Scientists have discovered that a very thin layer of liquid water exists at the outermost edge of ice — even ice that is extremely cold. For years, scientists believed that this thin layer of liquid water was the result of pressure being applied to ice.
Water has an unusual property: its solid form (ice) is less dense than its liquid form. This is why ice cubes float in a glass of water. The lower density of ice means that its melting temperature can be lowered by applying pressure.
So, for example, when an ice skate glides over ice, scientists once believed that the pressure applied by the skate would lower the melting point of the solid ice. This would cause the outermost layer to melt. The skate would then slide across on a thin sheet of water that would immediately refreeze as soon as the skate passed.
Unfortunately, scientists later discovered that the pressure applied by skates or people's shoes standing on ice was not great enough to cause these changes. So, they had to look for another explanation. In recent years, two other theories have been developed to explain the presence of the thin liquid water layer that exists on ice.
One of these theories is that friction causes the liquid layer of water to form on ice. Friction is the force that generates heat whenever two objects slide against each other. If you rub your hands together, you can feel them heat up. That's friction at work. When a skate moves over the surface of ice, the friction between the skate and the ice generates heat that melts the outermost layer of ice.
But ice is still slippery even when you're standing still. If you stand on ice without moving, no friction exists to generate heat, yet the ice is still slippery. So there must be something else going on.
The other theory is that ice is just slippery, because the outermost layer never turns to a solid. According to this theory, the water molecules at the surface of the ice move more, because they're at the edge and there aren't any molecules above them to help keep them in place. As a result, the outermost layer stays in a liquid state even at temperatures way below freezing.
So far, scientists haven't been able to decide which of these two theories is correct. Without a definitive answer at this point, it's safe to say that both theories play a role in keeping ice slippery.