Bubble Trouble with Hard Water 

    Kids, you may have heard that the chemical element CALCIUM is very important for strong bones and teeth. Terrific sources of calcium in our diets include milk, broccoli, salmon and sardines. Along with bones and teeth, calcium is also a major part of things like cement, seashells, limestone, chalk, marble, eggshells, and de-icer for icy roads. Sometimes when water flows over limestone or other materials with a lot of calcium in them, the calcium gets into the water. Water that contains a lot of calcium or other minerals is called HARD WATER. One characteristic of hard water is that it makes a soap scum when mixed with soap. It also makes a soap solution much less bubbly. But don't just take our word for it, let's check it out!

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

    Cover a work area with paper towels. Label 3 clear 8 oz plastic cups as "water", "water and plaster", and "soapy water". Pour 1/2 cup warm water into each cup. Add about 1/4 teaspoon of plaster of paris powder to the cup labeled "water and plaster". Stir thoroughly with a plastic straw. Grate 1-2 tablespoons of soap from a bar of soap. Put about 1 tablespoon into the "soapy water" labeled cup. Stir thoroughly with a new straw. Add 1 tablespoon of your soapy water to the "water" and "water and plaster" cups. Do not stir right away, and observe what happens closely. Is there a difference? What do you see happening in one of the cups? Now stir each cup with a separate straw. Do they still look different? Next use a new clean straw to blow gently into each cup. Do you notice a difference in the bubbling? What do you think is the reason? It will help you to know that plaster of paris is a chemical compound called calcium sulfate (made from gypsum).

    [Safety Tip: Please be sure to blow into the liquids. Do not suck the liquid into the straws at all.]


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 1995


    Reference: WonderScience 10/94, volume 8(6).