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Q & A: Ice melting rates with salt

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Most recent answer: 04/15/2013
Q:
I read one of the questions already on this site and i saw that a kid put salt on a ice cube and timed how long it took to melt and he also timed how long it took to melt a ice cube without salt on it. It ended up that the ice cube without salt melted fist. I did that same experiment and the ice cube with salt on it melted faster. Also on the ice cube with salt had a different surface than the other ice cube which was round. It looked like the salt dug into the ice cube. Why did my experiment come out like this? p.s Does salt absorb water?
- Michael (age 11)
Boston
A:

Hi Michael,

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.

Tom J.


(published on 10/22/2007)

Follow-Up #1: salt melting ice

Q:
We are about to do an investigation at school about what affects ice melting. My question i need to investigate is "Does adding salt to an icecube affect how long it takes to melt". We have come to the I.T room to do some research on our experiments - Can you give me some advice or tips on what i can do to make my test as accurate as possible please?
- Gemma (age 14)
South West England
A:
Sure, you should compare cubes of the same size and shape, sitting in similar dishes, and of course at the same temperature, one with and the other without some salt on it.
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.

Mike W.

(published on 10/22/2007)

Follow-Up #2: ice and salt

Q:
Hi there, does desolving salt in water before freezing it result in the same effects to its melting time as spinkling it ontop?
- Sam (age 15)
Australia
A:
If you freeze saltwater slowly, almost all the salt stays out of the ice as it freezes. So you're left with ice with salt on the surface. The effects are similar to sprinkling ice on the surface after it's frozen.

Mike W.

(published on 05/16/2009)

Follow-Up #3: melting salty ice

Q:
we are doing a experiment , we froze a cube of freshwater, then made some salt water and froze the to , we took it out after 24 hours and started timing it the saltwater melted a lot fasterwithin an hour it was unfrozen it took the fresh water 3 and ahalf hours why is that...
- james (age 11)
pleasant hill calif
A:
James- I've marked your question as a follow-up to a related thread. Our search engine will turn up many other answers on this site dealing with the whole question of salt's effects on freezing and melting water.

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.

Mike W.

(published on 02/12/2012)

Follow-Up #4: different salts melting ice

Q:
Does different salt types i.e. NaCl, LiCl or KCl have different effects on the rate of melting?
- James (age 23)
London
A:
Those rates are complicated things. The different salts will almost certainly have slightly different effects for some given arrangement. For example, if the melting ice sits in a cup of salty ice-water, the different densities of the different salt solutions will change the circulation patterns in the water, giving different melting rates. You can try this to see how it works.

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.

Mike W.

(published on 01/10/2013)

Follow-Up #5: Why do different salts have the same effect on melting?

Q:
Why do different salts have almost exactly the same effect on the melting temperature if the solutions are adjusted to have the same number of ions per unit volume?
- Oliver (age 24)
A:
Here's the simplest picture of why solutes affect the melting temperature, Tm. First, we should explain that Tm is the temperature at which the net entropy (system plus environment) of the solid and liquid states is the same. (Entropy is a measure of how many different quantum states are available to the system.) The entropy lost as the atoms or molecules line up to form a solid is just balanced by the entropy gained as the heat released warms up the environment at Tm. Adding solutes to the liquid causes more entropy loss when the solid starts to form, because the solutes have less room to run around in the remaining liquid than they had before. So that favors keeping the liquid state, and therefore the solid doesn't form until Tm is reduced enough to compensate.

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.)

Mike W.

(published on 04/15/2013)

Follow-up on this answer.