Abstract

Objective

The goal of this project is to use a homemade single-beam balance to directly measure the surface tension of a liquid.

Introduction

You've seen examples of surface tension in action: water striders walking on water, soap bubbles, or perhaps water creeping up inside a thin tube. What, exactly, is surface tension?

Surface tension is defined as the amount of energy required to increase the surface area of a liquid by a unit amount. So the units can be expressed in joules per square meter (J/m²). You can also think of it as a force per unit length, pulling on an object (Mellendorf, 2002). In this case, the units would be in newtons/meter (N/m). Since the forces are so small, you often see surface tension expressed in millinewtons per meter (mN/m — 1 mN is 1/1000 N). It's a good exercise to do the dimensional analysis and prove that both ways of expressing surface tension—J/m² and N/m—are equivalent. If you need a refresher on your units of energy and force, there is a good reference in the Bibliography.

The force arises from the mutual attraction between the molecules of the liquid. Do background research on the chemistry of water to learn more about its intermolecular attractions. In particular, you should study up on hydrogen bonding.

In this experiment, you will be making and using a single beam balance to measure the force exerted by surface tension on a needle, floating on the surface of the water. The needle will be attached to your balance, and you will measure how much force is required to pull the needle out of the water. The surface tension of the water is providing the resistance. From your measurements, you will be able to calculate the surface tension of water.

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• surface tension,
• water molecules,
• hydrogen bonding of water molecules,
• detergent,
• force.

Questions:

• Considering what you have learned about hydrogen bonding in your background research:
  • will adding detergent to water increase or decrease the surface tension?
  • will adding rubbing alcohol to water increase or decrease the surface tension?

Bibliography

• This is an archive of a physics discussion board with information on measuring surface tension:
  Calder, V. and D. Plano, 2002. "Measuring Surface Tension." Ask a Scientist Physics Archive, University of Chicago, Newton BBS. http://www.newton.dep.anl.gov/askasci/phy00/phy00455.htm
• This is an archive of a physics discussion board with information on surface tension units:
  Calder, V., K. Mellendorf and D. Plano, 2003. "Surface Tension Units." Ask a Scientist Physics Archive, University of Chicago, Newton BBS. http://www.newton.dep.anl.gov/askasci/phy00/phy00656.htm
• A great book with lots of project ideas on the properties of matter:
  Gardner, R., 2004. Science Fair Projects About the Properties of Matter: Using Marbles, Water, Balloons, and More, Berkeley Heights, NJ: Enslow Publishers.
• Gravitational force:
  http://www.glenbrook.k12.il.us/gbssci/phys/Class/circles/u6l3c.html
• The website Sizes.com has a huge index of units and systems of units:
  Editor, Sizes.com, "Index to Units and Systems of Units." [accessed December, 2005] http://www.sizes.com/units/index.htm

Materials and Equipment

To construct a homemade single-beam balance (see Figure 1 in the Experimental Procedure section), you will need the following:

• a beam (e.g., drinking straw, piece of stiff cardboard, wooden or plastic ruler),
• a fulcrum (e.g., a pin or nail),
• 2 supports of equal height (e.g., two books arranged back-to-back with a small space between them, two cans, two wood blocks),
• pan for weights (you can make this from foil),
• needle (or 5 cm length of straightened paper clip wire),
• thread (for attaching pan and needle to balance),
• small bit of modeling clay to counterbalance the empty pan.
  
  

• a small bowl,
• water,
• liquid detergent,
• plus any other liquids whose surface tension you would like to measure (e.g., rubbing alcohol, cooking oil).

• weights (common pins, drops of water from an eyedropper),
• and a way to calibrate them (self-service scale at post office, 10 ml graduated cylinder).

