Mpemba effect: warmer water can freeze faster than colder water | Hacker News Reader

Mpemba effect: warmer water can freeze faster than colder water

en.wikipedia.org

32 pointsmike_esspe25 comments

I have no idea what this has to do with HN, but, one of the best "life hacks" I've ever learned was from my Iraq tours: evaporative cooling.

If you're outside on a hot day and your drink is hot (in our case, it was palettes and palettes full of bottled water), get a wash rag or a skivvy shirt wet, wrap it around your drink container, and let it sit out in the sun. In a little while, voila, cold drink.

Apropos of nothing.

It works better in the shade, and best of all in the shade exposed to a breeze. You don't need to warm the water in the rag to evaporate it. It will evaporate in the dry air, even if cool. Putting it in the sun raises its temperature as the evaporation lowers it. If there's a nice breeze, evaporation will still win, but it will get cooler if you don't add energy at the same time you are removing it.

That's it, it's been awhile (6 years... wow). Thanks for the correction. Heat + breeze (even if the only breeze is warm).

+1 thanks for your service

In Spain traditionally was used The botijo ( en.m.wikipedia.org/wiki/Botijo ), for keeping water fresh at home and while working on the fields (along with the "bota" for the wine).

When I was a kid growing up, it was common to see vehicles from the outback with a square water bag suspended from a frame in front of the grill.

The bag was canvas and leaked slightly--enough to keep the outside moist so that the contents would chill from the wind as the vehicle drove along. The water was nice and cold, just right for a hot summer's day.

Oddly, I don't see them any more. Perhaps there are enough roadhouses along the highways that anyone can pull in and buy a chilled soft drink when they want.

And nobody understands exactly why.

The British Royal Society offered a 1,000 pound prize recently for an explanation, so far this is the winner, though it's not complete:

http://www.rsc.org/mpemba-competition/mpemba-winner.asp

I can perceive this as nothing but a horrible, horrible failure to account for unseen variables.

AKA, every unsolved problem in science.

Didn't they prove this happens only if the water is impure?

If the water is purified, ie. steam distilled, the warm vs cold effect disappears?

Back in my high school chemistry class this effect was noted and explained in the following way:

Ice needs to form a particular crystalline structure when it freezes. This is what makes ice float in water. The molecules in warm water are moving faster and therefore have a greater chance to wander into this correct formation. The cold water molecules are moving more slowly and simply take more time to find the correct alignment.

There wasn't really any proof to this theory, but it was followed with a bunch of anecdotal evidence, like Canadian's washing their cars with cold water, etc.

Newton's cooling law can be written as t = -ln(T(t) - Te)/k + ln(To - Te)/k. T(t) is the object's temperature at t, Te is the surrounding environmental temperature that's assumed constant, To is the object's starting temperature, and k is a constant scaling factor related to the material of the object in question. Since this effect is contrary to that law, the law is wrong, i.e. too simplistic. It's not hard to imagine that the surrounding Te is changed by the object or that k of water+container can change depending on its temperature or purity...

I don't quite follow your reasoning of why the law is wrong. You say yourself "Te is assumed constant", yet you imagine that converting the Te from a constant to a variable could affect the result. However, if you change Te from a constant to a variable, Newton's Law of Cooling is null and void, because a simplification is that Te is constant, and that allows a lot of /dt's to cancel.

The law is correct for when you can assume a constant Te, for example an object within a fluid flow (like an airplane wing during flight). The problem is, for stagnant situations, convection plays a huge role in heat transfer within the air. Controlling convection is really really hard, and even harder in a scientific setting that needs to be reproducible. Also, when dealing with transition temperatures, various things need to be taken care of, such as supercooling. This can literally stop working if your beaker is slightly dirty. So the problem is there isn't a way to universally control the conditions for this experiment. However, I did find that the winner of the RSC paper had a suitable explanation that involves the presence of supercooling for the colder sample of water. Essentially, if the colder sample supercools, it will take slightly more time to freeze, because while it should have frozen, the ice does not have a nucleation site. On the other hand, the warmer sample does not supercool, and freezes upon reaching 0 C. This experiment only works if the temperature difference is slight, probably within 15-30 C. Quite ingenious really!

Edit: After reading the rest of the comments, air bubble/filtered water/de-ionized water all have great effects on ice nucleation and supercooling, which for me supports the winner's paper.

The controller/thermostat on a stabilized freezer turns on and off in a steady state as it reaches setpoint and then drifts away from it when the motor is off. Filling your ice cube trays with hot water forces the thermostat on sooner, makes it stay on longer because of hysteresis in the controller settings and so freezes faster. That is the only demonstrable reason I have ever seen.

Warm water has less disolved gas, and provides less bouyancy so particulates that would lower the freezing point settle to the bottom. These two factors make the water freeze faster.

This isn't "unexplained" unless you never took a college level physics course, which is pretty typical of the people who edit WikiPedia pages.

Then why did the Royal Society create a contest to explain it?

RSC 1000 GBP contests are to encourage youth to develop science experiments. Did you think that amount was enough to finance a study? The 1000 GBP contests are all things for which RSC already has answers.

A friend of mine swears that if you're making say a gin & tonic, filling the glass with ice will result in less ice melting and therefore your drink will be colder but less watery.

That makes sense - the ice cubes are below freezing, so it will take less melting to reach melting point equilibrium temp. In this case you are simply using the heat storage of ice rather than the phase transition energy required to melt the ice.

That's an easy hypothesis to test:

1. Prepare a glass with a little ice

2. Prepare a glass with a lot of ice

3. Pour the same amount of cocktail into each glass (make sure it's identical)

4. Wait

5. Simultaneously strain both glasses into measuring cups

6. See how much liquid came out of each glass

Yes, and don't shake it or your drink gets watered down. The James Bond shaken theory was that he would get a watered drink so he'd stay alert. Personally I think the author thought it just sounded fancy.

If I recall correctly, ice frozen after being heated has fewer air bubbles. I wonder if there is a correlation.

The explanations I heard were that heating the water drove off trapped gases or the warmer water took longer to freeze giving them longer to escape.

This is clearly madness.

All the best science is.

Sadly, most of the worst pseudo-science is too, which makes it largely useless as an indicator.

Ah well.

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