We know that we cannot live without water. Thankfully, the Earth is covered by 71% water. Only 3% of that, however, is freshwater, and to make matters worse, just 0.5% is available to us. The remaining 2.5% is locked up in glaciers, polar ice caps, soil, and the atmosphere. If we could represent all of the world's water by one liter, only 5 mL (1 teaspoon) would be available for us to drink. And yet to drink that water, we must first purify it from the lakes and rivers. Let’s take a hint from nature to see how we can do that.
Materials
- Coffee filters
- Container of water with impurities (dirt, twigs, etc.)
- Gravel
- Plastic bottles, 1- or 2-liter size, 2
- Rubber bands
- Sand
- Scissors
- Optional: Activated charcoal, food coloring
BE SAFE: Adult supervision is necessary when cutting the bottle.
Preparation
Obtain water in a container and artificially “pollute” it by adding dirt, twigs, food coloring, etc. (Be creative!) Carefully cut two plastic bottles in half using scissors.
Experiment
Fold one of the coffee filters in half. Place the filter on top of the bottle-cap opening on one of the plastic bottles and secure the filter in place using a rubber band. For the second bottle, fold the coffee filter three times before securing it to the bottle-cap opening. (How many layers of paper are there for the water to flow through in each bottle?)
Place each filtering unit upside down into the bottom half of one bottle. To each filtering unit, add enough sand so it is about 2 - 3 cm high. Then add 2 - 3 cm of gravel on top of the sand. Shake the container of dirty water and pour about 1 cup of “dirty” water on top of the gravel in each unit. Observe the appearance of the water as it passes through the filters.
What’s happening?
Filtration is a simple concept but it is not necessarily obvious the order in which the filtering agents (gravel, sand, and filter paper) should be used. Having the gravel on top, sand in the middle, and filter paper at the bottom allows large particles such as twigs to be removed first, followed by finer particles. If the order were reversed, with filter paper on top and gravel on the bottom, large particles would get caught on the filter paper and cause it to become plugged. This would prevent water from flowing through the filter.
Tripling the number of layers of filter paper in the second filtration unit should result in cleaner water passing through it compared to the first. (Would you expect similar results using 6 - 9 cm of sand rather than 2 - 3 cm?) Additionally, the slower the water travels through the filters the more time the particles have to interact with the filter materials and get removed from the water. Using activated charcoal can add another layer of filtration to help remove charged particles.
The filtered or purified water obtained is still not clean enough to drink! Bacteria and other pathogens in water that can make you ill are NOT removed by simple filtration like in this activity. Outdoor water is normally boiled to kill bacteria before you can drink it.
References
https://kids.nationalgeographic.com/books/article/water-wonders
To view all past “ChemShorts for Kids” articles, go to:
https://chicagoacs.org/ChemShorts