Experimental Procedure

1. Do your background research.
2. Gather the materials and find a good place to work.

   Figure 1: Diagram of a simple single-beam balance

3. Constructing the balance (refer to Figure 1).
   1. Take your time and work carefully. You'll get better results.
   2. First construct the beam.
      1. There are many choices for materials. You just need something stiff enough to support a few grams at each end.
      2. You'll need to mark the center point for the fulcrum. Depending on your choice of material, either drill a hole for the fulcrum (e.g., for wood), or simply push it through (e.g., pin through a drinking straw). The beam needs to rotate freely about the fulcrum.
      3. You'll also need to make holes at each end of the beam, equidistant from the center. Attach loops of thread through the holes (paper clips, or ornament hangers could also work), as shown.
      4. Push the fulcrum through the center hole of the beam, and place it on the supports.
   3. Next construct the pan.
      1. This can be a simple box or dish folded from aluminum foil. (It's square in the diagram only because it was easier to draw.)
      2. If you make a round pan, three strings will work fine for supporting it.
   4. Tie a thread to the center of your needle or paperclip wire. Adjust the thread so that the needle or wire hangs horizontally.
4. Measuring surface tension.
   1. Hang the pan from one end of the beam and the needle from the other. Use a small piece of modeling clay as a counterbalance (as shown in Figure 1) to balance the needle and empty pan.
   2. Place your container of water so that the needle (or wire), still hanging horizontally, is submerged in the water.
   3. 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.
   4. It will not take much weight, so you need to add it in small increments. Here are two different methods you could try.
      1. Use common pins as your weights, adding them one at a time. Calibrate them by weighing a bunch of pins on a postal scale, and dividing by the number of pins to get the weight per pin.
      2. Use drops of water from an eyedropper or plastic transfer pipette. You can calibrate the water drops by counting how many drops are needed to make, say, 5 ml. Each ml of water weighs 1 g, so with your count you can calculate how much each drop weighs.
      3. Try both methods and see how your results compare!
   5. Repeat the measurement (steps 1–3) at least 5 times (more is better), to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the water), repeat the trial from the beginning.
   6. Average your results.
   7. The force you will be measuring can be expressed by the equation:
      F = 2sd, where
      • F is the force, in newtons (N),
      • the factor of 2 is because the film of water pulled up by the needle (or wire) has 2 surfaces,
      • s is the surface tension per unit length, in units of newtons/meter (N/m), and
      • d is the length of the needle (or wire), in units of meters (m).
   8. To convert grams to the force, F, you have to account for gravity pulling down on the mass in the pan. Do this by multiplying the mass (in grams) by 9.83×10^-3 N/g (for more information, see the link on "Gravitational Force" in the Bibliography).
   9. You can rearrange the equation above to solve for s, the surface tension of water. Measure the length of the needle (or wire), and you'll have all the information you need to calculate the surface tension of water.
   10. How do you know that you are measuring surface tension, and not an attractive force between the needle (or wire) and the water? Here's a good tip from Robert Gardner's book (Gardner, 2004). Surface tension is the cohesive force between water molecules. Observe the needle (or wire) carefully after it is pulled out of the water. If it remains wet, then it must be the water that pulled apart, and this is the force (surface tension) that you measured. If it is dry, then the adhesive force between the water and the needle broke first, and this is what you measured, not surface tension.

Variations

• Add a drop of liquid dish detergent to the water in your dish, mix it by stirring gently (you don't want a lot of bubbles), and measure the surface tension again. Do you think it will be higher or lower than for plain tap water?
• Try measuring the surface tension of other liquids, (e.g., rubbing alcohol, cooking oil). Remember note 2j, in the Experimental Procedure section.
• For another method of investigating surface tension, see: Build a Motorboat Powered by Surface Tension.
• For a project on the chemistry of surface tension, see: Measuring Surface Tension of Water with a Penny.

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies
Gardner, R., 2004. Science Fair Projects About the Properties of Matter: Using Marbles, Water, Balloons, and More, Berkeley Heights, NJ: Enslow Publishers.
Calder, V. and D. Plano, 2002. "Measuring Surface Tension." Ask A Scientist Physics Archive, University of Chicago, Newton BBS. http://www.newton.dep.anl.gov/askasci/phy00/phy00455.htm.

Last edit date: 2007-03-26 12:00:00