its an ideological parable with an axe to grind. Mpemba stated an observation that he knew well, and no one believed him. Often, people bring up the Mpemba effect, just to say that science can’t explain everything. It’s already happened in this thread.

My pure wag … I assume means wild arbitrary guess: hot, not boiling, water, in a wooden bucket, will ice over on top, faster than a similar bucket of colder water, when left outside at sub freezing temperatures. Because the evaporative cooling cools the surface of the water, while the wooden bucket insulates both well enough to keep the interior from cooling quickly. Those are the conditions Aristotle, Roger Bacon, Francis Bacon, and very likely Descartes were working under. None of them had cycling refrigerators, thermocouple temperature probes, plastic containers, distilled water, and most of them were working without the scientific method.

I’ve heard the difference is because the hot water is evaporating more than the cool water. So the hot sample may freeze faster, but it ends up being smaller.

But it seems you could easily eliminate the evaporation variable. Take two plastic soda bottles and pour the same amount of water in each with some air space at the top and seal them up. Put one bottle in the microwave for a short while to heat up the water. Put both bottles in the freezer and see which freezes first.

I have heard from people who heard from plumbers that hot water pipes freeze more often than cold water pipes. However, I contend the water in hot water pipes when they freeze is previously heated water that was at the same temperature as the water in the cold water pipes long before the pipes froze.

Even on a warm day, one generally has to wait a bit for the hot water from the water heater to reach the spigot. The idea that the water in hot water pipes is always hot is false.

No. I freeze soups in zip locks and then boil them in the same bags all the time. The heat and superheated steam of the food inside the bag will usually (most of the time) open up or break the zip lock seal when the soup is boiling but the plastic isn’t “melting”.

So, the property of heat loss has momentum?

Just foolin’. I deliberately edited out “The temperatures rapidly equalize and the two samples freeze at the same time” because I am a douchenozzle.

Apparently, Erasto Mpemba was turning the handle of an ice cream maker. He noticed that boiling hot ice cream mix froze up and solidified faster than ice cream mix that had cooled to room temperature. In order to reproduce this at home and claim TriPolar’s money, we need to find out what kind of ice cream it was, and get to cranking.

Way back when in my college physics class, my professor pointed out that radiation heat loss is proportional to the absolute temperature (in Kelvin) of the first surface raised to the fourth power minus the absolute temperature of the second raised to the fourth power. As a result, the rate of heat loss from the higher temp water is significantly higher than from the colder.

Do not heat a sealed container.

Just because this point keeps coming up, what do you suppose happens after the hot water cools to the same temperature as the cold water?

Also, I bet the cold water is used more often; therefore, the water in the ‘cold water’ pipes moves more often than the water in the ‘hot water’ pipes. Still water freezes faster than moving water.

Would gravity have any influence (i. e., convection)? I wonder if performing the experiments in zero-G would narrow the list of possible causes. Anybody know an astronaut on the ISS?

Let’s say we wanted to perform this experiment in our own freezers. How do we determine when water is frozen? Poke it with a toothpick? In ice trays, water usually freezes around the edges first, right? Do we have to establish that it’s frozen solid for it to be a valid test? Is opening the freezer door every couple of minutes and poking the samples with toothpicks going to mess up the results?