hii how can we design an experiment to prove that indoorplants remove formaldehyde benzene ethylene like gas?

Question

this is for science fair high school projects just need help to design experiment

Answer 1

Not easy.
You must have the gas you want to test, and you'll have to be able to detect it.

Once you have that, it's easy:
Two glass containers (or transparent plastic), one with plants, the other with only pots and soil, but no plants.

Inject your gas into both containers, or give both a gas-source that produces the gas.

If your assumption (plants remove the gas) is correct, you should be able to detect a difference in concentration between the two containers.

The pots&soil in the second container are there to make sure that the only difference between the two containers is the presence of plants. Hence, if the concentration of gas is different, it must be because of what makes the 1st container different from the second one.