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Q & A: Why does ice melt faster in tap water than in salt water?

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Q:
Hi, your answer for why ice melts faster in salt water vs. tap water is not correct. It does have to do with density, but not in how high something floats. Try the experiment with one normal ice cube and one colored ice cube, both in salt water and in tap water. You will find the results interesting.
- Greg Baxley (age 31)
Bakersfield College, CA
A:
Greg -

Thank you for your correction! We have removed the original answer from our database.

It is true that an ice cube will melt much faster in tap water than in salt water. And although there is a difference in how high the ice cube will float in each (as I’d said before), it is not enough to make the major difference.

When you think of a regular ice cube melting in a regular glass of water, you have to remember that cold water (like the water from the ice cube) is actually denser than warm water (like the water in the cup). This is because in the cold water, the molecules have less energy and are actually closer together than in warmer water. So as the ice cube melts, the cold water coming off of it sinks to the bottom of the glass and the warm water from the bottom comes up to take its place. The water in the glass is actually constantly moving, keeping the ice cube warm by something that scientists call ’convection currents.’

But salt water is much denser than tap water, warm or cold. So when you put a freshwater ice cube in a glass of salt water, the cold water coming off the ice cube doesn’t sink at all. Instead, the dense salt water stays at the bottom of the glass and the cold water stays on the top. Without any convection currents to carry the cold water away from the ice cube, the ice cube melts much more slowly.

Thank you again for this correction... please feel free to let us know if you see any other errors on our page.

-Tamara

If you’re interested in finding more information on experiments to use in your class on the role of convection in melting ice, you can also check out this article: ’Denison, R.F. Agricultural physics: a laboratory mystery to learn about convection. Golden Slate (Calif. Agric. Teachers’ Assoc. newsletter), March, 1999.’ (Submitted by Ford Denison, Age 47 from UC Davis.)

(republished on 07/25/06)

Follow-Up #1: salt and melting ice

Q:
The answer you give (below) is also incorrect. When you put a freshwater ice cube in a glass of water, it melts faster, not slower! Thats why they use ice to melt snow off roads. The reason has little if anything to do with convection currents, as is demonstrated by the effect of sprinkling dry salt on to the top of an ice cube... you can see the ice cube melt away before your eyes, much more rapidly than an unsprinkled ice cube. The reason has to do with the effect of the salt molecules on the vibration of the water molecules, in the liquid salt solution and the solid freshwater. At the interface, an equilibrium is established between ice and water - water is constantly freezing and melting, so molecules are moving back and forth between the states. The ratio of the speeds of freezing and melting, (which at the molecular level is a change in vibration speed)determines whether the overall trend is towards liquid or solid. This equilibrium is changed by the presence of an impurity in the liquid, but not the solid, so the ratio is changed, hence the overall melting rate changes. This is only a short summary of a fairly complex phenomena, the maths of which is quite involved. "But salt water is much denser than tap water, warm or cold. So when you put a freshwater ice cube in a glass of salt water, the cold water coming off the ice cube doesn’t sink at all. Instead, the dense salt water stays at the bottom of the glass and the cold water stays on the top. Without any convection currents to carry the cold water away from the ice cube, the ice cube melts much more slowly. "
- Marcus (age 41)
Australia
A:
Marcus- You have confused two things. The first is the effect of salt on the melting temperature of ice and the second is the rate at which a (melted) equilibrium is reached. There is no doubt at all that salt lowers the freezing point of water. So does any stable solute which doesn't freeze into the ice. One can prove that rigorously via a thermodynamic argument. That's why salt is used to melt snow.
    By the way, the main reason for the lowering of the freezing point by solutes is the effect of the changing liquid volume on the entropy of the solute. This effect is rather easy to calculate, and turns out to agree well with the observed effects.  Other effects can matter at higher solute concentrations, and can either increase or decrease the lowering of the freezing point.

The second point concerns the rate at which ice will melt to liquid in a glass of salty or pure water. Rates are much more complicated things than thermodynamic equilibria, and it turns out that in this case the actual effect is typically as described in our old answer.

Mike W.

(published on 04/05/08)

Follow-Up #2: cooling ice with salt

Q:
Does the addition of salt to ice actually make the ice warmer or just lowers the melting point, which would make it really cold water?
- Jeff
New Jersey
A:
The presence of salt lowers the melting point, as we discuss elsewhere. Unless the ice is very cold, some of it will then melt. The liquid state has higher energy density than the frozen state, so that energy has to come from the thermal jiggles in the ice and salt. They will actually cool down as they melt, although gradually heat will leak in from elsewhere.

