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    Testing Water for Hardness Using Soap Bubbles
    K-12 Experiments & Background Information
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    Water Hardness Experiments
    This experiment is courtesy of 




    George Hight
    Drexel Hill, Pa. 19026

    Michael J. Gavaghan
    Dr. Peter a Yarnell
    Rohm and Haas Company
    Spring House, PA 19477


    3rd to 5th Grade



    Physical Science


    1. To broaden a students' knowledge and understanding of water
    2. To determine what materials cause water to become hard.
    3. To understand the function and limitations of soap in mineralized water.
    4. To determine the hardness of a mystery sample of water


    1. Students will discuss water quality and usefulness to various industries.
    2. Students will observe that soap reacts differently with different water samples.
    3. Students will conclude that while all water samples look the same to the naked eye there is something different in each one that prevents soap from working the same in each sample.
    4. Students will discuss the nature, cause and effect of hard water.
    5. Each student will conduct a soap test on a sample of hard water where the hardness is known measuring the height of the bubbles produced.
    6. Students will conduct a soap test on an unknown sample of tap and attempt to ascertain its approximate hardness.


    Not all water is created equally. Water has the unique ability to dissolve a wide variety of materials. Some Industries can not use water with materials that may be present naturally because they would cause adverse effects to the machinery of the facility, the product,or the environment. A power plant is an example in point. The presence of such dissolved solids as Magnesium, iron, calcium and others require the power company to expend more energy than normal to raise water to its boiling point. In addition to energy waste these materials can quickly lead to a mechanical hardening of the arteries as they precipitate out inside steam tubes.

    People are also affected by this problem. When using soaps in water with a high mineral content it will be difficult to raise a lather because compounds in the water must be precipitated from the solution before sudsing can occur. In addition, mineralized water will leave a film or "soap scum" around the washing surface that will be difficult to remove.

    Water that contains large amounts of calcium and magnesium is said to be "hard water". Hard water usually occurs in areas where limestone rock can be dissolved by water that is slightly acidic. The acid in water comes from carbon Dioxide. A waste product of animal respiration, CO2 makes up 0.03 percent of the air we breathe. When combined with water, carbon dioxide forms a weak acid. This Carbonic acid eats away at limestone or magnesium rich earth and produces calcium or magnesium carbonate. These are the compounds that are most frequently associated with the production of water hard. Iron, aluminum and other minerals may cause hardness if enough of them are present in a given quantity of water. However, large amount of these minerals are not usually found in natural waters.

    Hardness tests are frequently performed in industries that rely on pure water. These tests measure the total amount of calcium and magnesium in a water sample and express the results in metric units as milligrams per liter (mg/L) of calcium carbonate (CaCo3), or the English units of grains per gallon (gpg) of calcium carbonate. To convert from metric units to the customary English unit simply apply the formula :

    mg/L = gpg X 17.1

    Water is said to be hard when its total hardness exceeds 121 milligrams per liter of calcium carbonate.(condensed from Water,Water Everywhere, Teacher's Guide and experiments by Cliff Jacobson)

    Not all materials dissolved in water are necessarily harmful. Some elements in small amounts can be quite beneficial. Small amounts of fluoride can help prevent tooth decay, and water companies frequently add chloride to cut down on the number of micro organisms that that would reproduce in our drinking water and make us sick.

    Advanced prep:

    Teachers will need to make up a stock solution of hard water and prepare several dilutions of the stock in specific concentrations for children to test. Students should bring in samples of water from their home, rain water or other sources to check for hardness.


    1. Obtain several 1 and 2 liter containers. If none are available two liter soda bottles will work. Make a mark on a full bottle of each to use as a measuring container.
    2. Place 1/4 table spoon or 1.4 grams of calcium chloride in to a two liter container and fill to the mark with distilled water This is your stock solution and should have a total hardness of 391.8 parts per million as calcium carbonate. Label this container A.
    3. To make a 0.1 dilution, take 100ml of A and place in another 2 liter container. Add one liter of distilled water. Label this container B.(to make a 0.2 dilution use 200ml) If you do not have 100ml graduated cylinders available a 4oz baby food jar will hold just about 100ml.
    4. Take 100ml of B and add to l liter of distilled water labeling this C. This is a 0.01 dilution of the stock.



    Distilled water (can be purchased at the food store)

    Magnesium Chloride crystal or Epsom Salts to make a 1.0 stock solution.

    Calcium Chloride or calcium chloride based ice removal

    material to make a 1.0 stock solution.

    Liquid hand soap (detergent will not work)

    Medicine Droppers

    Felt marking pens

    test tubes (at least 4 per group)

    Metric rule

    stoppers for test tubes



    Part A

    Does all water behave the same way when soap is added?

    1. Arrange test tubes, label each A,B,C, and so on.
    2. Place equal amounts of the appropriate solution in each test tube, filling each about 1/3 full.
    3. Place a line on each tube at the height of the sample, this becomes the base line.
    4. Add one drop to liquid hand soap to each tube
    5. Stopper and shake each tube 10 times, recording the height of suds from the base line to the top of the suds. Record this number.
    6. Compare group results.

    Part B

    Finding the relative hardness of a mystery sample of water

    1. Arrange the test tubes in order from most bubbles to least amount of bubbles.
    2. Compare the results of each student group.
    3. Place the concentrations of each dilution on the board and their appropriate total hardness.
    4. Prepare a test tube as described above and fill to 1/3 with the mystery sample.
    5. Add one drop of liquid soap solution.
    6. Shake 10 times and measure the height of bubbles formed.
    7. Compare the results of the mystery sample with those of the known dilutions from part A.
    8. Determine which test dilution is closest to the mystery solution.

    This experiment is courtesy of 

    My Dog Kelly

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    Last updated: June 2013
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