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You, too, can grow ice-cube spikes in your very own freezer!

Q: Why do the ice cubes in my ice tray occasionally have spikes on them? Why would they defy gravity and spike upwards? (Bert, Massachusetts)

Q: I found a spike on a cube in my ice-cube tray. What causes such a thing? (L. A., Sandia Park, New Mexico)

A: I’ll answer your questions by discussing a video* taken of an ice spike growing in a freezer. I will explain how the spike formed.

Reader’s ice spike, grown in his kitchen freezer [Courtesy of Lanney Atchley]

A clear plastic cube filled with distilled water sits in the middle of a steel wire rack in a freezer tank whose air temperature is 11.3 degrees Fahrenheit (-11.5 degrees C). It’s hand-sticking cold inside the freezer. The water-filled cube chills. As the temperature drops, the water molecules move slower. In about an hour and a half, the water molecules close to the edges of the cube on top, where it’s the coldest, jiggle less.

"We know there are forces of attraction between the atoms so after a while they will not be able to jiggle so well, writes Nobel-prize winning physicist Richard P. Feynman in his Lectures on Physics. At very low temperatures, "the molecules lock into a new pattern which is ice." They freeze.

The molecules rearrange their bonding structure into tiny ice crystals that take up more space than the liquid structure does. They expand. The crystals grow in their interlocking way into ice sheets and spread from the sides towards the center. In a minute or two, the cube-top surface is ice bound except for a small hole.

More than one hole can remain free and "once we found three spikes emanating from a common base," says Ken Libbrecht, physics professor at Caltech who investigated the phenomenon in his lab

The ice continues to grow outward from the sides just below the top surface. Since newly formed ice has more volume than it did as a liquid, the expanding ice pushes against the surrounding sub-surface water. The water moves in the easiest direction — up and out through the open center hole.

The displaced water piles up around the hole edge and freezes, forming a rim, which is the beginnings of a hollow tube. The rim grows into a mound and the mound bulges upward (this is where the video starts) as more water wells up from below. A hill protrudes. More water pushes up the hollow tube and the baby ice spike grows along a sharp angle from the vertical. This happens probably because the sides of the spike are continuations of subsurface-crystalline arrays that extend from the surface at a sharp angle.

"The spikes form at apparently random angles with respect to the vertical and with various lengths," says Libbrecht.

Up and up it goes — the video shows the action at 50 times normal speed. Until, in about five real minutes the narrowing tube freezes across its diameter, closing the hole. A sharply-angled spike is born.

That’s how it happens. I tried it in my freezer and got an itsy bitsy tiny spike. But, as the reader’s image shows, you can build big spikes (over 2 inches or 5 cm) in your own freezer. Essential: use distilled water.

Physicist Ken Libbrecht’s spike, grown in his kitchen freezer. [Courtesy of Kenneth G. Libbrecht, © Used with permission.]

Minerals and salts found in ordinary tap water usually block a baby ice spike’s growth. Ice crystal structure does not normally include impurities (which is why sea ice is drinkable). So tap-water impurities get squeezed into the tip of a baby spike, which reduces the freezing rate at the tip and, therefore, the growth of the tube. As a result, the baby ice-spike tube grows so slowly that it freezes shut before the spike achieves any size.

With ideal conditions (distilled water, a fan promoting evaporation and rapid cooling, -7 degree C temperature, an empty freezer) "about half the cubes in an ice-cube tray will form spikes," says Libbrecht.

By the way, the fact that ice crystals take up more room than the original volume of water means that ice is less dense than water. (The ice has the same mass but more volume.) "This is why ice floats and why lakes freeze from the top," emails physicist Steve Morris of the University of Toronto.

* Video made by Edward Lozowski (and Lesley Hill and Russ Sampson) in Lozowski’s ice lab at the University of Alberta.

Further Reading

University of Toronto: Got spikes on your ice cubes? by Stephen W. Morris

California Institute of Technology: An investigation of laboratory-grown "ice spikes" by K.G. Libbrecht and K. Lui.

SnowCrystals.com: Ice spikes by Ken Libbrecht

WonderQuest: Ice (almost) always floats

Craig F. Bohren, What light through yonder window breaks? "Frozen dew and frost". New York: John Wiley & Sons, Inc. 1991.

(Answered Oct. 25, 2005)