e. Thus lime (calcium oxide) is made by strongly
heating calcium carbonate:

CaCO_{3} = CaO + CO_{2}.

~Acid carbonates.~ Like all acids containing two acid hydrogen atoms,
carbonic acid can form both normal and acid salts. The acid carbonates
are made by treating a normal carbonate with an excess of carbonic acid.
With few exceptions they are very unstable, heat decomposing them even
when in solution.

~Action of carbon dioxide on calcium hydroxide.~ If carbon dioxide is
passed into clear lime water, calcium carbonate is at first
precipitated:

H_{2}O + CO_{2} = H_{2}CO_{3},

Ca(OH)_{2} + H_{2}CO_{3} = CaCO_{3} + 2H_{2}O.

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:

CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}.

If now the solution is heated, the acid carbonate is decomposed and
calcium carbonate once more precipitated:

Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}CO_{3}.

~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:

C + H_{2}O = CO + 2H.

2. _By the partial reduction of carbon dioxide._ When carbon dioxide is
conducted over highly heated carbon it is reduced to carbon monoxide by
the excess of carbon:

CO_{2} + C = 2CO.

When coal is burning in a stove or grate carbon dioxide is at first
formed in the free supply of air, but as the hot gas rises through the
glowing coal it is reduced to carbon monoxide. When the carbon monoxide
reaches the free air above the coal it takes up oxygen to form carbon
dioxide, burning with the blue flame so familiar above a bed of coals,
especially in the case of hard coals.

3. _By the decomposition of oxalic acid._ In the laboratory carbon
monoxide is usually prepared by the action of concentrated sulphuric
acid upon oxalic acid. The latter substance has the formula
C_{2}H_{2}O_{4