Incidentally, adding ordinary table salt (NaCl) to liquid water soaks up some enthalpy, so if the salt and water started at the same temperature, they get cooled a bit by forming a solution. Some other salts release some heat as they dissolve.

Mike W.

(published on 01/26/09)

Follow-Up #3: salt water freezing again

Q:
Ok. all you little kids are wrong! I did a big huge project on this and i won 1st place in the science fair. If you put water and ice together in a cup and a cup of just water in the freezer at the same time then the one with salt will freeze faster. This happens because when salts solute in water they break apart as ions for example table salt, NaCl, becomes Na+(aq) Cl-(aq) water, which is already somewhat polarizes, reacts to the soluted ions by polarizing further, thus making the bonds between water molecules more powerful stronger bonds means more it takes more energy to keep them apart, means they'll solidify at a higher temperature. So yeah u little kids dont know anything bout this stuff. well mmmkay have a good day! XD
- Binky (age 15)
california
A:
Wow, where to start? 

First, I assume that by "water and ice" you meant "water and salt", otherwise it doesn't connect with the rest of your discussion.

Let's start with the obvious parts based on familiar facts.

If salt tended to stabilize ice more than water, by the fancy mechanism you describe, adding salt to ice would help keep it frozen. Somebody better tell the Highway Department, because they've been using salt to melt ice all these years.

Working our way toward slightly less familiar facts, the mechanism you describe would only work if the salt was actually in the ice, helping line up those water molecules. However, virtually none of the salt goes in to the ice. You can check this by partially freezing some salt water. You'll find that the non-frozen liquid is saltier than what you started with, because salt was excluded from the ice.

I'm trying to see if there is any grain of truth in what you've written. It is true that dissolving table salt in water will lower the temperature of the water, because it takes energy (more precisely, enthalpy) to pull those ions apart. That could help freeze the water. However, that effect amounts to only about 0.9 K cooling for 1 M salt concentration. (That cooling is temporary and has no effect on the freezing time for the salt water if you let it stand at room temperature before putting it in the freezer.) The same 1M NaCl lowers the freezing point of water by about 3.7 K, so it still needs to cool more than plain water before it freezes, even if you didn't let it reach room temperature first.

So I have to conclude that everything you wrote about the science of water and salt is incorrect.

Is it possible that your glass  of salt water froze first? Maybe- freezers don't cool evenly, some spots already have frost, some are in good contact with metal, so it takes a lot of care to check whether the result depended on details of placement, etc. However if you got the water really salty, stirring in NaCl until no more would dissolve, the water wouldn't freeze until it was cooled to -21.1 °C. Ordinary home freezers don't usually get quite that cold, so unless you have a very good freezer on the coldest setting you would never have seen it freeze.

For anybody who wonders who is right: do the experiment yourself.

Mike W.

(published on 09/12/09)

Follow-Up #4: experiments on melting ice

Q:
We do an experiment in my online chem class where we put food coloring into water and then freeze it. Then we put the colored ice into tap water (fresh water) and water with either salt or sugar in it. The tap water melts the cubes in about half the time as the salt/sugar water. It is a matter of bringing warm water into contact with the surface of the cube (this happens with the tap water because of convection) and layering of the cold, colored melt on top of the denser solution (little convection of the bulk fluid here). We also use the layers on top of the solutions to test diffusion--which turns out to be both surprisingly slow (takes several days on the counter top and over a week in the refrigerator) and smooth. Most of the time diffusion measurements are confused by convection but these don't seem to be bothered. The nice thing about using the color is that you can see the currents (and lack of currents) and you can add the diffusion activity. Tall thin (parfait) glasses work best for both parts of the experiment.
- Bennett Willis (age 68)
Lake Jackson, TX, USA
A:
Thanks, I bet some  of our other readers will want to use this technique.

I wonder if we can use the technique to demonstrate diffusion in class. Our other diffusion demos are messed up by convection, as you say. The issue will be if we can get the apparatus to sit untouched for a few days between classes.

Mike W.

