Water Purification Experiments: How Filtration, Distillation, and Chemistry Clean Water
Water contamination falls into three broad categories: particulate matter (dirt, sediment, and debris), dissolved chemicals (salts, heavy metals, pesticides, and organic compounds), and biological contaminants (bacteria, viruses, and parasites). No single purification method removes all three categories effectively, which is why water treatment plants use multiple steps in sequence. Filtration removes particulates, activated charcoal adsorbs dissolved organic chemicals, and chlorination or UV treatment kills biological contaminants. By testing each method individually, you can understand its strengths and limitations, and appreciate why combining methods produces safer water than any single method alone.
Understand Water Contaminants
Before purifying water, you need to understand what makes it impure. Particulate contamination includes visible materials like dirt, sand, plant matter, and rust particles. These are the easiest contaminants to remove because they are physically large enough to be trapped by filters. Dissolved chemical contamination includes substances that are molecularly dispersed in the water, such as chlorine, fluoride, lead, pesticide residues, and dissolved organic matter. These pass through physical filters because the individual molecules are far smaller than filter pores. Removing dissolved chemicals requires either adsorption (where contaminant molecules stick to a surface like activated charcoal), distillation (where water evaporates and leaves contaminants behind), or chemical treatment (where a reagent reacts with and neutralizes the contaminant). Biological contamination includes living organisms like bacteria, protozoa, and viruses that cause waterborne diseases. These can be killed by chemical disinfection (chlorine, iodine), heat (boiling), UV radiation, or removed by ultrafine filtration. For your experiments, you will create safely contaminated test water using soil, food coloring, and other harmless materials. Never test purification methods on water that may contain actual pathogens.
Build a Layered Filtration Column
Cut the bottom off a large plastic soda bottle and turn it upside down so the neck points downward (this becomes your filter column). Place a coffee filter or a piece of cotton cloth inside the neck to prevent materials from falling out. Add layers of filter media in this order from bottom to top: two centimeters of small pebbles or aquarium gravel, three centimeters of coarse sand, three centimeters of fine sand, and two centimeters of activated charcoal (available from pet stores as aquarium filter carbon). Each layer removes different contaminants. The gravel catches large debris, the coarse sand traps medium particles, the fine sand catches smaller particles, and the activated charcoal adsorbs dissolved organic chemicals and some inorganic contaminants. Prepare test water by mixing a cup of tap water with a teaspoon of soil, a few drops of food coloring, and a pinch of powdered drink mix. Pour this muddy, colored water into the top of your filter column and collect what drips out the bottom into a clear glass. The filtered water should be noticeably clearer than the input, though it may still retain some color because food coloring molecules are small enough to pass through sand. Compare the input and output water side by side to assess how much the filter improved clarity.
Perform Solar Distillation
Distillation purifies water by exploiting the fact that water evaporates at a lower temperature than most dissolved contaminants, leaving impurities behind. Build a simple solar still by placing a shallow bowl of contaminated water (use salt water made with two tablespoons of table salt per cup) inside a larger bowl or pot. Cover the entire assembly tightly with plastic wrap. Place a small weight (a coin or pebble) on the plastic wrap directly above the inner bowl so the plastic dips toward the center. Set the apparatus in direct sunlight. As the sun heats the water, it evaporates, rises, and condenses on the underside of the plastic wrap. The condensed droplets run downhill along the sagging plastic toward the weight in the center, where they drip into the inner collection bowl. The collected water is distilled, meaning it has left the dissolved salt behind in the outer bowl. Taste a drop of the collected water (if you used only table salt as the contaminant) and compare it to the salt water. The distilled water should taste flat and salt-free. This method removes dissolved salts, chemicals, and biological contaminants because none of these evaporate with the water. The tradeoff is speed: solar distillation produces water very slowly, typically only a few tablespoons per hour in direct sunlight.
Test Chemical Purification
Chlorination is the most widely used chemical water disinfection method in the world. Municipal water treatment plants add chlorine to drinking water to kill bacteria, viruses, and parasites. You can demonstrate this at home using regular unscented household bleach, which contains about 5% to 8% sodium hypochlorite. The recommended emergency disinfection dose is two drops of bleach per liter of clear water (or four drops per liter of cloudy water). For this experiment, fill two identical clear jars with tap water. Add a pinch of dried oregano or basil to both jars to simulate organic matter (this makes the chlorine effect visible because chlorine will bleach the plant pigments). Add two drops of bleach to one jar and leave the other untreated as a control. Stir both and let them sit for 30 minutes, the standard wait time for chlorine disinfection. After 30 minutes, compare the two jars. The chlorinated jar should show lighter-colored herb pieces because the sodium hypochlorite has oxidized and bleached the plant pigments. The untreated jar retains its original color. This bleaching action is a visible proxy for the oxidation reactions that chlorine performs on bacterial cell membranes and viral protein coats, destroying their ability to function. Note that chlorination does not remove dissolved chemicals or particulates, which is why it is always used in combination with filtration in water treatment systems.
Compare Purification Methods
Prepare four identical samples of test water by mixing tap water with soil, food coloring, and a tablespoon of salt per cup. Treat each sample with a different purification method: pour one through your filtration column, distill one using your solar still (or stovetop distillation for faster results), add bleach to one for chlorination, and leave one untreated as a control. After each treatment, evaluate the water for clarity (hold it up against a printed page and see if you can read through it), color (compare to the control), taste (only if you used food-grade contaminants and are confident the treated water is safe), and dissolved salt content (use a conductivity meter if available, or taste for saltiness). Record your observations in a comparison table. You should find that filtration removes particulates and some color but not dissolved salt, distillation removes everything but is slow, and chlorination does not affect clarity or dissolved chemicals but would kill biological contaminants. This comparison demonstrates why water treatment plants use all three methods in sequence: first sedimentation and filtration to remove particulates, then activated charcoal to adsorb chemicals, and finally chlorination to disinfect.
Build an Activated Charcoal Adsorption Test
Activated charcoal deserves special attention because it works through an interesting physical-chemical process called adsorption (not absorption). In absorption, a substance soaks into a material like water into a sponge. In adsorption, molecules stick to the surface of a material through weak chemical attraction. Activated charcoal has an enormous internal surface area because it is riddled with microscopic pores, giving a single gram of activated charcoal a surface area of about 1,000 square meters. To demonstrate adsorption, prepare three glasses of water colored with food coloring. To the first glass, add one tablespoon of regular charcoal briquette fragments (crushed with a hammer, wrapped in a cloth). To the second, add one tablespoon of activated charcoal from an aquarium store. Leave the third as a control. Stir each glass and let them sit for 30 minutes, then compare the color intensity. The activated charcoal should remove significantly more color than the regular charcoal because its greater surface area provides more adsorption sites. The regular charcoal may remove some color, but much less effectively. This difference illustrates why the activation process (heating charcoal with steam or chemicals to create internal pores) is essential for making charcoal useful as a water treatment medium.
Water purification experiments demonstrate that no single method removes all contaminants, and that effective water treatment combines physical filtration, chemical adsorption, and disinfection to address particulates, dissolved chemicals, and biological threats in sequence.