Introduction
This straightforward hands-on exercise demonstrates and experiments with the capacity of activated charcoal (carbon) to remove objectionable colors, odors, and tastes from water. Commonly used in municipal and home water purification systems, the properties of this remarkable substance make for fascinating study.
Background
Water treatment and purification is one of those “behind-the-scenes” processes that few of us know much about and nearly all of us take for granted. We just assume that when we turn on a faucet the water that comes out will be clean and drinkable and not look or smell like it came directly from a river or a sewer.
To gain a little insight into what goes on “behind the scenes” we can carry out one or two simple experiments with a truly remarkable substance—activated charcoal.
As already mentioned briefly, activated charcoal (alternatively referred to as activated carbon) is becoming increasingly common in municipal (large scale) and home (small scale) water purification systems. Perhaps you have an activated charcoal cartridge filter system under your kitchen sink already! What exactly is this material?
Regular charcoal or carbon is obtained by the destructive distillation of wood, nut shells or animal bones. This charcoal is then “activated” by heating to 800 to 900 °C with steam or CO2 in the absence of air. This activation process results in an extremely porous internal structure and an internal surface area averaging 10,000 to 20,000 square feet per gram! Activated charcoal works by “adsorbing” chemicals that may impart unwanted colors, odors, or tastes. Adsorption differs from the more familiar process of absorption in a fundamental way. In absorption one substance actually penetrates into the inner structure of another substance—such as water being absorbed by a paper towel or cotton ball. Adsorption, in contrast, is the adherence of one substance onto the surface of another. The adsorbed substance may be in the form of atoms, ions, or molecules of a liquid or gas. An analogy of this process would be a man in a Velcro® suit adhering to a Velcro wall—he is adsorbed onto the surface of the wall due to the properties of Velcro.
Due to its almost inconceivably large internal surface area, activated charcoal has a remarkable capacity for adsorbing unwanted chemicals. In addition to its use as a water purifier, its ability to deodorize, decolorize and purify mixtures has applications in air purifiers, gas masks and aquarium filters—among others. It is also used commercially in the purification and decolorization of sugar and artificial sweeteners.
Materials
(for each lab group)
Activated charcoal (carbon)*
Food coloring*
Imitation strawberry or mint extract†
Water, tap
Beakers, 150-mL, 6
Beaker, 400-mL
Filter paper, 11-cm diameter (medium speed)*
Funnels, 60–70 mm, 3
Funnel support or iron rings
Mortar and pestle
Ring stand
Stirring rods
Transfer pipet or eyedropper†
*Materials included in kit.
†Optional
Safety Precautions
Charcoal dust is potentially toxic by inhalation. Dispensing and grinding of the charcoal should be performed under an operating fume hood or in a well-ventilated area. If further experimentation with other odors is undertaken, teachers and students should be aware of the hazards, if any, associated with odor-producing chemicals or mixtures. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The filter paper containing the activated charcoal may be wrapped in newspaper and placed in the trash according to Flinn Suggested Disposal Method #26a. The filtrates may be poured down the drain with excess water according to Flinn Suggested Disposal Method #26b.
Procedure
Part One
- Add one drop of the food coloring to approximately 200 mL of tap water in the 400-mL beaker. Distribute this colored water to three 150-mL beakers—approximately 50 to 60 mL in each.
- Designate and label one of the three beakers as a control, the remaining two as “granules” and “ground.”
- To the beaker labeled “granules,” add approximately one to two teaspoons of the supplied activated charcoal.
- With a mortar and pestle, grind approximately one to two teaspoons of the activated charcoal into a fine powder (see safety precautions). Add this ground charcoal to the beaker labeled “ground.”
- Gently but thoroughly stir the contents of both charcoal beakers for several minutes. To control for time, try to equalize the adsorption period in the two charcoal beakers either by timing them or by carrying out all the steps concurrently.
- Separately filter the contents of all three beakers into the remaining three 150-mL beakers that should be labeled in a corresponding fashion. Carefully compare the colors of the filtrates.
Part Two (optional)
- Steps 1–6 in Part One may be repeated with other colored solutions such as acid–base indicators, inks and stains. A 1% solution of methylene blue in water works extremely well.
- To test for odor adsorption place 25 mL of tap water in each of two 150-mL beakers. To both beakers add an “eyedropper” of imitation strawberry or mint extract. Add two teaspoons of finely ground activated charcoal to one of the beakers and stir for several minutes. Filter the contents of each beaker into two clean 150-mL beakers. Compare the filtrates by alternately sniffing the contents of the two beakers.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
MS-ESS2.A: Earth’s Materials and Systems
MS-ESS2.C: The Roles of Water in Earth’s Surface Processes
HS-PS1.A: Structure and Properties of Matter
HS-ESS2.A: Earth’s Materials and Systems
Crosscutting Concepts
Systems and system models
Structure and function
Performance Expectations
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
MS-PS1-3. Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
MS-ESS2-4. Develop a model to describe the cycling of water through Earth’s systems driven by energy from the sun and the force of gravity.
HS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
Discussion
In part one the results should be fairly clear. Students should see no change in the control beaker and varying degrees of decolorization between the “granules” and “ground” beakers. Ideally the “ground” charcoal should completely decolorize the water. Grinding the charcoal into a powder effectively increases the available surface area and thus increases the adsorptive capacity of a given quantity—in this case one or two teaspoons—of charcoal.
The comparison of odors seems to be somewhat more subjective than the comparison of colors as the nose can easily become desensitized to more subtle aromas. Perhaps the odor evaluator(s) can be “blindfolded,” either literally or figuratively, to render the results of odor experiments more scientifically valid.
References
DeWayne Lieneman, Glenbard South High School, Glen Ellyn, Illinois 60137
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