# Question #3795a

Jun 22, 2017

Here's what happens here.

#### Explanation:

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.

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

${\text{CO"_ (2(g)) rightleftharpoons "CO}}_{2 \left(a q\right)}$

you get an acidic solution because some of the dissolved carbon dioxide will react with water to form carbonic acid, ${\text{H"_2"CO}}_{3}$, a weak acid.

${\text{CO"_ (2(aq)) + "H"_ 2"O"_ ((l)) rightleftharpoons "H"_ 2"CO}}_{3 \left(a q\right)}$

So depending on the actual concentration of carbon dioxide in solution, the $\text{pH}$ of the solution can be low enough--the $\text{pH}$ 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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