Melting Point of Ice????

[emdee3]

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Here's something that I don't quite understand. If the addition of ions increases the melting point of solids (because the number IMF's increases via ion-dipole interactions), then why does adding salt lower the melting point of ice? Can someone please explain this to me clearly?

 

[kraskadva]

Because additional ions disrupt the dipole-dipole interactions of water molecules, interfering with the crystal structure of ice, introducing additional entropy, which makes it melt faster/freeze slower.

 

[emdee3]

So what about the ion-dipole interactions between the Na+/Cl- ions and water molecules? I thought that increasing the strength of IMF's increases the melting point of a substance. Since ion-dipole interactions are stronger than hydrogen bonding wouldn't this mean that the melting point of ice should increase with the addition of salt?

 

[kraskadva]

So what about the ion-dipole interactions between the Na+/Cl- ions and water molecules? I thought that increasing the strength of IMF's increases the melting point of a substance. Since ion-dipole interactions are stronger than hydrogen bonding wouldn't this mean that the melting point of ice should increase with the addition of salt?

Because additional ions disrupt the dipole-dipole interactions of water molecules, interfering with the crystal structure of ice, introducing additional entropy, which makes it melt faster/freeze slower.

^ crystal beats weak attractions

also a tiny number time a bajillion is bigger than a slightly larger tiny number. In any amount of water there are many orders of magnitude more hydrogen bonds than any number of ion dipole interactions you could introduce. So mass effect still wins.