Water Water Everywhere

    Kids, did you ever wonder how much of the water on the planet is available to drink? Although 75% (three-quarters) of the Earth's surface is covered with water, 97% of it is too salty to drink. Another 2.5% is either frozen or too deep to reach, leaving just 0.5% of Earth's water for drinking, washing, cooking, and irrigation. Here we have an activity so that you can see for yourself how little water this is. Because proper proportions are so important for this demonstration, we will use the accurate measuring "cups" available in a standard science lab: a 1-liter beaker filled to the liter mark with tap water, 10- and 50-ml graduated cylinders, 3 smaller beakers (about 50 -ml size), a dropper, wax paper, measuring spoons, and sodium chloride (table salt).

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

    Imagine that the water in the 1-L beaker represents all the water on Earth. Now pour 28 ml (using the 50-ml cylinder) into a small beaker labeled "A". Stir 1 tablespoon of salt into the large beaker of water. This big beaker now represents all of the salty, undrinkable ocean water on the planet, and the 28 ml in beaker A represents all of the Earth's freshwater. Pour 6.5 ml from beaker A into another small beaker labeled "B". Now beaker A represents all of the freshwater frozen in ice caps and glaciers (you can even freeze this now to make it more dramatic), and beaker B is the rest of the freshwater. Pour 3.4 ml from beaker B into the last small beaker, labeled "C". Now beaker B represents groundwater that is too deep to use, and beaker C is the entire freshwater supply available to us on Earth.

    Unfortunately, much of this freshwater is polluted. Use the dropper to remove just 5 drops from beaker C and drip them onto a piece of wax paper. These 5 drops are a reasonable estimate of how much drinkable water is actually available from the original 1 liter of water.

    As freshwater becomes scarce people are beginning to turn to seawater as a resource. However, before we can drink seawater the salt must be removed. (This is because too much salt will disrupt the normal balance of electrolytes in our bodies; cells with too much or too little will not function properly). Desalination plants purify water either by distillation (boiling off pure water leaves salts behind, but this is very expensive) or reverse osmosis using special membranes (see 5/01 issue for more on this).


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    December 2002


    References: M. Stewart in ChemMatters, Oct. 2002, pg 4 (American Chemical Society); M. Brennan, Chemical & Engineering News, 4/9/01, pg 32; D. Martindale, Scientific American, Feb. 2001, pg 52.