Articles

    Chasing Puddles

    November 2017:

    In the February 2017 edition of ChemShorts For Kids you investigated some unique properties of water like cohesion. I want to take a look at that again using a different experiment.

    Materials:

    Wax paper, Water, Food coloring, Toothpicks, Dropper, Dish soap

    Experiment:

    Have different colors of water available and using a dropper, put a few drops of water in different places onto the wax paper. Stick the toothpick into the middle of one of the droplets and move it around on the wax paper. You can move the droplet around to wherever you like and even join your droplet with another to make a droplet twice as big. Try mixing different colors of droplets and see what happens. Once you finally tire of dragging around droplets, put your toothpick into some soap and then try dragging your droplet. What happened?

    What’s happening?

    Just like last February’s article, water is a polar molecule which means that there is a positive and negative end to the molecule – similar to a magnet. And what happens when magnets are put together? They stick together like water molecules stick together. Wax paper on the other hand is nonpolar and so water has a much greater attraction for other water molecules than it does for nonpolar wax paper. Therefore the water molecules would prefer to hang out together (cohesion) than to be associated with the wax paper. Once you add the soap however, the soap molecules have the effect of breaking down those attractive forces that the water molecules have for each other and create what is referred to a as a micelle. In the diagram below you see the soap molecule has a nonpolar end and a polar end. The water (H2O) is attracted to the polar end while all the nonpolar ends hang out together. This nonpolar end is going to be attracted to the wax paper so the water tends to have more attraction toward the wax paper now since it is associated with it through the soap molecule.

    References:

    http://www.4cforkids.org/news/water-drops-on-wax-paper
    https://www.youtube.com/watch?v=nIqH34L-AC4

    To view all past “ChemShorts for Kids”, go to:
    http://chicagoacs.org/articles/article_category/1

    Paul Brandt