(published on 09/16/09)

Follow-Up #5: stirred salt water and ice

Q:
What if your two glasses of salt and fresh water were constantly stirred? Would the ice melt faster in the salt water? That would eliminate the density stratification that you described that causes the cold water to stay on top of the salt solution.
- James M (age 60)
Austin, TX
A:
The best way to tell is by experiment, but I'd lay good odds that you're right.

Mike W.

(published on 11/09/09)

Follow-Up #6: ice melting with salt or not

Q:
I read the discussion on ice melting in tap water vs. ice melting in salt water and I also researched other websites and I am confused. My 4th grader and I tested ice cubes melting in glasses of tap water vs. salt water, sugar water and baking soda water and found that by adding the various solutes we indeed saw a temprature drop of the water. We also observed that an ice cube in tap water melted at a much faster rate and the baking soda/ water cube melted the slowest. We then took ice cubes and placed them on individual plates. We sprinkled one cube with salt and left the other alone. We covered the cubes with glasses and observed that the cube sprinkled with salt melted faster. To me the results were confusing. Is this two different things taking place?
- Jim Houghtaling
Lancaster, SC 29720
A:
If I understand your descriptions right, what you saw fits pretty well with our old explanations. In a salty liquid, the melting is actually slowed down, because the cold melted water sits on top of the denser salty water. That keeps heat from getting quickly to the ice. On the other hand, direct contact with salt promotes melting.  Furthermore, actual dissolving of salt in water soaks up some energy and cools things down.  So yes, several effects are present, and what you saw fits expectations very well.

Mike W.

(published on 03/04/10)

Follow-Up #7: disputes about melting of ice by salt

Q:
It is my understanding that a solute, such as salt (NaCl) melts ice not by the process listed above but by one slightly different. In this process, the salt ionizes at the surface of the ice where an equilibrium between ice and water exists. The salt ionizes into Na+ and Cl- within the water at the surface (NOT the ice). Thus this water at the surface has solutes in the solution and a lower freezing point than the water originally on the surface, and consequently does not refreeze, Shifting the equilibrium to make more water, which will in turn be removed from this equilibrium by the salt. This is the result of a colligative effect. One major assumption that is incorrect in seemingly all of these answers is that the melting point and freezing point are the same. The salt Ions lower the freezing point by nature of their interruption of the formation of hydrogen bonds settling into a crystalline structure (colligative effect of solute in solution). However, once frozen, the ions dispersed within the water in a crystalline state actually RAISE the MELTING POINT by a completely different mechanism. This is a product of intermolecular forces. The intermolecular forces in pure ice crystal is solely hydrogen bonds. When salt is included into this lattice, the intermolecular forces increase, as a result of an increased attration between ion-dipoles interactions as opposed to simply dipole-dipole interactiom (pure ice). An increase in intermolecular forces is not the same as the colligative effects of adding a solute, while both raise the boiling point of a solution, intermolecular forces increase the kinetic energy necessary for a vapor molecule to escape the liquid, thus lower the vapor pressure, whereas colligative bP elevation/ vapor pressure depression is a result of less solute particles taking the place of solvent particles on the surface of the liquid, decreasing the likelihood of a vapor molecule escaping the liquid and as a result has a quantifiable effect directly related to the concentration of the solute.
- Billy (age 21)
Birmingham, AL
A:

In looking over your discussion, there seems to be one point on which we clearly disagree. You write "One major assumption that is incorrect in seemingly all of these answers is that the melting point and freezing point are the same. " The equality of the thermodynamic melting and freezing points is not just a feature of some detailed model. It follows directly from the fundamental laws of thermodynamics. For a given solution concentration one phase or the other has the lower free-energy at each temperature, and that phase is the stable one. There's just one temperature where the free-energies are equal, and that's the melting/freezing point. For non-saturated solutions, the concentration changes as the freezing or melting occurs, giving a melting/freezing range. However, at each temperature in that range a precise fraction of the material is melted in equilibrium, regardless of previous history.

Interactions between ions of salts do change the magnitude of the freezing point depression and boiling point elevation, giving a correction to the simple colligative values except for very dilute solutions. The type of argument you give, trying to pick up various terms in the dynamic behavior separately, is tempting to use for pictures but famous for leading to wrong conclusions. The deep and rigorous rules of thermodynamics are a reliable guide to equilibrium properties, although not to rates.

Mike W.


(published on 07/28/13)

Follow-up on this answer.