Perhaps the most basic version of an experiment that compares faucet and bottled water involves comparing the bacteria levels in each. Collect samples on microscope slides, and then examine them under a microscope. Use a field guide to identify any bacteria cells that you detect and count the number present in a measured drop. To increase the complexity, include more water sources, perhaps testing the tap water from several different towns, or include samples from a local pond or river. You might also compare several different brands of bottled water. The results of this experiment, although varied based on which brands and locations you sample, usually prove to be very interesting.
Another great option for experiments that involve comparing water aims to determine the most effective water for simple seed germination. Most likely your third grader will already have experience with sprouting seeds, which most early-elementary curricula include. Build on this prior knowledge by attempting to sprout seeds using different sources of tap water, bottled water and water found in nature. Radishes, marigolds, beans, wheatgrass and alfalfa are some the fastest sprouting seeds and will yield the quickest results. Measure your results by recording observations of height, rate of growth and color. To increase the complexity of this experiment, sprout several different types of seeds using several different water sources.
A fun way to involve a class or audience is to perform blind tasting experiments of various sources of water. Try to have about five samples which may include local tap water, tap water from another town, hose water, inexpensive bottled water and expensive bottled water. It is important not to use water from nature for this experiment as it could contain dangerous bacteria that could make your test subjects sick. Be sure all your samples are at the same temperature and served in identical, unmarked cups. First, ask your participants for their own hypotheses regarding which sample they think will have the best taste. Then ask them to taste all five samples and choose which they liked the best. When you display your results, most people will be surprised at the outcome.
For a more scientifically advanced experiment, compare the chemical compositions of tap water and bottled water. Purchase a water testing kit and compare the minerals, contaminants and pH levels of your water samples. Some kits will even test for additives like arsenic, fluoride and chlorine. Again, to increase the complexity of your experiment, test more samples, or purchase a more comprehensive water testing kit.