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

Answer

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Hint: 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”.

C a {(O H)}_{2}_{(a q)} + C {O_{2}}_{(g)} \to C a C {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

C a {(O H)}_{2}

(Calcium hydroxide) reacts with

C O_{2}

to form

C a C 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 (

H C l

), of 2 M concentration and add few granules of

C a C O_{3}

in to a test tube and close it by bung (a small rubber cork).

C a C O_{3 (s)} + 2 H C l_{(a q)} \to C a C l_{2 (a q)} + 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 $C a {(O H)}_{2}$ .