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The NaCl "salts out" the glycerine in crude soap leaving behing hard soap at the surface of the water. I believe that NaCl increases the ionic character of water which drives the dense non-polar glycerine molecules to the bottom of the water. The soap molecules, being polar and less dense, will then float to the surface for extraction.

 

And no, I don't believe that the K+ or Na+ cations dissociated in water though the soap molecule itself is soluble in water.

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I actually don't know of any reaction between glycerine and salt. When the fat and lye are mixed together, the soap forms as the lye saponifies the fat. This leaves you with pure glycerine and sodium stereate chains. The sodium sterate is VERY soluble in water. If it wasn't, then the soap would be useless.

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actualy the addition of NaCl to soap in manufactiring is quite common, I`m not entirely sure of the purpose, but it IS to do with ions, and Hard and Soft water, something to do with the Lathering.

and don`t ask, I don`t know why, I just know that in all the recipes that I`ve seen/used call for it :)

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Also, the strength of the alkali hydroxide determines what soap you make. LiOH doesn't really produce a clasical soap. The resulting product of mixing fats with LiOH is more of a viscous oil that isn't all too soluble in water. NaOH results in the formation of a solid soap. KOH creates a liquid soap. RbOH and CsOH don't really form soaps, if I recall correctly, as they break down the fat to a much greater extent.

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I have made soap with KOH and i can come out to be hard if you let it sit for a while.

 

 

That's because you most likely didn't finish making it. ;) When you've made soap with KOH, it will gel up into a very thick paste as it cools down. After it has cooled, you then heat it up again and add some water to the heated soap. Mix it up and you then have a liquid soap that won't solidify. The KOH based soap is much more soluble in water because the fat is broken down to a much greater extent. As a result, it can be dissolved into water much easier because the fat chains are shorter which results in a much more polar molecule.

 

With NaOH based soaps, the solubility isn't nearly there and you won't get a liquid soap. If you heat it up and add water and mix it together, once it cools down it will be a solid again. That's not the case with KOH soaps.

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Like I said, the NaCl in creases the ionic character of the water. Glycerol and soap both have an affinity for water, which is polar but not ionic. This allows glycerol, which is pretty much non-polar, to mix with the water along with the soap. You usually don't want glycerol in your soap, however, so you add the salt to separate the two. By increasing the ionic character of the water, the non-polar glycerol will have less affinity for it and will then be driven away from the ionic layer. The salt also removes water from any solvated glycerol which then forms a layer of high purity glycerol beneath a layer of salt water and soap, which, at this point, still contains many impurities.

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Glycerol is VERY polar. It has three -OH groups, one on each of the carbons. It is incredibly polar and VERY miscible in water. Glycerol has such a strong affinity to water that if you leave it out it will actually absorb water from the air. There is nothing about it that doesn't cry out 'polar compound miscible with water'.

 

Also, the addition of a salt does nothing to affect a substances solubility in water. If something will mix with water, it will mix with water regardless of whether or not there is salt in there. The salt and the ions aren't what's responsible for the dissolution of another substance.

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it IS important in the lathering process tho.

take a bar of ordinary soap and wash your hands with it, note the lathering.

add some calcium hydroxide to a bucket of water and do the same again (little to no lather).

add NaCl to your hand and try once more, the lather comes back :)

 

it`s all about Hard and Soft water, that`s all I rem :)

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Also' date=' the addition of a salt does nothing to affect a substances solubility in water. If something will mix with water, it will mix with water regardless of whether or not there is salt in there. The salt and the ions aren't what's responsible for the dissolution of another substance.[/quote']

 

Addition of one substance always affects solubility of other substances. What is soluble stays soluble but amounts may change dramatically. Take for example mixture of acetone and water. If you add table salt to it and mix well it separates into two layers upper being mostly acetone and lower mostly water + NaCl.

 

This applies to complex salt solutions as well. For example solubility of KNO3 in NH4Cl solution is not the same as in distilled water and solubility of CaSO4 in water increases quite a lot if some NH4Cl is added.

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A lot of that has to with side reactions going on. My statement is just countering the one poster's belief that adding salt to a mixture of two 100% miscible liquids will result in the separation of the substances. That's just not true.

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