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Name: Rabab
Status: student
Grade: 9-12
Country: Canada
Date:  Fall 2011  

Question:
Hi, I have an assignment for applications of intermolecular forces and I've chosen to research up on road salt. I was just wondering, when salt is added to water, do the intermolecular forces weaken or strengthen, and how would I be able to explain this? From my point of view, the forces would strengthen because it becomes harder to separate the molecules since the boiling point increases. Also, how would i be able to explain the increase or decrease from the view of melting and freezing points?

Replies:
You have to explain why the liquid phase is stabilized at the expense of vapor and solid. You may need to invoke different mechanisms for the different cases.

You may also want to approach of from the perspective of what the water molecules actually do when they freeze to ice.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Rabab,

You are essentially correct that the fact that boiling point increases can be related to an increased interaction between ions and water versus that of water alone. We do know that ion-dipole interactions are stronger than hydrogen bonding. However, and I would ask you to read up on "colligative properties" - a stronger intermolecular attraction between solute and solvent is not necessary in order for boiling point to elevate. Boiling point will always go up with the addition of a solute for as long as the solute has no vapor pressure regardless of whether the solute-solvent interaction is stronger than the solvent-solvent interaction.

Read up on "colligative properties".

Greg (Roberto Gregorius) Canisius College


The sodium ions have a full positive charge. The chloride ions have a full negative charge. In that sense, the ions should have a greater attractive force for the partially positive and negative ends of a polar water molecule than water molecules would have for themselves.

You need to be careful, however, of how you explain this. You referred to the boiling and freezing temperatures. Well, one of the reasons that salt water has a higher boiling point and a lower freezing point has to do with the disruptions of the formation of a crystal structure in ice and the interference with the evolution of water molecules.

Some years ago, one of my students did some research and found that water with salt dissolved in it expands less than pure water when they both freeze. The model for this behavior is the disruption of the hydrogen bonded crystal structure of ice. Hydrogen bonded water molecules must be further apart than those water molecules that simply bump into each other with an orientation that does not allow for hydrogen bonding.

Ray Tedder


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