Preparation of Carbon Dioxide

Carbon dioxide is generally prepared in the laboratory by the action of somewhat diluted hydrochloric acid on marble:

CaCO₃ + 2HCl = CaCl₂ + H₂O + CO₂.

The appearance of the gas is preceded by the liberation of carbonic acid, H₂CO₃, which, being unstable, decomposes almost completely into water and carbon dioxide. The evolved gas, which contains traces of hydrochloric acid, may be purified by passing it through sodium hydrogen carbonate solution, or it may be collected over water instead of by the more usual method of upward displacement of air.

Dilute sulphuric acid cannot be employed in place of hydrochloric acid because the marble quickly becomes encrusted with almost insoluble calcium sulphate which stops the action; if, however, concentrated sulphuric acid, containing only a very little water, is used, the evolution of gas continues because an acid sulphate of calcium is formed which remains in solution.

If sodium carbonate is employed as the source of carbon dioxide the evolution of the gas is vigorous, whether dilute hydrochloric or sulphuric acid is employed.

Sodium hydrogen carbonate is a convenient source of carbon dioxide, which it evolves when gently heated in the solid state or in solution:

2NaHCO₃ = Na₂CO₃ + H₂O + CO₂.

Normal sodium carbonate is not decomposed by heat until a very high temperature is reached; calcium carbonate, being the salt of a less powerful base, begins to yield its carbon dioxide at about 550° C.; magnesium carbonate, in the form of the mineral magnesite, gives off carbon dioxide when gently ignited, and is therefore sometimes employed as a source of the gas.

Carbon dioxide may also be prepared by the combustion of charcoal or coke, and purified from the atmospheric nitrogen with which it will be mixed, by causing it to combine with an alkali carbonate, whence it is again set free by the action of acids.