Hard Water Test

    Kids, in this experiment you will make "hard" water from distilled water, which contains no minerals, and is therefore "soft" to start with. Tap water in many parts of the country (including Chicagoland) is hard and contains minerals that can interfere with the cleaning ability of detergents. Water softeners remove these minerals. You will also compare the sudsing ability of a detergent in soft and hard water. 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    You will need: 2 cups (500 ml) distilled water, 1 teaspoon (5 ml) epsom salts, 2 empty and cleaned 2-liter plastic soft-drink bottles with screw caps, and several drops of liquid dishwashing detergent. Pour 1 cup (250 ml) of distilled water into each of the empty soft-drink bottles. Add the epsom salts to one of the bottles and swirl until they dissolve. Add several drops of liquid dish detergent to both bottles. Seal the bottles with caps and shake. A large amount of suds will form in the bottle without epsom salts. Far fewer suds will form in the bottle containing the epsom salts.

    The suds formed in this experiment are made of tiny bubbles. The bubbles are formed when air is trapped in a film of liquid. The air is trapped when it is shaken into the water. The film of liquid surrounding each bubble is a mixture of water and detergent. The molecules of detergent form a sort of framework that holds the water molecules in place in the film. If there were no detergent, the bubbles would collapse almost as soon as they are formed. You can see what this would look like by repeating the experiment but leaving out the detergent. This experiment will not produce suds if detergent for a dishwashing machine is used. (Try it and see.) No suds are formed because automatic dishwasher detergent is formulated so that it does not form suds. Suds create major problems in a dishwasher.

    The minerals that make water hard usually contain calcium and magnesium. In this experiment you made water hard by adding epsom salt, which is magnesium sulfate (MgSO4). Calcium and magnesium in water interfere with the cleaning action of soap and detergent. They do this by combining with soap and forming a scum that does not dissolve in water. Because they react with soap, they remove the soap and reduce its effectiveness. This could be overcome by adding more soap, but the scum will make what is being washed appear dingy.

    Water can be softened in a number of ways. An automatic water softener connected to water supply pipes removes magnesium and calcium from water and replaces them with sodium. Sodium does not react with soap or detergents. If you don't have an automatic water softener, you can still soften laundry water by adding softeners directly to the wash water. These softeners combine with calcium and magnesium, preventing the minerals from forming a soap scum.


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    January 2003


    Reference: B. Shakhashiri at