Carbon filter

Purify water with a do-it-yourself filter!

15 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

I poured carbon into the pipette over the 3 mL mark. What do I do?

Simply pour the excess back into the bottle of activated carbon and continue with your experiment.

Some of the carbon is sticking to the filter neck. Is this okay?

Absolutely, feel free to continue the experiment.

A blue solution is dripping from the pipette.

Make sure you inserted two cotton cylinders: one before and one after adding the carbon. It’s also possible the carbon was not tamped down properly. Repeat the experiment, paying attention to these details.

Another possible reason why the filter isn’t working is that the pieces of carbon are too big. If this seems to be the case, pour your carbon into a clean plastic cup and grind them into finer pieces using a wooden stick. Alternatively, you can do this in the filter, but only after removing the upper cotton cylinder.

I’ve set up the filter, but water doesn’t seem to be passing through it.

If this happens, or if the water is moving too slowly, there is probably too much air inside the carbon part of the filter.

First, press down on the upper cylinder a bit with a wooden stick. Then, squeeze the filter walls with your fingers to force the air out. This action is rather similar to the one you would make when squeezing toothpaste from a tube. Just be careful not to spill the liquid. This should work!

If this still doesn’t help, try to repeat the experiment.

The water in the vial is gray! What’s wrong?

Actually, nothing! This grey substance is none other than some activated carbon particles. Some of them are very tiny and just passed through the cotton cylinders while the organic contaminant (methylene blue) was filtered out.

This same thing happens in everyday household carbon filters as well. Normally, you need to cycle water through them 2-3 times before they are ready to use!

When you finish filtering the first part of the solution, just wash the vial and try to filter some more – the water will gradually become clear and free of carbon pieces.

Step-by-step instructions

Assemble a carbon filter. To make it more effective, compress the activated carbon a bit using cotton cylinders and a wooden stick.


Set up the test stand.


Make an organic pollutant solution using water and methylene blue. Methylene blue is widely applied in industry to test the ability of filters to effectively clean drinking water.


The filter adsorbs the organic pollutant, and purified water fills the vial.


To repeat the experiment, wash the vial.

Expected result

Coal sorbs methylene blue molecules from water leaving the water clear and colorless.


Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

Pesticides used to treat crop fields, chemical manufacturing residue, and other manmade pollutants first seep into the soil, then trickle into rivers, lakes and into our drinking water. Since many of these substances are harmful, our water must be purified before we can drink it.

Among these pollutants there are rather big organic molecules . Methylene blue  has a similar structure and gives water a strong color, making it a good substance for testing filters.

Particles with such a structure readily “stick” to the charcoal surface (adsorbing to it). Activated carbon is made of charcoal, but unlike ordinary charcoal it has a much larger surface area due to its huge amount of cracks and pores. This is why so many pollutant molecules “stick” to it, while water molecules  pass through easily.

The carbon filter deals with many organic pollutants well, but can’t do much with some other harmful substances, like heavy metal ions . To get rid of these, ion-exchange resins are used, as we do in another experiment in this set.

Why does the solution become colorless?

The molecules of methylene blue, the coloring substance we use in this experiment, are adsorbed by the activated carbon, so the solution becomes less concentrated and almost colorless.

What is adsorption?

This experiment is possible due to a very important process called sorption. This process can be of either a chemical, physical or mechanical nature, but its working principle is always the same: we have a substance, which can “soak up” other substances, removing them from a solution or reaction mixture. In the case of the activated carbon, sorption takes place on its surface. This type of sorption is called adsorption, while activated carbon is the adsorbent. It “soaks up” methylene blue from the solution.

What does activated carbon do?

Activated carbon is a very porous substance. The molecules of methylene blue get “trapped” in the pores on the surface of activated carbon and can’t escape. As a result, less of the molecules of this coloring are left in the solution after it passes through the carbon filter, making the solution clearer.

Learn more

The filters that we use in everyday life also use activated carbon as an adsorbent. However, activated carbon on its own is not enough to purify water. It can adsorb organic pollutants but is less effective against bacteria and heavy metals. To get rid of the latter, the manufacturers use so-called ion-exchange resins. These materials can absorb (absorption is another type of sorption - when the process takes place all throughout the sorbent volume, not just on its surface) the heavy metals like iron, mercury, and cadmium. These absorbents are sometimes enriched with silver ions. This way they can also fight harmful microorganisms. We use such materials in the "Ion-exchange resin" experiment.

Why do we use methylene blue?

Firstly, its intense color helps us estimate visually how effectively the filter works. Secondly, the chemical structure of this coloring is very close to those of widespread organic pollutants such as dioxins (see below: dioxin, left, and methylene blue, right). So if a filter can clear a solution of methylene blue, it is expected to perform well regarding real pollutants too.


Can I drink the filtered water?

Despite filtering out most of the methylene blue, our filter is not yet effective enough, so do not drink the filtered water!