How to Identify Carbon Dioxide

How can we identify that the gas produced after reacting calcium carbonate with vinegar is carbon dioxide?
- Anonymous

A real chemist could give a much better answer, but we like playing with these "how to" questions.

If you happen to have access to a device for measuring the optical absorption as a function of frequency (a spectrophotometer)  CO₂ has a characteristic absorption spectrum in the infrared. That would be a good signature. 

You probably don't have a spectrophotometer. You could still get an infrared light and notice that a plastic bottle of this gas absorbs much more of that light (detectable as heat) than does plain air. (That's why it's a strong greenhouse gas, driving global warming.)  You could compare with a bottle of plain CO₂ and notice that it matches.

You could weigh a bottle of the gas at standard temperature and pressure and note that it weighs the same as one of CO₂ , more than one of air or water vapor.

You could note that the gas will put out a candle, unlike air. When mixed with air, it won't burn, unlike methane.

If you let some distilled water sit for a while in the gas it will become a little acidic. You can check that the pH is the same as what you get for CO₂ and different from what you get for some other gases. You could also check that the electrical conductivity of the solution matched that for CO₂, not for other familiar gases.

If you have a good source of cold, like some liquid nitrogen, you could see that the gas froze into dry ice. The general physical appearance, as well as the freezing temperature, would be good signatures of CO₂.

Maybe readers can come up with some more good tests.

Mike W.

(published on 02/02/2014)