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Water filter molecules

Dye is added to a few grains from a home water filter. After a minute of shaking the water is clear as the dye molecules stick to the filter grains. In a home water filter, the same process removes calcium, magnesium, chlorine, lead and copper from drinking water.
Science content (2016 curriculum): 
Chemistry: Atoms, Molecules (3-7)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Science topic (2005 curriculum connection): 
Earth and Space Science: Air, Water and Soil (grade 2)
Physical Science: Chemistry (grade 7)
  • microcentrifuge tubes
  • dropper bottle with methylene blue dye, diluted to 0.0075ml in 100ml water
  • water filter grains (from inside a home water filter)
  • 15 sec timer or stopwatch
  • dropper
  • scoop
  • tape
  • pencil

Do you have a water filter at home, school or work? Lots of people drink filtered water. How does a filter work? Do an experiment to find out.

Take a scoop of grains from the water filter. This is a Brita filter, used in many home water filters. What do the filter grains look like?
Put a full scoop of the filter grains in a tube. Add blue dye to fill the tube halfway. Snap the tube closed.
Half fill a new tube with blue dye only. Close the tube. This is your standard. Later you will compare it to the tube containing filter grains.
Start the timer. Shake both tubes hard until the timer is done. Compare the two tubes.
What have the filter grains done to the dye? Where has the dye gone?

How do the filter grains work?
The dye molecules stick to the filter grains: Look at the round beads in your experiment. You may be able to see the blue dye stuck to them.

What happens in a home water filter?
In our home water filters, the filter grains remove unwanted atoms from tap water. Like the dye in your experiment, the atoms stick to the filter grains:
Calcium and magnesium are natural, harmless atoms in water, but they cause blockage in pipes.
Chlorine is added to drinking water to kill harmful bacteria. Some people don't like its taste.
Lead and copper leach from old pipes and are toxic in large amounts.
Although you could see the dye in your experiment, you can't see these atoms in tap water — they are too small and are not colored like the dye.

How much dye can the filter grains remove?
Open your tube containing filter grains and water. Use a dropper to suck the water from above the filter grains. Squirt the water into the trash. Add more dye to the grains. Start the timer and shake the tube again. Do the grains remove these dye molecules as well?

The filter grains in your experiment should have removed more dye molecules from the water. The round beads should have more blue dye stuck to them. In the same way, the filter grains in home water filters can be used over and over to remove unwanted atoms from the drinking water. Eventually the filter grains get full and cannot remove any more atoms. Then you need to replace the filter with a new one.

1. How many filter grains do you need to remove all the dye molecules? Find out by adding different amounts of grains to different tubes. Make sure you add the same amount of dye to each tube. Make sure you shake all the tubes for the same amount of time.
2. How long do you need to shake to remove all the dye? Try shaking different tubes for different times. Make sure the only thing that is different between the tubes is the shaking time — put the same amount of grains and dye in each tube.
3. Try your own experiments. Remember to change only one thing at a time, so you know why you see a difference between tubes.

Attached documents: 

Tested with adults and children of all ages in family groups.

Grades tested: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Self guided
Teaching site: 
New York Hall of Science