Can Water Hardness be Determined by Soap Bubbles?

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To see if it is possible to estimate the hardness of a variety of waters from various wells and public water supply sources solely by their bubble activity after being mixed with soap and shaken.


Hard water has a high dissolved mineral content of calcium, magnesium, and other minerals. Water hardness is often expressed in grains per gallon (gpg), milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l. Water is considered hard when it contains between 7-10.5 gpg or 120- 180 mg/l of these minerals. Hard water is generally not considered a health risk but it is not desirable to have hard water because the minerals can build up on pipes and leave film and stains around sinks and tubs, and hard water tends to not mix well with detergents. Therefore, since it is known that hard water does not mix well with soaps or detergents, it should be possible to estimate the hardness of water by testing how many bubbles are produced when a mixture of a water sample and soap is shaken.

Materials Needed

  • 5 1-liter containers
  • 5 liters distilled water
  • calcium chloride (magnesium chloride can be substituted)
  • test tubes (one for each benchmark solution and one for each water sample tested)
  • pipettes
  • liquid hand soap
  • stoppers for the test tubes
  • black marking pen
  • test tube racks
  • tap water from various locations that include either a well or a public water supply
  • hard water test kit (drop titration kit)


Various benchmark solutions of hard and soft water containing specific amounts of calcium chloride added to distilled water (to imitate soft water, slightly hard water, moderately hard water, hard water, and very hard water) will be prepared, added to test tubes with a drop of liquid hand soap, and then tested for the amount of soap bubbles they produce when shaken. Then actual samples of tap water from wells and various public water supply sources will be tested with the liquid hand soap to see how many bubbles they each produce. The results will then be compared with the results from the bench-mark solutions and a guess or estimate will be made as to how hard the actual water samples are. Finally, each sample will be tested with a hard water test kit to measure the actual amount of minerals present in each sample. These results will be compared to the hardness estimates to see if soap bubble formation is a reliable indicator of water softness or hardness.


  1. Prepare 1 liter of each of the hard and soft water benchmark solutions. The benchmark solutions will be of 5 different grades of distilled water containing the calcium chloride—a soft water solution (containing 0–17.1 mg/l), a slightly hard water solution (17.1–60 mg/l), a moderately hard water solution (60–120 mg/l), a hard water solution (120–180 mg/l), and a very hard water solution (180+ mg/l). Note: you can convert mg/l into grains per gallon by dividing the amount of mg/l by 17.1.
  2. Pour each solution into a separate test tube, filling it  of the way up.
  3. Drop 1 drop of liquid hand soap into each test tube with the pipette. Cap the tubes with the stoppers and shake vigorously for 10 seconds each. Then use a black marking pen to indicate the highest point on the test tube that the bubbles reached. These will become your benchmark samples. Place them in a test tube rack.
  4. Take the first sample of tap water to be tested and pour it of the way up in a test tube. Drop 1 drop of liquid hand soap into the test tube and shake vigorously for 10 seconds. Then, with the black marking pen, indicate the highest point on the test tube that the bubbles reached. Repeat this step for each water sample.
  5. Once all of your water samples are tested, compare the amount of bubbles generated in each sample test tube with those of the benchmark test tubes and match the sample results with similar results obtained from the benchmark group. Record your observations. For example, if one water sample produced the same amount of bubbles as a benchmark tube that contains minerals that are between 120–180 mg/l, then record this observation with your estimate that the water in your sample is probably hard water.
  6. Once you have compared all of your water samples, perform the actual hard water test on each sample with the test kit. There are various test kits on the market for this procedure. Probably the easiest one to use is a drop titration kit. Follow the directions for testing hard water that comes with the test kit. Record your results.
  7. Now compare the actual results with the results you obtained from the bubble test and note your results.


  1. In the benchmark group of water samples was there a measurable difference between the amount of bubbles formed in the soft water and in the hard water?
  2. How did the bubbles from the tap water samples obtained from well water sources compare to the benchmark group?
  3. How did the bubbles from the tap water samples obtained from public water supply sources compare to the benchmark group?
  4. What were the actual results obtained from the hard water test kit? Did these results corroborate the results you obtained from your bubble test?
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