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Name: Rachel
Status: student
Grade: N/A
Location: N/A
Country: N/A
Date: 4/29/2005


Question:
I am a grade 12 student working on a chemistry project on the surface tension of water. I performed a lab measuring the surface tension of water on a base solution, an acid solution and an alcohol solution. My results were the alcohol increased the surface tension while the acid and base slightly lowered it, do these results make sense? I have read on the site that soap will lower the surface tension. Does this mean that surface tension refers only to the hydrogen bonding and not how much the solution can stretch? Also, what effect does air pressure have on surface tension?


Replies:
Not sure, Rachel.

Alcohol is a bit like soap; I think it lowers the surface tension. Acid and base I do not know; you will have to correct your measurement technique until soap reads "lower", and check whether alcohol goes in the same direction, and then measure acids and bases.

There is no "limit" to how far water (or any liquid) _can_ stretch. It can stretch almost forever, until it is only 1 molecule thick. Surface tension means: when you do stretch it, how hard does it try to pull back?

All liquids have surface tension. Any substance that likes itself, and holds itself together, has a "surface energy". In liquids, increasing energy is free to create a pulling force, a tension. Hydrogen bonding is not required, it is just another means of self-cohesion. When it is absent, there are a couple of weaker forces providing some self-cohesion. Polar Attractions, and Van-Der-Walls forces. Water does have higher surface tension than most other liquids. Try to look up the numbers of several; it would give you some perspective. You could include weird extreme liquids like liquid helium and mercury and melted iron.

Air pressure has little or no effect on surface tension. Whatever the pressure is outside the drop because of the air, inside the drop has the same pressure plus a little bit more added by the surface tension. Actually it is proportional to the surface tension divided by the radius of the drop. Smaller drops have higher pressure inside than large drops. Same with bubbles! That is one way surface tension is often measured.

I would have enjoyed hearing how you are measuring it...

Jim Swenson


The best way for me to answer your questions is to explain surface tension. You are correct in saying that surface tension has to do with hydrogen bonding. It is not talking about how much a solution will stretch. Your results are correct, but here is an explanation for you as to why they turned out that way. The base decreased the surface tension because it added more OH- to the solution, pulling the hydrogen molecules in water toward itself. This would stop those hydrogen molecules from forming hydrogen bonds to other hydrogen molecules. The acid would work in a similar way, except the acid (H+) would draw the oxygen in the water molecule towards itself. It short, both the acid and the base "stool the attention" of part of the water molecule so it could not be attracted to the other water molecules to form hydrogen bonds. Alcohols contain carbon atoms and an OH- group in place of a Hydrogen atom. This makes alcohols nonpolar and hydrophilic. The water squeezes these non-polar compounds together and will not be attracted to them. Because of this, the hydrogen bonds would still occur since the alcohol did not "get the water molecules attention." I hope this helps. If you are still confused, remember to go over what acids and bases are and recap on polar/non-polar, hydrophobic/hydrophilic.

Grace Fields



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