Measuring Surface Tension

MeasuringSurface Tension Najee Quashie Orange Preparatory Academy 8th Grade Ms. Brown

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

Research The cohesive (attractive) forces between liquid molecules are accountable for the event known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and therefore they join together more strongly to those openly linked with them on the surface. This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely covered. Surface tension is the reason why some bugs (water striders) are able to “stand” on water.

Research Molecules can be grouped as polar or non-polar molecules. Some are also in between. The arrangement of the atoms in some molecules is such that one end of the molecule has a positive charge and the other side has a negative charge. If this is the case, the molecule is called a polar molecule, meaning that it has electrical poles. Otherwise, it is called a non-polar molecule. Whether a liquid is polar or non-polar helps to determine how strong the surface tension will be between its molecules. This is why water (a strong polar molecule) generally has a very high surface tension.

Polar Water Molecule

Hypothesis There will be more surface tension with the hand soap because it is a thicker substance.

Materials a beam (drinking straw) a fulcrum (nail) 2 supports of equal height (two wood blocks) pan for weights (made from aluminum foil) 5 cm length of straightened paper clip wire) thread (for attaching pan and needle to balance) Petri dish • small bit of modeling clay to counterbalance the empty pan • water • Hand soap • Rubbing alcohol • Vegetable oil • weights (common pins) • a way to calibrate pins (scale) • Lab notebook (for results).

Procedure • Background research. • Gather materials and find a good place to work. • Construct the balance. • Take your time and work carefully. You'll get better results. • First construct the beam. • There are many choices for materials. You just need something stiff enough to support a few grams at each end. • Mark the center point for the fulcrum. The beam needs to rotate freely about the fulcrum. • Make holes at each end of the beam, halfway from the center. Attach loops of thread through the holes. • Push the fulcrum through the center hole of the beam, and place it on the supports. • Next construct the pan. • This can be a simple box or dish folded from aluminum foil. • Tie a thread to the center of your paperclip wire. Adjust the thread so that the wire hangs horizontally.

Procedure • Measuring surface tension. • Hang the pan from one end of the beam and the wire from the other. Use a small piece of modeling clay as a counterbalance to balance the wire and empty pan. • Place your container of liquid to be tested so that the wire, still hanging horizontally, is submerged. • You will add small amounts of weight to the pan, and measure the force needed to pull the wire free from the surface of the liquid. • It will not take much weight, so you need to add it in small increments. • Use common pins as your weights, adding them one at a time. Adjust them by weighing a bunch of pins on a scale, and dividing by the number of pins to get the weight per pin. • Repeat the measurement (steps 1–3) at least 2 times, to assure consistent results. If something goes wrong (e.g., you accidentally tap the pan and pull the needle out of the liquid), repeat the trial from the beginning. • Average your results. • 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 the liquid being tested 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).

Procedure • 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. • You can rearrange the equation above to solve for s, the surface tension of the liquid. Measure the length of the wire, and you'll have all the information you need.

Results

Conclusion I found that water had the greatest surface tension of 0.043 N/M, vegetable oil had the second greatest surface tension calculated to be 0.033 N/M, hand soap had the third greatest surface tension (0.026 N/M), and rubbing alcohol came in last with a calculated surface tension of 0.024 N/M. This data disproved my hypothesis because I believed that hand soap would have had the greatest surface tension. In fact, the surface tension of water was 0.017 N/m greater than that of the hand soap. I also questioned how I knew that I was measuring surface tension , and not an attractive (adhesive) force between the wire and the tested liquid. In order to answer this, I observed the wire carefully after it was pulled out of the liquid. It remained wet, which means that it must be the liquid that pulled apart, and this is the force (surface tension) that was measured. If the wire was dry, then it would be the attractive (adhesive) force only that I was measuring.

References Science Buddies: http://www.sciencebuddies.org/science-fair projects/project_ideas/Phys_p012.shtml http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html WWLPT Biology Institute: http://www.woodrow.org/teachers/bi/1998/waterstrider/student_lab.html Teach Engineering: http://www.teachengineering.org/view_lesson.php?url=collection/duk_/lessons/duk_drops_mary_less/duk_drops_mary_less.xml Ron Kurtus’ School For Champions: http://www.school-for-champions.com/chemistry/polar_molecules.htm

Measuring Surface Tension