Science Buddies: "Ask an Expert"

Measuring the Surface Tension of Water: How do you build the model for measuring surface tension using 2 books as support, eyedroppers as weights, pin as fulcrum, and a ruler as a beam? The direction/ procedure and picture confused me!

Hello sciencekids4,

The project description has fairly complete instructions. I will need to have a better description of what you don't understand in order to help you.

Figure 1 shows the balance that is described in the test. The two tall rectangular blocks are the books or whatever blocks you use for support. The cylinder running left to right is the soda straw used for a beam. The right end of the straw is connected by thread to a needle in contact with the water, and a pan to hold weights is shown at the left end.

Please write back with a more specific question.

WW

I don't quite understand how to measure the surface tension of water. Please dumb it down for me.

I also dont understand how the needle has to do with surface tension either.

sciencekids4,

The project description says, "You will add small amounts of weight to the pan, and measure the force needed to pull the needle (or wire) free from the surface of the water. "

That's your measurement: the force needed to pull the needle free from the surface of the water. Surface tension means the force water exerts on something at its surface.

The needle is the object used to contact the water. You can use other objects. You might try objects to which water sticks well and objects to which it doesn't stick (wax, for example) and measure the difference in surface tension.

Be sure to read the project description and understand how the surface area of the object influences the contact force.

WW

how do you measure the force (in newtons)?

The Newton scientific unit definition is based on the relationship that in earth's gravity, 1 kgf (kilogram force) = 9.80665 N, or 3 significant digit approximations are 1gf = 0.00981 N, or 1mgf = 0.00000981, or 1ugf = 0.00981 uN.

So you weigh the weights needed to break the surface tension. If you do the measurement in grams, you multiply by .00980665 to get Newtons. For surface tension measurements, you are more likely to be in the micro gram range (ug) so multiplying by 0.00981 will give you micro Newtons (uN).

thanks!