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Some gentle help with Soap questions... O Chem. Thanks


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#1 greengoat

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Posted 8 February 2009 - 09:48 PM

I am struggling with fully answering two questions that are fairly similar.

1. Why should the potassium salts of fatty acids yield soft soaps? - I know that sodium yields what is considered as hard soap and potassium yields soft soaps. I also know that potassium is more soluble in water than sodium, but is this in fact that entire answer?

2. Why is soap derived from coconut oil so soft? Again I realize that water solubility is high here but I'm just not confident in understanding why.

3. Why does adding a salt solution cause soap to precipitate? Here is my answer for this:
It is a general rule that a fatty acid salt (soap) becomes less soluble as a salt is added to solution. The soap will precipitate because the decrease in solubility. The loss in solubility is because the positively charged ion from the salt will attach or attract to the negative head group of the salt and will precipitate out of solution. Under normal conditions, the negatively charged head group would be attracted to the polar water molecule and dissolve in water, but this canít happen if the positive ion (say Ca¨2+) has already attached.

Thanks everybody I'm SO glad I found this place. Our school has no tutors and our teachers are rarely available. THANKS!
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#2 hermanntrude

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Posted 9 February 2009 - 04:17 PM

I suspect (although i'm not sure) that your ideas about parts and 2 are correct and probably at least part of the answer. Part 3 would probably be helped if you looked up the "common ion effect"
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#3 John Cuthber

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Posted 9 February 2009 - 08:38 PM

You might want to look at the fatty acids contained in coconut oil and, for exaple, tallow or olive oil.
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