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Top Comments: Why is Ice Slippery? A Paradigm Shift [1]

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Date: 2025-09-04

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Phse diagram for a typical pure substance.

I taught chemistry at the undergraduate level for 30 years, both freshman chemistry and physical chemistry. In both courses, there was discussion of the physical phases of substances, that is, solid, liquid and gas, how they are related to each other, and how each phase can transform to another phase. These concepts are summarized in what’s called a phase diagram, which is essentially a map showing the physical conditions of temperature and pressure under which each of the phases is stable. The phase diagram for a typical substance looks like the diagram on the right. The x-axis is temperature, and the y-axis is pressure. The blue region is solid, the violet region is liquid, and the brown region is gas. The lines are called coexistence curves, and they specify the physical conditions required to maintain equilibrium between any two phases. They also define the borders between the various phase regions. To transform the phase of a substance requires crossing the corresponding coexistence curve. So, for example, boiling a liquid involves transforming a substance from its liquid form to its gaseous form, so, in this diagram, we’d have to cross the liquid-vapor coexistence curve, on the right, between the violet region and the brown region, from left to right, which requires increasing the temperature (which should not come as a surprise).

The phase diagram for water, however, diverges from this typical diagram in a significant way. Water’s phase diagram is shown at the top. You will note that, while in the typical diagram, the solid-liquid coexistence curve leans to the right, in water’s phase diagram, this curve slants to the left. (For the record, both of these slants are exaggerated for the sake of being able to see the slant.) This is because the crystal structure for ice is less dense than liquid water. For almost any other substance, when you freeze it, its constituent molecule arrange themselves is a crystal structure that brings them closer together, such that the solid is more dense than the liquid. For such substances, when you freeze or melt them, you will note that the solid is at the bottom of the container, while the liquid is on the top. We know from common experience that water doesn’t behave that way. Ice cubes float on top of liquid water demonstrating that water in its solid form is less dense than liquid water.

So, why is ice slippery? I spent my teaching career giving the reason that I learned myself as a freshman, and which makes sense from looking at water’s phase diagram. For example, consider ice skating at a temperature just below freezing. Suddenly, the full weight of your body is compressed upon a very narrow blade that is pressing down on the ice. This causes a dramatic increase of the pressure on the ice, and because of that back-slanted solid-liquid coexistence curve, under these conditions, the coexistence curve is crossed vertically to the liquid phase. The ice under the skate is transformed to liquid, which is slipperier than solid ice under ambient conditions. Once the skate has passed, pressure returns to normal, and the water refreezes into ice. (There is no change in temperature.)

This is what I’ve taught, and what I’ve thought was true for the past 50 years. However, new research has shown in detail that this is not what’s actually happening.

Water molecules are what we call polar molecules because the oxygen atom pulls more strongly on the electrons bonding it to the hydrogen atoms its bonded to, resulting in a slight positive charge on the hydrogens and a slight negative charge on the hydrogens. Further, because the water molecule is bent, the whole molecule has an electric dipole, corresponding to the green arrow in the figure on the right. In ice crystals, the molecular dipoles arrange themselves so that the negative end of one water molecule is close to the positive end of an adjacent water molecule. (Recall, opposite charges attract, while like charges repel.)

Under pressure, and in contact with substances with other polar molecules (such as those used to make shoe soles) destroy the symmetry of the molecular arrangement in the surface of the ice. (Steel skate blades have no polarity, but shoe soles generally do.) This disruption causes a phenomenon called “frustration” among the disturbed water molecules in the ice, where they are incapable of rearranging themselves in a way that satisfies the condition where each charged side of the molecule is pointing toward the opposite charge. The molecules can’t reorient themselves to reduce their overall energy. Under these conditions of frustration, the surface water molecules become mobile, that is they collectively behave as a liquid. This in turn makes the surface of the ice slippery. And then you slip and fall down.

So this is big news (or at least ought to be) for those who teach chemistry, and want to lie as little as possible (in the interest of simplification). The pressure doesn’t have to rise to the level where it crosses the solid-liquid coexistence curve. All that has to happen is frustration of the water molecules on the surface of the ice due to being disturbed by a foreign object.

Comments are below the fold.

Top Comments (September 4, 2025):

Highlighted by madmann1 with an assist by thespohyinx:

This comment by doncoolidge in irontortoise’s recommended post on the death of tickets sales at the Kennedy Center.

Top Mojo (September 3, 2025):

Top Mojo is courtesy of mik! Click here for more on how Top Mojo works.

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