Adding chlorine to drinking water has virtually eliminated waterborne diseases such as cholera in North America by destroying disease-causing microorganisms. You can safely and easily demonstrate how chlorine protects public health by removing microscopic life from a water culture that you create.

Materials

• Tap or fresh water (untreated water will yield more microorganisms)
• Wide-mouth glass vessel that holds about a quart of water (e.g., Mason jar) and several smaller glass vessels
• Container and heat source to boil water Handful of dry hay or straw (dry, untreated grass will do)
• Microscope (minimum 100 power)
• Glass slides
• Liquid laundry bleach (5.25% sodium hypochlorite)
• Measuring cup
• Eyedropper
• Plastic stirrer
• Thin rubber gloves
• Safety goggles or glasses
• Paper towels

Safety

Wear rubber gloves during the experiment. Do not allow culture to contact your body (people with cuts should not touch the culture).

Follow all label directions on bleach (it will bleach clothing).

Keep hands away from face during the experiment. Wear safety goggles/glasses when handling the bleach.

Thoroughly clean all equipment, gloves and hands when finished

Dispose of the culture in the toilet. Dispose of bleach solution in sink followed by a five-minute flush with cold water, and dispose of other materials in a clean trash bag.

Procedure

• Boil about one quart of water (tap or freshwater) for two minutes, let cool to room temperature, and pour into a thoroughly clean, wide-mouth glass vessel. Put a handful of dry hay, straw or grass into the vessel and set it aside in a darkened place at room temperature for seven to nine days. You are creating a water culture that will teem with microscopic life.
• After the wait, place a glass slide on the microscope and use the eyedropper to put a drop of the water culture on the slide. (Place a cover slip on the water culture drop). You and your students will be able to view many microscopic life forms, including protozoa (amoebas, parameciums) and nematodes (microscopic worms) moving around. These are the same types of organisms which live in surface waters and are removed during the water-chlorination process. In fact, certain protozoa are responsible for amoebic dysentery and giardia, which are common illnesses associated with water that is not properly chlorinated. Use several slides to sample water and view life forms from different parts of the culture.
• Now pour about one cup (eight ounces) of the water culture (being careful not to spill) into another clean container. Make up a solution of one part bleach to nine parts water in another container. After stirring the solution and rinsing the eyedropper, place about 20 drops of bleach solution into the cup of water culture; stir the water culture/bleach solution and let sit for about one minute. Thoroughly rinse the eyedropper in tap water and examine several drops of the bleach-treated culture. You should see no animals still alive in the water.

Suggestion

Arrange a field trip for your class to the local water treatment facility to learn where tap water comes from and why water disinfection and source water protection are important. Discuss tradeoffs involved (e.g., disinfection by-products vs. microbial contamination).

Instructor's Notes

If microscopes are not available, use pond water and add chemical fertilizer and set in direct sunlight, allowing eutrophication to take place.