Why Is Ice Slippery? It’s Not What You Learned at School

Step onto a frozen pavement and your feet might suddenly shoot out from under you. Skaters glide across rinks with almost no effort. Ice is one of the most slippery things we meet in everyday life. But why? The answer turns out to be more interesting than the one many of us learned at school.

The old explanation: pressure melting

For a long time, textbooks explained that a skate’s narrow blade puts enormous pressure on the ice. Pressure lowers the melting point of ice, so, the story went, the ice under the blade melts and the skate rides on a film of water.

The problem is the numbers. Pressure does lower the melting point, but only a little. Even under a skate blade, the effect is only enough to lower it by a few degrees at most. Yet people slip on ice at −20 °C, and a flat shoe sole spreads its weight over a much larger area than a blade. Pressure melting cannot explain why ice is slippery in most conditions.

The real reasons

1. A liquid-like surface layer. Inside an ice crystal, water molecules are locked in a neat, rigid pattern. At the surface, molecules have fewer neighbours to hold them in place. They are looser and move about, forming a very thin layer that behaves partly like a liquid, even well below 0 °C. This is called premelting. As long ago as 1850, Michael Faraday suggested such a layer exists, after noticing that two pieces of ice pressed together freeze into one. Modern experiments using X-rays and other techniques have confirmed it.

2. Friction heat. When something slides on ice, friction produces heat at the contact. That heat melts a tiny amount of ice, creating a film of water that lubricates the movement. The faster you slide, the more heat is produced, which helps explain why skating works so well once you get moving.

As physicist Robert Rosenberg summarised in 2005, the evidence favours these effects over pressure melting. Researchers still argue about the exact balance, and studies continue, but the classic school explanation is no longer accepted as the main reason.

Not all ice is equally slippery

Temperature matters. Ice is often most slippery just below 0 °C, where the surface layer is thicker and meltwater may be present. Very cold ice is less slippery because the surface layer is thinner and friction melts less. Speed skating rinks and ice hockey rinks are kept at slightly different temperatures, around −5 to −9 °C, tuned for the best balance of glide and grip.

Staying on your feet

• Walk like a penguin: take short, flat steps and keep your weight over your front foot.
• Watch for black ice: thin, clear ice on dark asphalt is nearly invisible, especially on bridges and in shaded spots early in the morning.
• Salt and sand: salt lowers the freezing point of the water layer, helping ice melt; sand adds friction.
• Choose good soles: soft rubber soles with deep treads grip much better than hard, smooth ones.

In short

• The school idea that pressure melts ice under a skate explains very little.
• Ice has a thin, liquid-like surface layer, even well below freezing.
• Friction heat from sliding melts a little ice and lubricates the movement.
• Michael Faraday proposed the surface layer as early as 1850.
• Ice is often most slippery near 0 °C; very cold ice grips better.
• Short steps, salt, sand and good soles help prevent falls.