|MadSci Network: Chemistry|
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.  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. 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. Regards Charlie A. Crutchfield
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