Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: npeters on April 28, 2009, 02:14:58 PM
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I just read an article at this address
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml
and I am now thoroughly confused.
I understand that adding salt to an ice/water mix will lower the freezing point, thereby making the ice melt. Melting ice absorbs heat from the surroundings, thereby lowering the temperature of the solution, in turn slowing the melting of the ice. The ice/water mixture without salt will also be melting and thereby lowering its temperature. In addition, as this pure ice/water solution gets colder, the ice in it will refreeze more easily than in the salt/ice/water solution because in the pure mixture there are no solute particles to interfere with refreezing. Thus, the salt/ice/water solution should have a greater rate of melting compared to refreezing than the pure solution.
Why on earth would the ice in the salt/ice/water solution melt more slowly?
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Look into Colligative Properties (http://en.wikipedia.org/wiki/Colligative_properties)
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I have looked into colligative properties. Based on what I read, ice melting slower in salt water just doesn't make sense. That's why I posted to begin with.
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I may be wrong here:
Adding salt to the water will lower its freezing temperature. Therefore, the water will be liquid at a lower temperature.
We can agree that water at a temperature of 40 degrees centigrade will melt ice slower than water at a temperature of 50 degrees centigrade.
It probably depends on your setup, but if you had enough ice in the water, and added salt, the water temperature would be able to get colder without freezing.. while the water that is locked up in the ice cubes will have no salinity, and a normal melting point.
Something like that?
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Consider the following:
1.) Ice floats in water, due to lower density of the ice. A salt water solution will have a higher density. A pure ice cube should float more in a salt water solution. This could change the surface area of the ice exposed to the salt water.
2.) Is the salt solution being mixed? The melting ice will form a concentration gradient.
3.) What is the quantity of ice compared to quantity of salt solution?
4.) How much salt is in solution?
5.) Will the salt solutions temperature drop to a point, where salt will come out of solution?
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Dear npeters;
IMHO: I agree with you that slower melting ice doesn’t make any sense to me too.
Why?:
The force that controls the speed for melting is the energy-flux from surrounding to the ice/water mixture. And the measure of this flux is the Temperature Difference/Gradient (TDG) between surrounding and the ice-mixture, at least as long as we hold the active surface constand (a little more precise: dE/dt da = k * dT/dt * A).
Conclusion:
The DTG for ice/water/salt (25°C - -21°C) is nearly twice as much as for only ice/water (25°C – 0°C), and that means, that the ice in ice/water/salt must melt faster and not slower.
And that’s also what my experience tell me.
(IMHO: The opposite is against second thermodynamic law)
Good Luck!
ARGOS++
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1.) Ice floats in water, due to lower density of the ice. A salt water solution will have a higher density. A pure ice cube should float more in a salt water solution. This could change the surface area of the ice exposed to the salt water.
I bet that's it. It would be interesting to observe the melting rates if both cubes were kept under water the whole time