How to Measure Surface Tension

Step-by-Step Guide

1. 1

   Step 1: Define the equation to solve for surface tension.

   In this experiment, the equation for surface tension will be determined by the equation F = 2sd.
   
   F is the force in newtons (N), s is the surface tension in (N/m), and d is the length of the needle used in the experiment.
   
   Rearranging the equation to solve for surface tension yields s = F/2d.
   
   The force will be calculated at the end of the experiment.
   
   Measure the length of the needle in meters using a ruler before starting the experiment.
2. 2

   Step 2: Construct a small balance beam.In this experiment

   The balance beam needs to be well constructed so that you can get an accurate result.
   
   You can use many different types of materials for this, just make sure the center beam is something sturdy like wood, plastic, or dense cardboard.
   
   Mark the center of the material to be used for your beam (straw, plastic ruler) and drill or poke a hole through it; this will be the fulcrum point (the point that allows the beam to rotate freely).
   
   If you are using a plastic straw you can just poke a pin or nail right through it.
   
   Drill or poke a hole at each end of the beam ensuring that they are the same distance from the middle.
   
   Thread string through the hole to serve as holders for the balance dishes.
   
   Support the fulcrum nail or pin horizontally on books or pieces of stiff material that won't collapse.
   
   You want the center beam to be able to rotate freely on the pin or nail poked through the middle. , The dish does not have to be exactly square or round.
   
   The dish will be filled with water or another weight, so make sure it is sturdy enough to support this.
   
   Hang the box or dish from one end of the beam.
   
   Poke small holes in the sides of the dish and thread the string through to hold up the dish. , On the opposite side of the beam, tie a paper clip or needle to the end of the string so that it lays flat.
   
   For the experiment to work, it is important that the paper clip or needle is horizontal. , Before beginning the experiment, you want to make sure that the beam is lying flat.
   
   The dish will be heavier than the needle, causing the beam to lower in the direction of the dish.
   
   Add enough clay to the opposite side of the beam so that the beam is level.
   
   This is called counterbalancing. , This step may require some extra effort to ensure the needle is resting just on the top of the water’s surface.
   
   You do not want the needle submerged in the water.
   
   Fill a container with water (or other liquid of unknown surface tension) and place it underneath the needle at a height that allows the needle to rest directly on top the surface.
   
   Make sure the string holding the needle in place remains taut once the needle is on top of the water. , You will be adding pins or drops of water one at a time to the aluminum dish you constructed earlier.
   
   For the calculation, it is important to know exactly how much weight is required to lift the needle out of the water.
   
   Count out a number of pins or drops of water and weigh them.
   
   Determine the individual weight of each drop or pin by dividing the total weight by the number of pins or water drops.
   
   For example, let’s say 30 pins weigh 15 grams: 15/30 =
   0.5.
   
   Each pin weighs
   0.5 grams. , Slowly add a pin or a drop of water to the aluminum dish one pin/drop at a time.
   
   Watch the needle closely to see if it comes out of the water with each new addition.
   
   Stop adding pins/drops once the needle is no longer in contact with the surface of the water.
   
   Count the number of pins or drops of water needed to remove the counterweight from the water's surface.
   
   Record each reading.
   
   Repeat the exercise several times (5 or 6) for more accurate readings.
   
   Calculate an average of the results by adding the total number of pins needed in each trial and dividing that by the total number of trials. , To calculate the surface tension, you need to know the total amount of force required to remove the needle from the surface of the liquid.
   
   Because you weighed the pins in the previous step, you can easily do this calculation using the conversion factor
   0.00981 N/g.Multiply the number of pins added to the dish by the weight of each pin.
   
   For example, 5 pins at
   0.5 g/pin = 5 x
   0.5 =
   2.5 g.
   
   Multiply the amount of grams by the conversion factor
   0.00981 N/g:
   2.5 x
   0.00981 =
   0.025 N. , Using the measurements you gathered throughout the experiment, you can now solve for force.
   
   Simply plug the numbers into the correct variable and solve using the proper order of operations.
   
   Continuing our example, let’s say the needle was
   0.025 m long.
   
   Plugging the variables into the equation yields:
   S = F/2d =
   0.025 N/(2 x
   0.025) =
   0.05 N/m.
   
   The surface tension of the liquid is
   0.05 N/m.
3. 3

   Step 3: you will use a balance beam and a small needle floating on the surface of water to measure surface tension.

4. 4

   Step 4: Fold a piece of aluminum foil to form a box or dish.

5. 5

   Step 5: Hang a needle or paper clip horizontally from the other end of the beam with thread.

6. 6

   Step 6: Place a piece of material such as clay on the beam to counterbalance the aluminum container.

7. 7

   Step 7: Place the needle or paper clip hanging from the beam into a container of water.

8. 8

   Step 8: Weigh a batch of pins or several measured drops of water on a small postal scale.

9. 9

   Step 9: Add the pins or drops of water one at a time to your aluminum foil container until the needle is freed from the water's surface.

10. 10

    Step 10: Convert the measurement of pins into force by multiplying the number of grams by 0.00981 N/g.

11. 11

    Step 11: Plug the variables into the equation and solve.

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