Soap and Viscosity of Water
How does soap affect the viscosity of water?
In fact, soap does not affect the viscosity of water in any
way. What soap (or detergents etc.) do is to reduce the
surface tension of water. This allows the water to spread
more easily over a surface, and to reduce or eliminate the
tendency for the water to bead up on a surface. But the
viscosity of the water itself is unaffected. This can easily
be proven by timing low long it takes for a measured amount
of water to flow by gravity through an orifice. Then compare
with a water soap solution. You will see that there will be
essentially no difference between the two situations. Since
viscosity is the main effect that limits the above flow rate,
and since adding soap to the water does not increase the flow
rate, it is clear that there is no difference in viscosity
between water, and soapy water.
In dilute solution soaps do not change the viscosity of
water (that is, its resistance to macroscopic flow). At high
concentration, such as in dish soap, liquid hand cleaners etc. that
is no longer the case. You can observe that increase in viscosity
by the syrup-like flow of such liquids.
In contrast, soaps do reduce the surface tension (or a
better term: surface energy) of water, which allows the water to
better "wet" the surface it might be on. Under certain conditions,
this "wetting" property can be confused with a decrease in
"viscosity" because "wetting" the surface makes it easier for the
water to flow across the surface.
A simple way to measure one type of viscosity is to take a
small can, say a 4 oz. tomato paste can, and carefully punch (or
drill) a small hole (1 to 1.5 mm dia., you will have to experiment)
in the bottom of the can. Fill the can to the brim (or some other
mark on the can wall, holding your finger over the hole so that no
water drains out.
Remove your finger and measure the time it takes for the can to
drain using a stopwatch. The number of seconds it takes for the can
to drain due to the force of gravity is proportional to the
viscosity, but is independent of the surface tension. Without some
more complicated physics, it does not measure the viscosity directly
but gives accurate comparisons. If you want to get fancier, make a
set of cans of the same size with different diameter holes. It is
important to be sure the hole has no jagged edges and is as circular
Your students can compare the viscosity of various liquids
and solutions using this simple apparatus.
Update: June 2012