What happens to water if it's not allowed to expand when frozen?

Marshall Brain

You Asked:

What happens to water if it's not allowed to expand when frozen? --- Tony, Cut Off, La.

Marshall Brain Answers:

In nature, or in your kitchen freezer, water expands when it freezes. It gets about 9% bigger. This video gives you a dramatic demonstration of what happens when water freezes in a closed container:

In the video, neither the plastic pipe nor the iron pipe can withstand the pressure that the freezing water creates. Both explode.

But what if you take a cup of water and put it inside a thick, incredibly strong stainless steel pipe? Let's say the pipe is 6 inches thick, with massive screw-on caps that can withstand huge pressures. Now you pour liquid nitrogen on that. Since the water can't expand inside this stainless steel pipe, what happens?

To understand the answer to this question, you have to realize that there is more than one kind of ice. The only kind of ice we normally see is the ice that forms at atmospheric pressures. But at higher pressures, different kinds of ice can form. You can see a table showing the different kinds of ice (there are more than a dozen) on this page (scroll down to the table).

Above that table is a graph called a phase diagram. If you have never seen a phase diagram before, this video explains what it is showing:

The vertical axis registers increasing pressure. The horizontal axis is showing increasing temperature. In the "normal" case on planet earth, the pressure stays the same as ice freezes. So if you have a container of water, it moves along a horizontal line on the phase diagram between liquid and solid as the temperature changes. [Normal pressure at sea level is 14.7 PSI or 101,325 Pa. Freezing point of water at that pressure is 32 degrees F, 0 degrees C or 273 degrees K]

You can see on this page that the phase diagram for water is a little more complicated than the idealized one shown in the video, but it works the same way. As pressure and temperature changes, you transition between different phases.

In the case of our water in the stainless steel pipe, as it cools it tries to expand. That drives up the pressure dramatically. On the phase diagram you can see that at higher pressures, the water might form Ice II or Ice VI, or even Ice XI if it gets cold enough.

The point is that there are forms of ice that do not have to expand 9% when they freeze, but they only happen at very high pressures or very low temperatures. If you look at this page, you can see that Ice II has a density of 1.16 grams per cubic centimeter, as opposed to normal ice (the kind we see in the freezer) which has a density of 0.92 grams per cubic centimeter. In other words, Ice II actually shrinks when it freezes rather than expanding because the molecules of water arrange themselves differently in the ice.

See also: How Water Works