Density Displays

    Kids, here you will be introduced to the concept of density, which is one property used by chemists to help identify unknown substances.

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

    Let's first make a liquid sandwich. Begin by getting a clean, narrow jar (a tall baby food jar or baby juice jar, or an olive jar) and adding 3 tablespoons each of vegetable oil, water, and honey or molasses to the jar. Watch and observe what happens. Eventually the honey or molasses will sink to the bottom, the oil floats on top, and the water is in the middle part of the "sandwich". They honey or molasses sinks the furthest because it is more dense (it weighs more for the same amount) than either water or oil. The oil floats because it is the least dense of the three.

    Now let's try a density column. Use a large test tube or another clean, empty, tall jar (olive or baby food jar). Add 3 tablespoons each, in order, of water with a drop of red food coloring, then vegetable oil, and then rubbing alcohol with a drop of blue food coloring. Pour these each slowly and carefully, and in the order of water, oil, alcohol. Wa-la, you now have a very patriotic display of your densities!

    Finally, to reward you for your efforts, you can try this density display that is good enough to eat. Put a 4-oz box of dry lemon gelatin in a bowl, have an adult add 1 cup of hot water, and stir until the gelatin is dissolved. Add 4-oz of whipped cream cheese (at room temperature) and stir again. It won't blend well but it will break into little bits. Does the cream cheese sink or float in the gelatin? Why? Add 1 cup of cold water to the gelatin and stir. Now add an 8-oz can of drained fruit cocktail and stir yet again. Does the fruit sink or float? Why? Pour the gelatin mixture into four small cups and refrigerate until it sets. Observe changes as they chill. Where do the cream cheese and fruit pieces end up?


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    January 1999


    Reference: M. Mandell in "Simple Science Experiments with Everyday Materials", Sterling Publ., 1989, p . 76 and ACS "WonderScience", February 1989 issue.