H2O + CO2 = H2CO3,
Ca(OH)2 + H2CO3 = CaCO3 + 2H2O.
Advantage is taken of this reaction in testing for the presence of carbon dioxide, as already explained in the chapter on the atmosphere. If the current of carbon dioxide is continued, the precipitate soon dissolves, because the excess of carbonic acid forms calcium acid carbonate which is soluble:
CaCO3 + H2CO3 = Ca(HCO3)2.
If now the solution is heated, the acid carbonate is decomposed and calcium carbonate once more precipitated:
Ca(HCO3)2 = CaCO3 + H2CO3.
Carbon monoxide (CO). Carbon monoxide can be made in a number of ways, the most important of which are the three following:
1. By the partial oxidation of carbon. If a slow current of air is conducted over highly heated carbon, the monoxide is formed, thus:
C + O = CO
It is therefore often formed in stoves when the air draught is insufficient. Water gas, which contains large amounts of carbon monoxide, is made by partially oxidizing carbon with steam: