Co2 is not a liquid unless under pressure, otherwise it is a gas. Products such as Excell are not CO2, but an organic carbon source that plants can use. I am not sure what product you are talking about, but if you have a link or a name I will be happy to take a look at it.
It is put into the tank as a gas and is dissperesed as bubbles and the longer it stays in contact the water the more is absorbed but you have to be careful becasue too much can kill the fish and it can also cause PH swings. I think over 30 something is bad.