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Q & A: Baloney with Saltwater

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Most recent answer: 01/19/2011
Q:
http://www.physlink.com/Education/AskExperts/ae643.cfm
Our first Baloney example (an answer with five major errors)comes from a generally good site which happened to slip pretty badly on a few questions. The site administrator was informed of the difficulties with the answer some weeks ago. More baloney will follow. We invite you to send in examples. The Web is a target-rich environment. "Question What does salt do to water that raises the boiling temperature? Is it a chemical reaction?"
- Mike W. (age 61)
A:
From the posted answer:

1. "...it is a conservation of momentum. "
Conservation of momentum has nothing to do with this question. Since there are external forces (e.g. with a pot holding the water) momentum wouldn't have to be conserved. At any rate, in this case there is no change in momentum as the water boils, since the molecules in either the liquid or the gas are randomly heading all directions, giving zero average momentum.

 2. "...for something to boil, enough energy must be absorbed by it to cause vibrations large enough to enhance the kinetic energy of each molecule to the point where they break away..."  "...the energy goes into not only exciting each water molecule to a higher kinetic energy but also each salt molecule..."
The whole issue of the solute soaking up energy is irrelevant, since the question is not how much energy is required to boil the water but rather what the boiling temperature is. If the solute did soak up some energy, that would temporarily lower the temperature, not change the boiling point.

3. "...more massive salt molecules themselves need a larger contribution of energy..." Although that whole issue is irrelevant, it's also handled wrong. More massive particles do not need more energy to be at a given temperature. In fact, the equipartition theorem says that they have the same kinetic energies as lighter particles at a given temperature.

4. The freezing point will be lowered by "...even better, calcium chloride, an even heavier compound..." Not that it's relevant, but it's not even true that dissolving salts in water consistently soaks up energy (enthalpy, to be precise). Some salts release enthalpy (exothermic), some soak it up (endothermic). Oddly enough, the particular salt mentioned (CaCl2) is one of the exothermic ones, releasing heat as it dissolves.

5. "...even better, calcium chloride, an even heavier compound..."  It's not close to being true that more massive ions raise the boiling point more than less massive ones. In fact, there's a pretty good rule of thumb that the boiling point effect depends only on the number of solute particles, not their detailed properties, and certainly not particularly on the mass. The rule becomes exact for dilute solutes.

The real physical chemistry involves a fundamental thermodynamic argument that any stable solute will raise the boiling point, regardless of microscopic details. At the microscopic level, the reason that most solutes have about the same effect per solute particle is that the main effect usually has to do with how the entropy of the solute goes down as the solvent evaporates, not the detailed and variable ways in which the energies change. A search of our site will turn up some info on this issue.
e.g.



Mike W.


(published on 01/19/2011)

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