Why does adding salt lower the freezing point of water?

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tammie94
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Why does adding salt lower the freezing point of water? I read that salt doesn't stick as well in ice which is one of the reasons, but why does this happen?
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username913907
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A proper explanation requires the use of chemical potentials... Are you actually an undergraduate student? I ask because so many A-level students put undergrad because they think their problem is just hard.
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jacksonmeg
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(Original post by tammie94)
Why does adding salt lower the freezing point of water? I read that salt doesn't stick as well in ice which is one of the reasons, but why does this happen?
I would of said that the ions stop hydrogen bonds forming between water molecules or something like that. I'm only doing a level though
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charco
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(Original post by JMaydom)
A proper explanation requires the use of chemical potentials... Are you actually an undergraduate student? I ask because so many A-level students put undergrad because they think their problem is just hard.
Please elaborate.

I would have said that the hydrolysed ions prevent the hydrogen bonding lattice from forming.
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acciolucy
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I'm only A level but doesn't the salt ions prevent the lattice forming as they disrupt the uniform structure, preventing it from forming at that temperature. It also means that not all bonds are hydrogen bonds so they are easily overcome, meaning it doesn't freeze. When it's colder the forces are effectively stronger allowing ice to be formed.
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username913907
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(Original post by charco)
Please elaborate.

I would have said that the hydrolysed ions prevent the hydrogen bonding lattice from forming.
Yes that works as a qualitative answer (not saying it wrong, it isn't) but for a qualitative answer consideration of chemical potentials explains the depression freezing points and elevation of boiling points.
Comes about from considering this system. Obv a substance boils when the chemical potential is the same as the chemical potential of the vapour phase. Adding a solute lowers the chemical potential (see the equation for the chemical potential of a solution) of the liquid phase and hence a higher temperature is needed to boil the solvent.
Equally, the lower chemical potential of the solution means the solution needs to be cooled more than pure solvent as the boundry between the solid and liquid phase happens at a lower temp.
Afraid I don't have my lecture notes atm so I can't dig up the actual derivation but it's explained fairly well here (with a nice graph!)
http://www1.lsbu.ac.uk/water/collig.html
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tammie94
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(Original post by charco)
Please elaborate.

I would have said that the hydrolysed ions prevent the hydrogen bonding lattice from forming.
And is the reason why ice can form at lower temperatures than 0 degrees, is because the molecules have less kinetic energy so the hydrogen bonds are more likely to form? Is this right? Or are the hydrogen bonds stronger or something?
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