istance in writing
the equations for reactions of double decomposition. Thus, in the
general reaction between an acid and a base, the essential action is
between the univalent hydrogen ion and the univalent hydroxyl ion. The
base and the acid must always be taken in such proportions as to secure
an equal number of each of these ions. Thus, in the reaction between
ferric hydroxide (Fe(OH)_{3}) and sulphuric acid (H_{2}SO_{4}), it will
be necessary to take 2 molecules of the former and 3 of the latter in
order to have an equal number of the two ions, namely, 6. The equation
will then be

2Fe(OH)_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 6H_{2}O.

Under certain conditions the salts Al_{2}(SO_{4})_{3} and CaCl_{2}
undergo double decomposition, the two metals, aluminium and calcium,
exchanging places. The simplest ratio of exchange in this case is 2
atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).
The reaction will therefore take place between 1 molecule of
Al_{2}(SO_{4})_{3} and 3 of CaCl_{2}, and the equation is as follows:

Al_{2}(SO_{4})_{3} + 3 CaCl_{2} = 3CaSO_{4} + 2AlCl_{3}.

EXERCISES

1. Sodium, calcium, and aluminium have valences of 1, 2, and 3
respectively; write the formulas of their chlorides, sulphates, and
phosphates (phosphoric acid = H_{3}PO_{4}), on the supposition that they
form salts having the normal composition.

2. Iron forms one series of salts in which it has a valence of 2, and
another series in which it has a valence of 3; write the formulas for
the two chlorides of iron, also for the two sulphates, on the
supposition that these have the normal composition.

3. Write the equation representing the neutralization of each of the
following bases by each of the acids whose formulas are given:

NaOH HCl
Ba(OH)_{2} H_{2}SO_{4}
Al(OH)_{3} H_{3}PO_{4}

4. Silver acts as a univalent element and calcium as a divalent element
in the formation of their respective nitrates and chlorides. (a) Write
the formula for silver nitrate; for calcium chloride. (b) When
solutions of these two salts are mixed, the two metals, silver and
calcium, exchange places; write the equation for the reaction.

_5._ Antimony acts as a trivalent element in the formation of a
chloride. (a) What is the formula for antimony chloride? (b) When
hydrosulphuric acid (H_{2}S) is passed into a solution of this chloride
the hydrogen and antimony exchange places; write the