Maddenation

Why is the sky blue? and nine other important science questions

The Minneapolis Star Tribune asked ten prominent scientists what science question should every high school graduate know the answer to. They came up with some decent questions, but not all of them. And in some cases, the answers provided (by the columnist? the scientists?) were terrible. For instance:

Q. 10. Why do we put salt on sidewalks when it snows?

A. 10. Adding salt to snow or ice increases the number of molecules on the ground surface and makes it harder for the water to freeze. Salt can lower freezing temperatures on sidewalks to 15 degrees from 32 degrees.

Whuh? Increases the number of molecules on the ground surface, and this magically makes it harder for the water to freeze? First of all, that’s stupid. Second of all, even if it were correct, you’ve not answered the question except in a tautological way: “We put salt on the sidewalks to help prevent ice.” Is this the great scientific answer every high school grad should know?

I’m not the only one with doubts about the survey. Mark Dominus goes into greater detail about his problems with the survey, and I believe him.

PatrickQuestions04/26/06 1 comments

Comments

Dad • 04/27/06 6:12 PM:

I don’t know who Mark Dominus is, but I pretty much agree with his assessment. His answer on question 10 wasn’t that good, however. I’d have to look it up in textbooks to really explain it, but basically, salt depresses the freezing point because water has to kick out the salt when it freezes. Adding salt makes it easier for water to stay in the liquid state, because the salt ionizes into Na+ and Cl- ions which like to disperse in the polar (not referring to the cold weather but to the uneven charge distribution) water molecules.

While it’s in the process of dissolving, the solid salt acts as grit to offset the slipperiness of the ice. I think the salt also heats the water up a little bit while dissolving, so this will also help melt the ice. The salt can’t stay as a solid on top of the solid ice because its “chemical potential” (Free Energy) produces a driving force to enter the dissolved state.

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