Re: How do I trap a gas in a bottle? Also, how can I put gas in it?

I'm assuming you have a tank of each gas you wish to use. 

A general technique is to submerge the container you want to fill with gas in a
larger container of water. Invert the smaller container, then fill it with gas
from a tank using a plastic tube to carry the gas into the container. The gas
will displace the water. When the container is filled with gas, insert the
potted plant. Then put a lid on the container, all while underwater.

The potted plant can be kept from getting waterlogged by placing it in a plastic
bag before submerging it. Remove the plastic bag after getting it in the
gas-filled container and before putting the lid on.

Hershey (1992, 1995) has a system for growing plants in 2-liter soda bottles
without carbon dioxide gas. 

References

Hershey, D.R. 1992. Plants can't do without CO2. Science Teacher 59(3):41-43.

Hershey, D.R. 1995. Plant Biology Science Projects. New York, Wiley.