Saltwater DOES NOT Melt Faster!!!

Most recent answer: 10/22/2007

Q:
My 4th grader conducted a science project to time how long various amounts of coarse and fine rock salt would melt one measured cup of water frozen in 16 oz. plastic cups. He poured the salt onto the surface of the ice in each plastic cup. He also timed long it would take for one measured cup of water to melt without adding any salt. He found that the water without salt being added melted in less time. Why?
- Jim
Pittsboro, NC
A:
Jim -

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.

-Tamara

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.
mike w.

(published on 10/22/2007)

Follow-Up #1: salt melting ice

Q:
i understand that salt doesnt make water melt faster but what i dont understand is.. the salt making it melt at a lower temp.
- Danielle 11
king NC US
A:
Hi Danielle- We have many other answer on this topic, which can find by searching for "freezing saltwater".
Mike W.

Lee H

(published on 10/22/2007)

Follow-Up #2: salt water melting ice

Q:
Which one would melt more ice: a cup of crystallized salt or a cup of salt already dissolved in water?
- Lesley H. (age 24)
Phila, PA, USA
A:
If there’s a fixed temperature, the final concentration of the salt in the melted water is fixed. So a given amount of salt will be dissolved in some fixed amount of water, when the ice and water have reached equilibrium. If there’s already some water used in the salt applied, then less water will be melting off the ice to reach the fixed final amount. However, it’s possible that having the salt dissolved in water may speed up the melting, even though not as much ice will ultimately be melted.

Mike W.

(published on 10/22/2007)