frost

Freezing Fast

Hot water sometimes freezes faster than cold water. This sounds counter-intuitive but it’s true. This effect was uncovered by Tanzanian game park manager Erasto Mpemba fifty years ago; since then scientists have been trying to explain what is known as the Mpemba Effect.

First let’s try the experiment
I took 100mls boiling water and poured it into a purple glass tumbler, I then filled a blue glass tumbler with 100mls of water from the cold tap….

Boiling kettle

Timer

I put them both in my freezer like so…

Freezer

After 45 minutes in the freezer the purple tumbler (warm water) looked like this…

Warm water

and the blue tumbler (cold water) looked like this…

Cold water

The observant amongst you will notice that I have failed to demonstrate the Mpemba Effect; the cold water started to freeze faster than the warm water. I tried repeating this experiment with different variations (temperature of warm water, sample container, location in the freezer) but got the same result. That’s the joy of science, what you expect to happen doesn’t always happen. If you need convincing about the Mpemba Effect I suggest you watch this video.

Solids, Liquids and Gases

Water can exist as a liquid, a solid (ice) or a gas (water vapour). These different states exist at different temperatures and have different levels of energy depending on how much the water molecules are able to move around. This energy is known as kinetic energy.

Molecules of water vapour have very weak forces of attraction between them. Therefore they are able to move around more than the molecules in liquid water or ice; this gives them a higher kinetic energy.

Water molecules in ice have strong forces of attraction between them and therefore they can only really oscillate on the spot rather than sliding over each other like they do in liquid water or whizzing around freely like they do in water vapour. This gives them a lower kinetic energy.

solid_liquid_gas

SOLID LIQUID GAS     

When you want to melt ice you need to put energy in to the system, this energy is in the form of heat. As you heat the ice you give the water molecules more kinetic energy so they are able to move around more and break some of the forces that are holding them together in an ordered solid state.

When you want to change something from a higher energy state like liquid water to a lower energy state like ice you have to remove energy (and therefore heat) from the system. Water freezes at 0ᵒ celsius so you must remove enough heat energy to get it down to this temperature before it can turn into ice. Warm water has a higher temperature and therefore will require more heat energy to be removed from it before it turns in to ice. For this reason you would expect warmer water to freeze more slowly. This is not always the case.

The Mpemba Effect

Erasto Mpemba noticed a strange phenomenon during a cookery class whereby hot ice cream mix froze faster than cool ice cream mix. This is an effect that has stumped scientists for many years; in fact Aristotle noticed that warmed water often cooled faster than cold. The Royal Society of Chemistry recently ran a competition with a £1,000 prize for the clearest and most plausible explanation of the Mpemba Effect. After a surge of 22,000 competition entries the prize went to Nikola Bregovic, a physical chemist at the University of Zagreb.

Bregovic explained the Mpemba effect as the result of two phenomena; convection and supercooling. He measured and reproduced the Mpemba effect by recording the temperature of two samples of water as they cooled in his lab freezer, one with a starting temperature of 35ᵒC and one room temperature sample.

Convection

Convection-snapshot

(Image courtesy of Wikipedia)

When a beaker of warm water is placed in the freezer the water near the walls of the beaker are cooled more quickly and the water in the middle of the beaker remains warm. This provides a temperature gradient. Convection is the transfer of heat by movement of particles within a fluid, heated fluid moves away from the source of the heat, carrying energy with it. The greater the temperature gradient the more particle movement there is meaning that a warmer sample will cool to freezing point faster.

In order to see whether convection alone was responsible for the Mpemba effect, Bregovic repeated the experiment but this time the cooling water samples were continually stirred by a magnetic stirrer. This will have disrupted the effect of convection. In the stirred samples the Mpemba effect was significantly reduced but was still present, therefore other factors may also be influencing this effect e.g. supercooling.

Supercooling

Supercooling is the process by which you lower the temperature of a liquid below its freezing point before it freezes. It has been found that in most cases the temperature of water does actually have to first fall below 0ᵒC in order for freezing to occur. It is hypothesized that the cooler sample supercools more than the warmer sample before it freezes, therefore freezing more slowly than the warmer sample. However the mechanism for why this would occur is not known. Heating water prior to freezing has also been shown to have very variable effects on supercooling with some experiments showing increased supercooling in pre-heated samples and some showing less.

So it looks like this puzzle of phase transition still holds a little bit of mystery… and that is another one of the joys of science.

wallpaper_winter_freeze

Comments
One Response to “Freezing Fast”
  1. Lee says:

    I thought it was something to do with the decrease in density of water below 4 degrees, and the formation of those tetrahedral molecular formations in ice. I now realise this has nothing to do with it. It’s good to learn. Must be why the water freezes from top to bottom though.

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