Experiment 5

Surface Tension and Soap Bubbles

Procedure and Results

Procedure

Preparation of detergent solutions

Prepare four solutions having different concentrations of detergent. The compositions of each of these are:

0.25 g of liquid detergent in 100 mL water.

0.50 g of liquid detergent in 100 mL water.

0.75 g of liquid detergent in 100 mL water.

1.00 g of liquid detergent in 100 mL water.

For each one add exactly 100 mL of deionized water to a beaker. Place the beaker on the balance and carefully add the required mass of the detergent. Remove the beaker from the balance and stir well.

Measure the surface tension and bubble size of each solution as described in the sections, "Measuring Surface Tension" and "Measurement of Bubble Size" below.

To study the effect of added glycerin, prepare 500 mL of a detergent-water solution of the same concentration as one of your previous solutions. For example, prepare 500 mL of a solution containing 1.0 g of detergent in 100 mL of water, by adding 5.0 g of detergent to 500 mL of water, and stir well. Pour out exactly 100 mL of this solution into each of four beakers. To the first beaker add 2 drops of glycerin and mix thoroughly. Add 4, 6, and 8 drops, respectively, to each of the remaining beakers. Be sure that you mix these thoroughly also. Measure the surface tension and bubble size for each of these solutions.

Measuring surface tension

Make two marks on a disposable pipet as shown below:

With a rubber bulb, fill the pipet with water to a level above the upper mark. Using your finger let the level drop to the upper mark. Let the pipet drain slowly to the lower mark counting the drops. Record the number of drops and repeat one more time. Tabulate your results and calculate and record the average of the two trials.

The greater the number of drops the lower the surface tension. When repeating this with detergent solutions use air from the pipet to prevent foam from entering the pipet. Let this drain back into the beaker to conserve the detergent solution.

Measurement of bubble size

Attach a piece of tubing to the compressed air outlet. Roll up a small piece of tissue and insert into the tubing to act as a filter. Next attach the tube to a conical filter flask. Turn on the air flow to a small flow rate. Dip the top of your filter flask into the detergent water solution and set the flask in front of a measuring stick having 0.10 cm divisions. Watch the bubble growth and record the size reached when the bubble bursts. Repeat this several times adjusting the flow rate of the air so that the bubbles are formed reasonably fast but not so fast that they are not conveniently measured. Once the ideal flow rate is obtained do not change it for the duration of the experiment.

Measure and record the diameter of 5 bubbles for each of your detergent solutions and each of the detergent-glycerin solution. Calculate the average bubble size for each of the solutions from the five trials.

Results

Your results section should contain a table with each of the two values of surface tension (number of drops) for each of the detergent solutions and each of the detergent-glycerin solutions. The average values should also be entered in your table. There should also be a corresponding table containing the individual bubble sizes with their average values (in cm).

1. Make a plot of average surface tension (drops) vs. concentration (grams) for the detergent solutions and also for the detergent-glycerin (drops of glycerin) solutions. It may be convenient to put these on the same graph using different colors. Don't forget to plot the points with no detergent and with no glycerin!
2. Also make plots of the average bubble sizes vs. concentration of detergent and glycerin.
3. To test the effect of surface tension on bubble size, plot the surface tension (number of drops) against the corresponding values of bubble size (cm). Indicate as error bars the range of sizes of bubbles for each average.