Hard Water, Soft Water, by John Cowens

"Ring around the bathtub" will have a new – and scientific! – meaning for kids after this lesson

Two boys, one with suds and one without

Soap won't lather in hard water (left) due to the chemical salts contained in it, but soft water (right) makes mounds of suds.

There are two basic types of fresh water. Soft water contains very little or no calcium or magnesium. It's treated water in which the only positively charged ion is sodium. As a result, the water may taste salty and be unsuitable for drinking.

Hard water, on the other hand, contains an appreciable quantity of dissolved minerals. About 10,000 lbs : 1 lb (or 4535.92 kg : 0.45392 kg) ratio of water to calcium, magnesium or iron are present. The minerals give the water a distinct taste. Natural mineral waters are highly sought after by some people for their flavors and their health benefits. However, hard water can contain so many minerals that, when the water is heated, carbonates precipitate out of the solution, damaging plumbing as scale is produced. Hot water heaters can collect so much scale that they will need more energy to produce hot water.

Where's the lather?
Hard water also reduces the effectiveness of cleaning agents. Soap is less effective in hard water since it reacts to form a calcium or magnesium salt of the organic acid in the soap and forms a grayish soap scum with no lather. Detergents, on the other hand, lather in both hard and soft water. Calcium and magnesium salts of the detergent's organic acids form, but these salts are soluble in water.

Softening water
Today, water can be softened in a number of ways. Automatic water softeners can be connected to home water supply pipes to remove magnesium and calcium from water and replace them with sodium, which doesn't react with soap or detergents. People who don't have automatic water softeners can still soften their laundry water by adding a popular brand of softener directly to the water. This product also combines with calcium and magnesium which prevents soap scum from forming.

Making hard water scum

  • soft water (rain water or distilled water)
  • bar of Ivory® soap or equivalent
  • quart-size (0.95 liter) jar
  • plaster of paris
  • two clear 16 ounce (0.47 liter) drinking glasses or plastic jars
  • teaspoon


  1. Make soapy water by dissolving a few shavings from a bar of Ivory soap (not hard-water soap) in a 16 ounce (0.47 liter) glass of soft water, such as rain water or distilled water.

  2. Pour half of the soap solution into another 16 ounce (0.47 liter) glass and set both glasses on a windowsill where daylight will shine through the soapy water. This will enable you to see what happens when you mix hard water with the soapy water.

  3. Prepare hard water by adding one teaspoonful of plaster of paris to a clean, quart-sized (0.95 liter) jar of soft water. Let the mixture stand overnight and allow the plaster of paris (which will not dissolve) to sink to the bottom of the jar.

  4. NOTE: Plaster of paris (calcium sulfate) is only slightly soluble and it is best to prepare the hard water used in this experiment several hours in advance.

  5. Pour some of the hard water from the quart-sized (0.95 liter) jar into one of the 16 ounce (0.47 liter) glasses that contain soapy water. Use the second, undisturbed glass of soapy water as a "control" with which to compare the soapy water to which the hard water was added.

    The hard water will cause the soap dissolved in the soft water to leave its solution and form sticky white flakes of a calcium soap, which is insoluble. This substance is what "ring around the bathtub" is made of. No wonder you have to scrub that ring so hard!

For further reading

  • Plain Talk About Drinking Water: Questions and Answers About the Water You Drink by James M. Symons (American Water Works Association, 1997, ISBN: 0-898-67860-9)

  • Experiment With Water by Brian Murphy (Two-Can Publishing, 2001, ISBN: 1-587-28119-8)

  • Science Experiments With Water by Sally Nankivell-Aston and Dorothy Jackson (Franklin Watts, 2000, ISBN: 0-531-15432-7)

Online explorations

John Cowens teaches science at Fleming Middle School in Grants Pass, OR.