Soap Expansion, Then Contraction

name         Jessica
status       other
grade        9-12
location     Canada

Question -    Hello,
I am doing a project on soap! I placed four different brands of 
soap into fresh tap water at room temperature and left them there 
for four weeks taking each bar out every other day to weigh. I 
hoped this would determine which brand of soap is the longest 
lasting in regards to its dissolution. To my surprise each bar 
had a similar pattern with its weight and first expanded growing 
in size and weight and then slowly dissolved. Can you please 
explain why the bars all expanded ?
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The initial process is the absorption of water. The soap bar acts a 
bit like a sponge. Then the fats (which are a major component in 
bath soaps) dissolve which results in a loss of both weight and 
volume. Both processes are sensitive to stirring. To compare the 
dissolution of the soap bars you might try letting each one "dry" by 
placing them on a paper towel every other day, when you compare 
their weights. This will cancel out the water retained, but I would 
not be surprised if you found that they all "dissolved" at pretty 
much the same rate because the major components of the soap are very 
similar substances.

Vince Calder
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