Why do people put salt on ice? Why does salt melt ice?

A lot of people, especially from the tropical countries may ask: "How come people put salt on ice?", "Why the salt melt ice on the road?"

The simple answer is: "Salt lowers the melting/freezing point of water". Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10% salt solution freezes at 20 F (-6 C), and a 20% solution freezes at 2 F (-16 C).

When you put salt on ice, you can see the dissolving process happen - the ice immediately around the grain of salt melts, and the melting spreads out from that point. However, it does not affect broader if you don't put enough salt because the mixture of ice and salt is weaker and weaker. Moreover, if the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect - the solid salt cannot get into the structure of the solid water to start the dissolving process. For that, put sand on the ice for better traction is helpful.

Similarly for making ice-cream, you should put some salt to lower the meilting-point of ice-cream and it's easy to eat, otherwise the ice-cream is hard as ice.