Yttrium2
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What causes salt to melt ice?
I've heard something about the equilibrium
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gdflp
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Not all salts will melt ice, some will actually cause ice which is starting to melt to freeze. In general though, most salts will melt ice and the
few which won't have a highly endothermic enthalpy of dissolution. Ignoring the enthalpy of dissolution, a salt(say calcium chloride) will melt ice
due to a property called freezing point depression. This is a property of solvents that when a solute is dissolved in them, the freezing point is lowered by a certain
temperature determined from the following equation : delta T = -i*m*Kf where i is the Van't Hoff factor, or how many ions the solute
dissociates into, m is the molality of the solution, and Kf is known as the freezing point constant, or the cryoscopic constant, which is
unique to each solvent. In addition, some salts will dissolve in water exothermically, this helps to melt ice faster as the produced heat melts more
ice, which allows more salt to dissolve, which produces more heat, etc. in a chain reaction.
[Edited on 2-7-2015 by gdflp]
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Texium
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Thread Split
29-12-2015 at 17:37 |
SplendidAcylation
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I was just scrolling through Ytrrium2's post history, and I found this one interesting because it raises a question whose answer is perhaps obvious to
many...
Does salt melt ice in the absence of liquid water?
I can't see how it would, the salt is held in a tight ionic lattice and the water is held together by hydrogen bonds etc, I don't see how the ions
could possibly insert themselves into the structure of the water.
I can see, however, how a small quantity of water might start the process off.
I understand the concept of freezing-point depression in terms of Gibbs free energy:
deltaG = deltaH - TdeltaS
If we talk about the process of melting ice, the process is endothermic, so deltaH is positive, and entropy increases, so deltaS is also positive.
If we assign arbitrary values to these, for ease of calculation, let deltaH equal +100 (for melting) and deltaS equal +4:
deltaG = 100 -4T
In order for this change to be feasible (i.e. deltaG <=0), T must be >=25.
In other words you need a temperature of 25 to melt it.
In terms of freezing, the signs of deltaH and deltaS are changed, so deltaH = -100 and deltaS = -4 (as the process is now exothermic, with a decrease
in entropy), substituting these into the equation:
deltaG = -100 - -4T
deltaG = -100 + 4T
deltaG = 4T - 100
In order for deltaG<=0, T must be 25 or lower, i.e. the temperature must lowered to 25 in order to freeze it.
If, however, you add salt, you increase the entropy of the system, so deltaS increases (in the case of melting) and decreases (in the case of
freezing), so if you change deltaS to an arbitrary value of 10:
deltaG = -100 - -10T
...
deltaG = 10T - 100
In order for deltaG <=0, T must be <=10, i.e. the temperature must be 10 to freeze it, hence the freezing point has dropped from 25 to 10, thus,
freezing-point depression.
In a more intuitive explanation, you can imagine that, when water contains ions of a salt, the ions "get in the way" of the water molecules,
diminishing the effect of the intermolecular forces, and therefore requiring a lower temperature to make (or break) the bonds...
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Texium
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Thread Moved
9-2-2023 at 06:14 |
Mateo_swe
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Hmm, i have been thinking about this salt/ice mix.
We all know the temperature goes down if adding tablesalt to an ice/waterbath, but what would happen if surrounding temperature was, say -10°C and
you got a bowl and put some icecubes in it, then adds some common table salt on top of the ice cubes.
In this situation does the salt alter the temperature?
As salt cannot mix with any water (its already solid ice at -10°C) so the temp should not change in the ice/salt mix, or does it?
If surrounding temp increase above 0°C the icecubes will start to melt a little releasing liquid water that will then mix with the salt and this
solution will have lower freezingpoint (stay liquid) and also lower the temperature below 0°C and even below -10°C.
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Texium
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Why don’t you test it out? All you need is water, salt, and a freezer.
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DraconicAcid
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If solid salt contacts solid ice, they will react at the point of contact- water molecules at the interface will coordinate to ions at the interface.
Using the term "react" loosely, of course- thermodynamicists have a great deal of apathy towards the distinction between a chemical reaction and a
physical change, as long as they can measure their enthalpy changes.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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