How can ice freeze upwards

Contrary to what you might read in the YouTube comments , don't worry, these aren't supernatural in origin. In fact, they're the result of a very simply physical process that also makes it possible for life to survive on Earth, as Derek Muller explains in the latest episode of Veritasium. So why do they form? It all comes down to the fact that water expands by around 8 percent as it cools.

In your freezer, ice crystals first form around the outside edges of your cube, where the water molecules have the most contact with the cold air. The tallest spike we've seen growing in an ordinary ice cube tray was 56mm 2. Why Distilled Water? Most ice cube trays produce a few spikes, but usually only if distilled water is used. Millions of people make ice cubes every day using ordinary tap water, and most don't see ice spikes.

Oddly enough, some people often get plenty of ice spikes using ordinary tap water, but this appears to be rare. I have some ideas about why distilled water might make ice spikes more readily than tap water, but so far I cannot explain why some tap waters make copious ice spikes.

We measured the probability of ice spike formation as a function of salt NaCl concentration in distilled water, at a fixed temperature of -7C, which is shown in the graph at right.

Very little salt is needed to kill the ice spikes. All tap waters contain relatively high concentrations of various mineral salts, so apparently different impurities have different effects on ice spike formation. What Else Matters? Temperature is certainly a factor when growing ice spikes, and we have done experiments that show that spikes are more likely to form when the temperature is just below freezing see the graphs below.

I'm not entirely sure why this is so, but I suspect that the ice tube freezes shut before the tube gets very long if the temperature is too low. So, if your ice cream is really hard, your freezer is too cold to make lots of ice spikes. We've also found that ice spikes form more readily if we install a fan that blows the air in the freezer around. This is probably because the moving air provides some evaporative cooling that makes the top of the ice spike freeze more readily, so the tube can develop further before it freezes shut.

At the optimum temperature, and with air blowing over the freezing ice, about half the cubes in an ice cube tray will develop ice spikes. They can, however, sometimes form on falling sleet pellets. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American.

Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. Stephen Morris, professor of experimental nonlinear physics at the University of Toronto, maintains a Web page on just this topic.

So we asked him to tackle the question: Water is one of those rare materials that expands while it freezes. Get smart. Sign Up.