dilute solution sulphuric acid acts upon many
metals, such as zinc, forming a sulphate and liberating hydrogen. When
the concentrated acid is employed the hydrogen set free is oxidized by a
new portion of the acid, with the liberation of sulphur dioxide. With
copper the reactions are expressed by the equations

(1) Cu + H_{2}SO_{4} = CuSO_{4} + 2H,

(2) H_{2}SO_{4} + 2H = H_{2}SO_{3} + H_{2}O,

(3) H_{2}SO_{3} = H_{2}O + SO_{2}.

By combining these equations the following one is obtained:

Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.

4. _Action on salts._ We have repeatedly seen that an acid of high
boiling point heated with the salt of some acid of lower boiling point
will drive out the low boiling acid. The boiling point of sulphuric acid
(338 deg.) is higher than that of almost any common acid; hence it is used
largely in the preparation of other acids.

5. _Action on water._ Concentrated sulphuric acid has a very great
affinity for water, and is therefore an effective dehydrating agent.
Gases which have no chemical action upon sulphuric acid can be freed
from water vapor by bubbling them through the strong acid. When the acid
is diluted with water much heat is set free, and care must be taken to
keep the liquid thoroughly stirred during the mixing, and to pour the
acid into the water,--never the reverse.

Not only can sulphuric acid absorb water, but it will often withdraw the
elements hydrogen and oxygen from a compound containing them,
decomposing the compound, and combining with the water so formed. For
this reason most organic substances, such as sugar, wood, cotton, and
woolen fiber, and even flesh, all of which contain much oxygen and
hydrogen in addition to carbon, are charred or burned by the action of
the concentrated acid.

~Salts of sulphuric acid,--sulphates.~ The sulphates form a very important
class of salts, and many of them have commercial uses. Copperas (iron
sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium
sulphate) serve as examples. Many sulphates are important minerals,
prominent among these being gypsum (calcium sulphate) and barytes
(barium sulphate).

~Thiosulphuric acid~ (H_{2}S_{2}O_{3}); ~Thiosulphates.~ Many other
acids of sulphur containing oxygen are known, but none of them
are of great importance. Most of them cannot be prepared in a
pure state, and are known only through their salts. The most
important of t