It’s supposedly the same soap through and through, all the way to the core of the bar, right? Why does it react so differently to water and being rubbed on skin when it is reduced to a sliver?
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Soap maker here: the surface thing explained above is part of the answer but that’s not the main reason.
You are probably talking about commercial (industrial) soap. To reduce the cost of production, an industrial bar of soap mimic the behavior of natural soap using chemicals. For example, they remove glycerine, a natural moisturizer resulting naturally in the saponification reaction to sell it separately. They replace it with cheap chemical moisturizing agents. To keep the whole thing looking like soap (😂) they add more chemicals, preservatives… Each chemical has a role to look like that old fashion natural soap bar. So when it becomes older, its chemical composition becomes a little unbalanced. Why less lather? probably because sulfactants used to artificially create lather are less concentrated. By contrast, a soap molecule, the natural one, the one you can make at home with some oils and some lye, is uniform and stable, it’s one molecule and its behavior won’t change.
Next time you buy a bar of soap from the supermarket, before throwing the package, google that Exxx stuff in the components list 😉
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