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Name: Unknown
Status: educator
Grade: 6-8
Location: N/A
Country: Canada
Date: N/A

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.


Bob Wilson.

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 as possible.

Your students can compare the viscosity of various liquids and solutions using this simple apparatus.

Vince Calder

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