Invisibility Demos

    Kids, how can you make crystals and spheres disappear?   Start with ones that are made from polyacrylamide! Superabsorbent polyacrylamide crystals (e.g. Soil Moist Crystals®, among other brands) are small grains which, when placed in water, seem to disappear. To your friends, you can cast an “invisibility spell” on the small crystals to make them grow and disappear in a glass of water. Small spheres made of the crosslinked polyacrylamide polymer are also available.

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

    Place the hard crystals in a glass or bowl about ¼ full, add water, and they can expand by up to 300 times, producing gel-like spheres that are slippery (and safe) to the touch. These are extraordinary volume increases. Crystals starting with a diameter of about 3 mm end with a diameter of about 20 mm.  From a volume perspective, spheres can expand from 113 mm3 to 33,512 mm3, which is more than 29000%. Determine the change in volume yourself using V = 4/3 π(r)3.

    The wet crystals and spheres can be dyed using food coloring, then dried out and reused. Put them in the sun and they will become rock-hard again. For some other variations, tie a loop of yarn around one of the beads. When submerged in a glass of water, the bead disappears and the yarn forms a (seemingly empty) circle in the water.  Lifting the yarn out of the water results in the previously invisible bead magically appearing as it rises above the water. From a biology or earth science perspective, this is a great way for starting seeds and growing plants so that the roots can be seen.

    What’s happening? Light changes its speed when it passes into an optical medium of different density. This causes the light rays to bend (refract). Crystals and spheres refract light when it enters and leaves them. Furthermore, the various planes of the crystals scatter the refracted light at different angles. When left in water, the fully gelled super absorbent crystals contain up to 95% water, so they have the same optical density as water. This minimizes refraction when submerged in water and the crystals and spheres become invisible.

    Polyacrylamide crystals and spheres can be purchased from science supply companies, or from garden shops, where they are used to uniformly provide water to plant roots over watering/drying cycles. 


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    May 2009


    References:  ACS ChemMatters February 2009, Teacher’s Guide (Item #2 of “Demonstrations & Lessons” from “Science at Hogwarts: Chemistry in Harry Potter’s World” by Jane Snell Copes).
    “Growing Spheres” at
    Carl Ahlers - Prof Bunsen Science;