This is one of the biggest misconceptions about salt. Salt DOES NOT necessarily make ice melt faster. It makes ice melt at a lower temperature. The more salt, the lower the melting temperature. The reason that they use salt on roads is so that there will be more temperatures at which the ice is melted. For example, regular ice melts at 0 degrees C. By adding salt, you can drop that temperature to as low as -21 degrees C. So if it were -10 degrees C outside, salted ice would be melted but regular ice wouldn't. Below -21 degrees C, the salt would not make any difference. This has nothing directly to do with how quickly
the ice melts - just what temperatures it will melt at.
The cool thing about this is that this is nothing special about salt. Pretty much anything that dissolves in water will drop the melting point. For instance, you could use sugar or even alcohol. Salt is just used on roads because it's inexpensive.
So why did your student see that the regular ice actually melted faster when we'd expect them to have melted at the same rate? This is hard to say without seeing the experiment itself. Most likely it was something incidental to the set-up. It's possible that he used so much salt that the undissolved salt actually worked like an insulator around the ice, keeping it from melting as quickly.
It's also possible that as the ice started to melt, the salted cube was sitting high in a pool of very dense saltwater, while the unsalted one was sitting low in a pool of plain water. Then the newly melted (and cold) water would sit on top of the denser saltwater, keeping the ice cold. In the plain case, the ice would contact more water, probably helping conduct heat to the ice.
(republished on 07/25/06)