Question

How is $C{{O}_{2}}$ gas tested in the laboratory?

Answer 1

The chemical used for testing carbon dioxide in the laboratory is an aqueous solution of a hydroxide of an alkaline earth metal. Sometimes carbon dioxide ($C{{O}_{2}}$) can also be tested by using a flame. The formation of insoluble white precipitate indicates the presence of carbon dioxide.

Complete step by step answer:
There are two tests for the detection of carbon dioxide.
Firstly, $C{{O}_{2}}$ will extinguish a flame, though, as any oxygen free gas will also turn off a flame, this is a poor test and may possibly result in misidentification.
The best way of testing Carbon dioxide is to bubble it through “lime water”.

$Ca{{(OH)}_{2}}_{(aq)}+C{{O}_{2}}_{(g)}\to CaC{{O}_{3}}_{(s)}+{{H}_{2}}{{O}_{(l)}}$
(Lime water)

In the above reaction carbon dioxide reacts with calcium hydroxide and forms calcium carbonate and water.
After passing $C{{O}_{2}}$through lime water it turns into milky. Lime water turns milky as the $Ca{{(OH)}_{2}}$(Calcium hydroxide) reacts with $C{{O}_{2}}$to form $CaC{{O}_{3}}$(Calcium Carbonate) which is insoluble in water and thus forms a milky white precipitate.

Additional Information:
Let us see the ways to prepare $C{{O}_{2}}$.
Prepare a small amount of hydrochloric acid ($HCl$), of 2 M concentration and add few granules of $CaC{{O}_{3}}$in to a test tube and close it by bung (a small rubber cork).

$CaC{{O}_{3(s)}}+2HC{{l}_{(aq)}}\to CaC{{l}_{2(aq)}}+{{H}_{2}}{{O}_{(l)}}+C{{O}_{2(g)}}$

In the above reaction calcium carbonate reacts with hydrochloric acid and forms calcium chloride, water and carbon dioxide as a gas.
Now we can use the formed $C{{O}_{2}}$for testing purposes.

**Note:****Don't be confused between Calcium Hydroxide with Calcium oxide. Both are not the same. Calcium oxide has the chemical formula CaO while calcium hydroxide has the chemical formula $Ca{{(OH)}_{2}}$.