Ice is the common name used to designate water in a solid state (the word "crystal" itself comes from the Greek word meaning "ice"). It is a transparent crystalline solid: at standard atmospheric pressure (101 325 Pa) the phase transition occurs when liquid water is cooled below 0 ° C (273.15 K, 32 ° F).
The water can remain in the liquid state even below 0 ° C due to the supercooling phenomenon (up to -42 ° C) or with pressures higher than the normal one (up to -30 ° C); vice versa, ice can also form at temperatures above 0 ° C with pressures lower than normal. There are 15 different solid phases of water, but the most common is Ih, which is the only one present in the biosphere, apart from a small percentage of Ic found in the upper atmosphere. The various phases of ice formed at pressures different from the normal one have a crystalline structure different from that of ordinary ice.
Ice, water and water vapor can coexist at the triple point, which for this system is placed at a temperature of 273.16 K (0.01 ° C) and at a pressure of 611.73 Pa.
An unusual feature of ice is that the solid has a density that is about 8% lower than that of liquid water. At 0 ° C and at atmospheric pressure, ice has a density of 0.917 g / cm³, water 0.9998 g / cm³. Liquid water reaches its maximum density, exactly 1 g / cm³, at 4 ° C and starting from this value it becomes less dense while the temperature drops towards 0 ° C when its molecules begin to arrange themselves in the hexagonal geometries that will give rise to the formation of ice. This is due to the bonds that form between water molecules by means of hydrogen atoms, which align the molecules less efficiently, in terms of volume, when water freezes.
One of the consequences is that ice floats on water, an important factor for the Earth's climate and essential for aquatic life (and for life in general) because, by blocking convection phenomena, it prevents the underlying water from continuing to cool and freeze all.
A body moving on ice moves "sliding", that is, without significantly decreasing its speed. This is due to the fact that a body resting on the ice is subject to the weight force that pushes it downwards; this force manifests itself as a pressure that acts on the contact surface between the body in question and the underlying ice, and causes a partial melting of the ice, with the formation of a thin layer of water that adheres to the body and allows it to slide.
Thanks to the formation of the aforementioned thin layer of water, the two solid surfaces (the sliding body and the frozen surface) are not directly in contact, so the movement is slowed down by the viscous friction (which occurs between the water and solid surfaces), which is significantly lower than the sliding friction that would occur if the solid surfaces were in direct contact.
Furthermore, the pressure is given by the ratio between the applied force and the contact surface area (p F / A), so decreasing the contact surface area increases the pressure and consequently the underlying ice melts more easily. , so the friction is less. For this reason, the blades of the ice skates must be very thin.
The friction also generates heat, which partly contributes to the formation of the layer of water.
Finally, in order to slide on the ice, the surface of the ice must be smooth enough; this condition is satisfied, for example, if the ice is formed by slow solidification of a body of water.
However, the previous theory does not explain why even small light objects can slide on ice.