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Can I freeze ice cubes faster?

I started thinking about faster ways to make ice cubes after our automatic ice making contraption broke and the repair guy told me it wasn't worth fixing. So I won't go into all the things I didn't know about freezers, just the stuff I didn't know about making ice cubes.

The Tray

I remember having metal ice cube trays with a lever when I was a kid long ago and far away. Now they seem to be mostly plastic, but lately some are made of silicone. For speed, metals the way to go because of its little insulation, although it tends to cost more.

Water Quality

Dissolved minerals in water tend to lower the freezing point. The surfaces tend to freeze first because this is pure water. The impurities concentrate in cloudiness in the middle. Water in Vancouver is soft, so it has fewer minerals. Our ice cubes are still cloudy in the middle, but I see bubbles in there, probably from trapped gases. Commercially produced ice is made with a flowing source of purified water so the bubbles get washed away as the ice forms from below.

Water temperature

I assumed that colder water would freeze faster (and of course it is cold just before it freezes). But the Mpemba effect says that warmer water placed into the cold can freeze faster.

In the 1960s, a high school student in Tanzania named Erasto Mpemba wondered why, when his class made ice cream, warm milk froze faster than cold. He questioned his teachers and did experiments with water to verify that it happened. I think this is an awesome example of asking questions of authorities. The tricky thing is that it doesn't always happen and it seems difficult to control just the variables you want to compare.

Brownridge found that hot tap water had a higher freezing point and froze faster than cold distilled water, but that isn't just changing temperature. Here are some suggestions for why:

  1. Evaporation - If you use hot water, then it evaporates more quickly so you have less water left over, which can then freeze more quickly. But the Mpemba effect can occur with closed containers.
  2. Supercooling - Hot water is less likely to undergo supercooling when it is below 0 degrees Celsius. 
  3. Dissolved gases - Hot water may lose some of the dissolved gases, and the resulting water might freeze faster. If you used boiled water to make ice cubes they are supposed to be clearer.
  4. Convection - Hot water could result in more circulation of the temperature and result in more rapid cooling.
  5. Surroundings - If you have a freezer with frost and you put something hot on it, that will defrost it and then cool more effectively. This might be especially true with metal ice cube trays and old fridges with the little ice cube making compartments that get all frosted up.

So why warmer water would freeze faster is still open to discussion. I just found that the Royal Society of Chemistry in Britain and Hermes 2012 offered a prize of £1000 (over $1500 CDN) for the best explanation for the Mpemba effect (sorry, it's too late to enter!). They received 22,000 responses and at the time of this post, were still processing them.

All this investigation has given me pause. Am I so busy that I cannot even let my ice cubes freeze as they will? I've decided that it is me and not the ice cubes that need to chill.

Nonetheless, if you have ice cube freezing tips, let me know.

Take this a bit further and explore a great activity on Science World Resources: Ice Cube Towers