Representing, for brevity's sake, hydrogen by the letter H., and chlorine, bromine, iodine, etc., respectively by Cl., Br., and I., the constitution of the acids derived from these bodies may be conveniently represented by:—
| H Cl | H Br | H I |
| ┗━━┛ | ┗━━┛ | ┗━━┛ |
| Hydrochloric Acid[5]. | Hydrobromic Acid. | Hydriodic Acid. |
and the more complex acids, in which the electro-negative component is a compound, such as sulphuric acid (built up of 1 atom of sulphur and 4 atoms of oxygen, united to 2 atoms of hydrogen) or nitric acid (consisting of 1 nitrogen atom, 6 oxygen atoms, and 1 hydrogen atom), may advantageously be retained in memory by the aid of the abbreviations:—
| H2SO4 | HNO6 | |
| ┗━━━┛ | ┗━━┛ | |
| Sulphuric Acid[6]. | and | Nitric Acid[7]. |
When zinc does act on an acid, it displaces the hydrogen contained in it, and takes its place; the acid losing at the same time its characteristic sourness and corrosiveness, becoming, as chemists say, neutralized. One atom of zinc can replace two atoms of hydrogen, so that one atom of zinc can replace the hydrogen in two equivalents of such acids as contain only one atom of hydrogen.
This power of displacement and replacement possessed by zinc is not peculiar to this metal, but is
possessed also by many other bodies, and is of very common occurrence in chemistry; and may be roughly likened to the substitution of a new brick for an old one in a building, or one girder for another in an arch.
It will be well, therefore, to remember that in all batteries in which acids are used to excite electricity by their behaviour along with zinc, the following chemical action will also take place, according to which acid is employed:—
| Hydrochloric Acid | and | Zinc, | equal | Zinc Chloride | and | Hydrogen Gas. |
| 2HCl | + | Zn | = | ZnCl2 | + | H2 |