Salt will melt ice faster than the same amount of sugar because there are more molecules in salt than there are in sugar. It is the colligative property of salt that makes ice melt faster.
Pure water has a freezing point that is constant. It is 32 degrees Fahrenheit. When a substance is added to the water, it does two things.
- Lowers the freezing temperature of water
- Varies the freezing temperature of water - this is why the melting rate of water sprinkled with sugar is different from the melting rate of water sprinkled with salt
In both cases, the temperature is lowered, which is why salt is put on roads during freezing temperatures. It means the water on the road will not freeze, even though the air temperature is cold enough.
The chemical nature of the substance that is put on ice, however, is not an important factor. Instead, the number of molecules determines how fast or slow the ice melts. This is known as a colligative property.
Assume there is the same measurement of salt and sugar - one tablespoon. There are more molecules in the tablespoon of salt than there are in the tablespoon of sugar. This is why the tablespoon of salt melts ice faster than the tablespoon of sugar.