Describe what you see in a positive test for carbon dioxide?
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You can perform what is called the limewater test for carbon dioxide. Carbon dioxide is CO2 and when it is bubbled through water it will dissolve and form carbonic acid (H2CO3) which is mildly acidic. If CO2 is bubbled through limewater (which is Ca(OH)2 dissolved in water), the carbonic acid produced will react with the Ca(OH)2 to produce calcium carbonate (CaCO3). The reaction is shown below:
Ca(OH)2 + H2CO3 --> CaCO3 + 2H2O
Calcium carbonate is basically chalk and is poorly soluble in water. So it will precipitate from the solution and turn it milky white. So if an unknown gas is bubbled through limewater and the result is a milky white solution, you have a positive test for carbon dioxide.
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