Solid and Solid Equal Liquid!

The Observation:

Ice can be melted with the help of salt. When certain salts like NaCl (sodium chloride = table salt) or CaCl₂ (calcium chloride) are sprinkled on ice, the dry looking ice turns to liquid.

The Theory:

The states of ice and liquid water are in a dynamic equilibrium:

Water in ice (solid) + energy <-> water (liquid)

When salt is added to ice, it dissolves in the liquid water on the surface of the ice. During this process, water molecules surround the salt ions. These water molecules are prevented from rebuilding a crystal lattice at 0 °C. Therefore, the water gets eliminated from the equilibrium and has to be constantly replenished from the solid ice to continue the state of equilibrium. In this way, the ice gradually turns to liquid.

Salt for Safe Streets:

Salt is often used to fight black ice, but its success is temperature dependent:

Dissolving salt requires energy (the so-called heat of solution) and this energy is provided by the solution itself. For this reason, an intense cooling of the salt solution down to -21.3 °C can be observed on its surface. Below this temperature, no more table salt dissolves when in contact with ice. Therefore, no more water can be removed from the equilibrium and the ice remains solid.
As a result, dispersal of sodium chloride is of no use below -21.3 °C. Calcium chloride (CaCl₂), which also has less effects on the environment, can be used as an alternative because CaCl₂ solutions do not freeze until -55 °C.

In general, salt promotes the corrosion of iron, which means that cars rust faster. Furthermore, trees and plants are affected and dogs get sore paws, so salt is being used less and less. To keep streets safe, pebbles can be used instead, in order to increase friction.

Icy Cold Sea Water:

Salt-water can be cooled down far below 0 °C, the freezing point of pure water, without showing signs of freezing. Nevertheless, at the North and the South Pole it is cold enough for the water surface to freeze over. The ice that crystallises from the salt-water is pure fresh water.