Why does salt melt ice
Salt has a rather peculiar relationship with various forms of water. Apart from lowering the freezing point of ocean waters, salt makes it easier to scrape off snow from roads. It is important to know the chemical composition of the common form of salt. Chemically, salt is sodium chloride. Upon addition to water, salt separates into a positively charged ion of sodium and a negatively charged ion of chlorine. These charged ions interfere with the tendency of the surrounding water molecules to fix themselves into a stable structure of ice.
Not only sodium chloride, many other forms of salts perform similarly. For instance, potassium chloride, calcium chloride and urea also succeed in making snow melt. However, sodium chloride continues to be the popular form of salt because of its harmless nature and easy availability. The lowering of the freezing temperature of water with some presence of dissolved salt is logical extension of simple conclusions. In a sample of pure water, the density of water molecules is high. As we decrease the temperature, the motion of water molecules slows down and they organize themselves to acquire a solid crystal shape. Salt dissolution lowers the density of water molecules, and as a consequence, the temperature needs to be even lower for the molecules to slow down sufficiently for the solid crystal shape. As a thumb rule, greater the amount of dissolved salt, lower is the temperature of freezing of water.
This is precisely why salt succeeds in melting ice and snow. By altering the freezing conditions of water, salt actually makes ice start melting. Getting into deeper technicalities, it has been established that addition of one mole of ion releasing salts in one kilogram water results in a decrease of 1.8 degrees Celsius to the freezing point temperature of water. In other words, the effective temperature experienced by the already existing ice form rises up, and initiates the melting phenomenon. This can continue only till a certain saturation level of water, and not beyond that. The melting point lowering stops once water is not able to absorb further salt molecules.