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Melting Point of Ice As It Relates to Intermolecular Forces

Hi, I was taught that increasing the strength of intermolecular forces also increases the melting point of a solid. If this is true, then why does salt decrease the melting point of ice? I know that salt will dissociate into Na+ and Cl- ions and that these ions will form ion-dipole interactions with water. Moreover, ion-dipole interactions are stronger than hydrogen bonding so should this not increase the melting point of ice?

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Contributor

Re: Melting Point of Ice As It Relates to Intermolecular Forces

Freezing-point depression - Wikipedia

It’s an entropy effect:

“The explanation for the freezing point depression is then simply that as solvent molecules leave the liquid and join the solid, they leave behind a smaller volume of liquid in which the solute particles can roam. The resulting reduced entropy of the solute particles thus is independent of their properties. This approximation ceases to hold when the concentration becomes large enough for solute-solute interactions to become important.”

New Contributor II

Re: Melting Point of Ice As It Relates to Intermolecular Forces

Oversimplification is fine when teaching basic concepts, especially where the audience is not prone to critical thought, but you seem to be off on a tangent that is only peripherally related to the basic question at hand. Colligative properties are not dependent upon the properties of the solute, only it's quantity, as molality. In fact, the only manner that the dissociation of salt into sodium and chloride ions affects the freezing point is to double the depression by providing double the moles of solute.

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Contributor

Re: Melting Point of Ice As It Relates to Intermolecular Forces

Stephen,

I find the tone of your comments offensive.  They also add nothing to the information in Wikipedia.  I cited Wikipedia (a generally reliable source) to make the point that "colligative properties" are an entropic effect, not dependent on the strength of the solute interactions with the solvent.  In dilute solutions, they are (as you suggest) dependent only on molal concentration.

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