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I've seen it many times... Why does soap become foam-like when it reacts with water (moisture) or when we touch it with wet hands.. What compound actually causes this physical change? And why does this physical change occur?

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  • $\begingroup$ So you're thinking of solid 'bar' soap? $\endgroup$ – Aesin Aug 9 '12 at 10:46
  • $\begingroup$ yeah,..! Why are there any other different species in your mind.>? $\endgroup$ – user491 Aug 9 '12 at 11:09
  • $\begingroup$ Sorry, what I meant is that I guess you're in particular thinking about the physical change of the properties of solid soap when you get water on it. $\endgroup$ – Aesin Aug 9 '12 at 11:34
  • $\begingroup$ Yeah man.. I mentioned the word 'physical change' in the question $\endgroup$ – user491 Aug 9 '12 at 11:37
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The simplest kinds of soap are just a fatty acid chain with something more ionic on the end - something like the ubiquitous sodium laureth sulphate:

Sodium laureth sulphate

When it's in bar form, it behaves a bit like a solid block of plastic, because chemically speaking, those long chains of carbon are quite a lot like a common plastic such as polythene. These chains attract each other and repel and exclude water, keeping the block together. The sulphate group on the end is hydrophilic, though - it's chemically stable surrounded by water. This means that when you add water, it infiltrates and teases apart the structure of strands, getting to the ionic ends on each chain and forming a "solvation shell" of waters around each one. There's no real chemical reaction here - as you say, it's a physical process, which is why rubbing the soap helps - you're applying shear to the structure and helping to separate the strands.

Once they're separated from the bulk, the soap forms tiny bubbles called "micelles", which have the chains pointed inwards towards each other and the ionic sulphate groups meeting the water at the outside.

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While this question should get a more elaborate answer, mine will at least give you leads.

Soaps act as surfactants, that is, they are amphiphilic (both hydrophobic and hydrophilic) molecules that lower the surface tension of a liquid.

By lowering the surface tension of water, soap bubbles can form, the surface becoming more "elastic" - the molecules at the interface experience less inward/normal force than they would were surfactant not present. On the contrary, adding soap to a pond would prevent water striders from gliding on the surface of the pond.

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  • $\begingroup$ Thanks. But I didn't ask about surface tension. Its in my 10th grade syllabus. I'm asking 'why is this physical change?' Please read the question and its not necessary for ponds & surface tension $\endgroup$ – user491 Aug 8 '12 at 13:07
  • $\begingroup$ @CrazyBuddy Then your question is ambiguous. The question you ask in the main post and the one you ask in the comment are different. Consider clearing up your question. $\endgroup$ – CHM Aug 8 '12 at 15:31
  • $\begingroup$ One thing that should be made clear is the question you want to ask, and also what you mean by "solid-like". $\endgroup$ – CHM Aug 8 '12 at 15:32
  • $\begingroup$ sorry for that.. I told solid-like 'cause i thought soap as something like colloid due to its appearance $\endgroup$ – user491 Aug 8 '12 at 16:17
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The soap dissolves in the water. Dissolution is not a chemical change because the same chemicals are present, they just interact with each other differently. Like CHM said, soaps act as surfactants and the difference in surface tension improves the survivability of bubbles, thus creating a "foam".

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  • $\begingroup$ There. Is. No. Chemical. Change. We have all been trying to tell you this. We've been telling you about physical changes and you've been ignoring us. $\endgroup$ – Wegko Aug 10 '12 at 15:17
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    $\begingroup$ We're talking about physical changes and you keep going "no I mean the physical change man". $\endgroup$ – Wegko Aug 10 '12 at 15:23