Re: why does soap make water less buoyant (dense)?

Dear Mike,

Sorry for the delay in answering your interesting question.  It prompted 
me to perform my own experiment.  I prepared 10% weight/volume solutions 
of NaCl (table salt), NaHCO3 (sodium bicarbonate), and liquid dishwashing 
detergent.  I then weighed 100 ml. of each solution in a 100 ml. 
volumentric flask so I could calculate the denisty values for each 
solution.  I also checked regular tap water and looked up the published 
density value for water at the temperature of the test.  Following are my 
results:

SOLUTION            DENSITY        REFERENCE
Tap Water at 86 F.  0.995 gm/cc    0.995 gm/cc (CRC Handbook)
10% NaCl            1.063 gm/cc
10% NaHCO3          1.064 gm/cc
10% Detergent       0.999 gm/cc

The greater the denisty, the greater the buoyancy and the greater weight 
should be supported by floating.  From the above data, the pennies 
floating on the cut base of a Dixie cup should sink first in Tap Water, 
not in the detergent solution.  One other factor is involved.  The surface 
tension of the water makes a type of "skin" on the surface of plain 
water.  Detergent greatly reduces this surface tension and reduces the 
floating capacity of the solution.  You did not say if you weighed the 
pennnies and how much more weight could be floated by the Tap Water.  I 
hope this helps.