Testing for Gases

The Test for Carbon Dioxide Gas,  CO2(g).

1)  Carbon dioxide gas has no colour or smell.

2)  Carbon dioxide gas will put out a lit splint.

3)  Carbon dioxide gas will turn moist litmus paper from blue to red,
and moist universal indicator paper to yellow.

Carbon dioxide dissolved in water forms carbonic acid (H2CO3).
Carbonic acid is a weak acid (see the carbon cycle).


Specific Test for Carbon Dioxide Gas.

Carbon dioxide will turn lime water 'milky' or cloudy.

Lime water is a solution of calcium hydroxide (slaked lime).
When carbon dioxide is bubbled through it,
a solid precipitate of calcium carbonate is formed.
Calcium carbonate is chalk or limestone,
and it is this that makes the lime water cloudy.

calcium hydroxide  +  carbon dioxide  arrow  calcium carbonate  +  water.
  Ca(OH)2(aq)          +      CO2(g)          arrow      CaCO3(s)            +  H2O(l)

Collecting Carbon Dioxide Gas.

Carbon dioxide gas is more dense than air and can be
collected by downward delivery or using a gas syringe.

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