Question #3795a

Keep in mind that carbon dioxide itself does not change the color of dry litmus paper.

However, a solution of carbon dioxide will turn blue litmus paper red, which implies that a carbon dioxide solution is acidic.

You can get the same result by exposing carbon dioxide to moist litmus paper--the gas will react with the moisture to produce an acidic solution, thus turning blue litmus paper red.

![https://science7acidbase.wikispaces.com/Litmus+Paper?responseToken=33ddb9af2f206f24b2c8f94066be55c6]()

The idea here is that when you dissolve carbon dioxide in water

`"CO"_ (2(g)) rightleftharpoons "CO"_ (2(aq))`

you get an acidic solution because some of the dissolved carbon dioxide will react with water to form carbonic acid, `"H"_2"CO"_3`, a weak acid.

`"CO"_ (2(aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 2"CO"_ (3(aq))`

So depending on the actual concentration of carbon dioxide in solution, the `"pH"` of the solution can be low enough--the `"pH"` of the solution must be lower than `4.5` in order for the color of the litmus paper to change--to turn blue litmus paper red.

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