Question #3795a

1 Answer
Jun 22, 2017


Here's what happens here.


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

#"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.