Ice Melting Rates With Salt
Most recent answer: 10/22/2007
- Michael (age 11)
Your experimental outcome sounds more plausible and is easier to explain than the other one. (but see http://van.physics.illinois.edu/qa/listing.php?id=13496 for why the opposite can also happen)
Dissolving salt in water lowers the temperature at which the water freezes, or at which the ice melts. Salt, when placed on top of a melting ice cube, will dissolve in the little bit of water that melts first, and the dissolved salt lowers the melting temperature of the ice it's in contact with.
Melting ice takes heat energy from the surroundings -- 80 calories per gram of water, salt or no salt. The rate at which the ice melts depends on the rate at which heat energy flows in. Heat flows by conduction, convection, and radiation, and the flow rate depends on lots of external factors. You can change the heat flow rate by adding insulation, by moving the air with a fan, or by changing the temperature of the surroundings, among other things. A properly controlled experiment has all factors the same for the ice cube with the salt and for the ice cube without the salt, so that the effect just of the salt can be determined. If more than one factor is changed, one is never sure which factor caused the difference in the outcome.
Heat travels from hot stuff to cold stuff, and the greater the temperature difference, the faster the heat transfer will be. If the ice has salt on it, it will melt at a lower temperature, and will be colder than ice melting without salt on it. Heat will then travel faster from the warm air to the colder ice than to the warmer ice.
The salt will only affect that part of the ice with which it comes into contact. It may make a little pool of saltwater on the top face of the ice cube, which will "dig into" the ice cube. This is a common observation -- if I sprinkle salt crystals on ice on the sidewalk in the winter, it will look like the salt crystals dig little holes in the ice.
(published on 10/22/2007)
Follow-Up #1: salt melting ice
- Gemma (age 14)
South West England
To be sophisticated, you should try this at several different temperatures.
Perhaps I shouldn't say anything about what results to expect, but it's tempting to say a little. There's a temperature range (on the cold side) where the salt-free ice just won't melt but the salted ice will. We talk about the reason in lots of answers here. On the other hand, when the temperature is high enough to melt the ice without salt, you may find more complicated effects. The melting rate can depend on how the ice sits in the melted water, and that depends on whether you have salt as well as what type of dish it sits in.
(published on 10/22/2007)
Follow-Up #2: ice and salt
- Sam (age 15)
(published on 05/16/2009)
Follow-Up #3: melting salty ice
- james (age 11)
pleasant hill calif
The main point is that salt causes ice to melt at a lower temperature than it would without salt. That's why salt is sprinkled on icy streets- to melt the ice. So the frozen salt water, which has little pockets of salt scattered around near the ice, melts at lower temperature (and thus sooner) than pure ice.
Some of our older answers discuss why salt or sugar or other things that can dissolve in water always lower its melting temperature.
(published on 02/12/2012)
Follow-Up #4: different salts melting ice
- James (age 23)
Perhaps more interesting, the different salts you mention have nearly the same effect on the melting temperature if the solutions are adjusted to have the same number of ions per unit volume. Searching around on this site for "saltwater", "freeze", and "entropy" should turn up an explanation. The word "colligative" should help you search on the Web. Of course, this means that per gram the different salts have different effects on the melting temperature.
(published on 01/10/2013)
Follow-Up #5: Why do different salts have the same effect on melting?
- Oliver (age 24)
Notice that this effect depends only on how the available liquid space is reduced as the solid forms. To be more specific, entropy is the logarithm of the number of available states. If you take away 1% of the volume a particle had, its entropy will go down by ln(0.99) (times a conventional constant in some units). The type of particle doesn't matter at all.
This argument turns out to be exact so long as the solute particles are very dilute. Once the particles are concentrated enough to interact significantly with each other, some entropy and energy changes from the changing interactions can also have an effect, changing Tm one way or the other. For uncharged solutes, the interactions tend to be weak and the effect on the melting temperature has hardly any dependence on the type of solute, just the concentration of particles. For salts, I exaggerated a bit before, since the ions do interact significantly even at moderate concentrations. Thus you only get about the same effect for different salts at the same concentration. In particular, divalent ions (e.g. Mg+2) interact more strongly than monovalent ones (e.g. Na+), so say MgCl2 and NaCl will have different effects for the same number of total ions.
(Now that you ask, I've gone back to soften the wording in the previous answer.)
(published on 04/15/2013)