MadSci Network: Chemistry

Re: How does anyone verify a carbon dioxide (CO2) gas in an Experiment?

Date: Thu Oct 29 01:38:12 1998
Posted By: Charlie Crutchfield, , Retired, Retired
Area of science: Chemistry
ID: 909277345.Ch

Dear Sam:
   Here is a simple way to determine if a gas is Carbon Dioxide. It is 
based on two properties of the gas. It is very soluble in water and many 
carbonates are insoluble in water. And all carbonates dissolve in stronger 
acids. Also, remember that Carbon Dioxide is nearly odorless. Any gas with 
a pronounced odor is not carbon dioxide.

[1] First,for either of these methods, you must have some apparatus to 
catch the gas as it is formed and pass it into pure water [distilled or 
deionized]in which the gas will be dissolved. You are probably using some 
acid to generate the gas, if so, you must be very carefull to prevent any 
of this acid from being carried over with your unknown, either as a gas 
itself, or as droplets.

[2]Prepare some Lime-Water as follows: To about a liter [or a quart] of 
pure water add about one teaspoon [or about 5 grams]of Calcium Hydroxide 
[Lime].Shake well and leave stand until most of the insoluble powder 
settles out- the clear liquid is the "Lime-Water". Cap the container until 
you want to use it. It wil last a long time is you keep it sealed

Set up a filter with paper filter, filter the clear Lime-Water into the 
vessel into which you will pass and catch the unknown gas.
Pass the gas into the Limewater. Carbon Dioxide will react with the 
dissolved Lime to form white, insoluble, Calcium Carbonate which will 
appear as a white cloudiness in your Lime-Water.

If this white cloud appears, there is a second confirmation test. Add some 
strong acid to the cloudy test solution.[Hydrochloric is preferred, but 
Citric or Tartaric will work just as well] Calcium Carbonate will dissolve 
rapidly when acid is added, but [say] Calcium Sulfate will not.

Caution: Remember, normal air contains enough Carbon Dioxide to interfere 
with your test.

An easy way to check and try out this test is with your own breath - which 
contains several percent Carbon Dioxide. Blow through a tube - or soda 
straw-into some filtered Lime Water and observe the white cloud. Add acid 
and watch the cloud vanish. Do this again with a fresh solution, blow your 
breath into it and after the cloud forms, keep blowing. After a time you 
will find the cloud has vanished -the Calcium Carbonate has changed to 
Calcium Bicarbonate.
                      Charlie A. Crutchfield

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