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Name: Blair F.
Status:   student
Age:   12
Location: N/A
Country: N/A
Date: 1999-2001


Question:
We have been doing some expeiments with salt and ice. We know that salt will melt ice and that the saltwater solution then goes below 32 degrees F. I understand why the ice melts but I do not understand why the temperature goes down. Where does the cold come from?


Replies:
Blair,

Cold does not "come from" anywhere. Cold is the absence of heat. As the salt dissolves in the meltwater, intermolecular bonds between water molecules in the ice lattice are broken. Bond breaking processes always require energy. The energy flow to enable the breaking process comes from the surroundings. If you were to be holding the glass containing the ice-salt-water mixture, heat energy would flow out of your hand into the glass. Heat flow away from you is perceived as cold.

Regards,
ProfHoff 290


Blair F.,

In general, as salt (the solute) is added to the water (the solvent), the resulting solution will have a lower freezing point than the pure solvent, in this case- water. In chemistry, the properties that vary according to the ratio of the weights of the solute and solvent are known as colligative properties. I hope that this answers your question.

Sincerely,

Bob Trach


It takes energy to change a solid to a liquid. That energy has to come from somewhere, right?

Tim Mooney


To "explain" the lowering of the freezing point of ice "properly" requires knowledge of an area of physical chemistry called thermodynamics, which I am assuming you are not ready for, so I will try to put the concepts across as well as I can using as little of the jargon as possible.

Pure water and ice at 0 C. are said to be "in equilibrium" which means simply that they will co-exist indefinitely so long as the temperature remains at 0 C. This equilibrium is written as if it were a chemical reaction:

ice(pure) = water(pure) at 0 C.

If the temperature is raised above 0 C. the ice will eventually melt, and the ice and water are no longer "in equilibrium". If the temperature is lowered below 0 C. (Ignore the fact that water can sometimes be super-cooled below 0 C. and remain a liquid, at least for a while because that is an unstable condition.) If the temperature of the ice/water mixture is lowered below 0 C. the water will eventually all freeze leaving only ice, and the ice and water are no longer "in equilibrium."

There is a quantity called the "chemical potential" that is a property of the pure ice and the pure water. At 0 C. this "chemical potential" of the ice and the water are equal to each other. (Don't read too much into the term "chemical potential" literally because it is a jargon word. Think of it more as some measurable quantity that depends on temperature, pressure, and the composition of the things involved.The only thing you have to know about this "chemical potential" is that it increases with temperature, and it decreases as some substance is dissolved in the water. For solids and liquids like ice and water, this quantity, this "chemical potential", is independent of the pressure unless the pressure is very high. For our purposes you can say pressure has no effect. That leaves only the temperature and the composition of the things involved (ice and water).

In place of the pure water, suppose we have some salt dissolved in the water.

ice(pure solid) = salt water (liquid solution)

The presence of the salt lowers the "chemical potential" of the water. But the "chemical potential" of the ice remains the same. If we were to hold the temperature of the ice and salt solution at 0 C., the ice would all melt because it has a higher "chemical potential" than the salt solution. Remember I said that the addition of salt reduces the "chemical potential" of the liquid water. If the pure ice is to remain in equilibrium with the salt solution, it must lose some of its "chemical potential". Since pressure has no effect (you have to trust me on that), and no salt can dissolve in the solid ice, the only way for the ice to lose "chemical potential" is for the temperature to be lowered. Remember, I said that "chemical potential" increases with increasing temperature. And that is what happens. The temperature of the ice = salt solution decreases until the "chemical potential" of the ice and NOW the salt solution become equal.

For water, this requires that the temperature drop of (delta T)= -1.86 * m, where m is the number of moles of ions (both Na+ and Cl- for salt) per 1000 gm. water.

I know this sounds pretty complicated, but your question is really not as simple as you might think and your asking it shows your perception and curiosity about what is happening. That is why I tried to give you more than just the "standard answer" which really isn't true.

Vince Calder


Good question!

When salt is added to ice, nothing happens at first. You need for a little of the ice to melt, so that some salt can dissolve in the meltwater to make saltwater.

When ice melts and no salt is around, ice and its meltwater are constantly exchanging. All the time, water molecules from the ice are detaching from the surface and diffusing into the liquid water, and water molecules from the liquid are attaching to the solid ice surface. If the ice is melting, the molecules detaching from the ice slightly outnumber the molecules attaching to it. If the water is freezing, the balance is slightly in the other direction.

Things get interesting when the liquid water is impure, for instance when it is saltwater. Then, the number of water molecules in a given volume of liquid is a little less than it would be in pure water. This means that fewer water molecules from the liquid phase will attach to the solid, because there are fewer of them around. Since the solid ice hasn't changed any, however, the rate of molecules detaching from the surface and going into the liquid phase also doesn't change. So, the rate of detachment falls below the rate of attachment, and overall the solid melts.

When water molecules attach to or detach from the ice surface, something else happens. It turns out that a water molecule in ice has a lower potential energy than a water molecule in liquid water. This is because it is surrounded by other water molecules in a very precise arrangement. When a water molecule leaves the ice surface, it has to pick up some energy to break out of its potential energy "hole." Going the other way, when a water molecule attaches to the ice surface from the liquid, it must give up some energy to allow it to stay in its new potential energy "hole."

Pure ice (no salt around) melts at a very specific temperature because at that temperature the kinetic energy of the molecules is great enough that they won't stay in the precise ice arrangement. When salt is added to the liquid phase, making it more difficult for water molecules to attach onto the solid ice, the water molecules that detach from the ice still need to pick up energy. They get this energy from their surroundings, which are the rest of the liquid and solid water. In other words, as the water molecules leave the ice and go into the water, they absorb energy from the ice and water, making it colder.

The ice will continue to melt, that is, the water moecules' detachment rate will be larger than the attachment rate, until the temperature is low enough that it's harder and harder for the water molecules in the ice to pick up the energy they need to detach. Finally, the attachment and detachment rates will be the same, so the energy flow will again be in balance. Then the temperature won't drop any further.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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