they are called electrolytic conductors or more briefly electrolytes (from a Greek verb, luo, I loose). When an electric current is passed through a solution of hydrochloric acid, for example, chlorine is set free at one electrode and hydrogen at the other. Again, when the current is passed through sodium chloride solution, chlorine is set free at the same electrode as in the case of hydrochloric acid; at the other electrode sodium is, no doubt, set free, but it immediately reacts with the water, producing hydrogen and sodium hydroxide. What we observe at the latter electrode, therefore, is that hydrogen gas is given off and that the water becomes alkaline, due to the production of sodium hydroxide. When a current is passed through an aqueous solution of sulphuric acid, the products obtained are hydrogen at one electrode, oxygen at the other. (See Expt. 16, p. 12.) This is quite consistent with the supposition that the primary products are hydrogen and the sulphuric acid (or sulphate) radicle =SO4, and that the latter, being incapable of independent existence, reacts with water:

=SO4 + H2O = H2SO4 + O

Thus, oxygen is liberated and the sulphuric acid is regenerated. The net result is, therefore, the decomposition of water, and we ordinarily speak of the process as the "electrolysis of water."

The passage of a current through sodium hydroxide solution likewise yields hydrogen and oxygen as final products. The primary products are (a) sodium, which reacts with water, liberating hydrogen and regenerating the sodium hydroxide:

(b) the hydroxyl radicle, OH, which is immediately converted into water and oxygen:

2. Reactions between dissolved acids and bases (neutralization) are instantaneous. So, also, are reactions between two salts (precipitation reactions), e.g.:

=

Silver nitrate + Sodium chloride Silver chloride + Sodium nitrate Reactions between non-electrolytes in solution are usually much slower.

3. All the electrolyte chlorides give the same precipitate (silver chloride) with all silver salts. All sulphates (including sulphuric acid) give the same precipitate (barium sulphate) with all barium salts.

Experiment 57.

Materials:

Solutions of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, aluminium chloride.

Solutions of silver nitrate and silver sulphate.

Solutions of sodium sulphate, potassium sulphate, magnesium sulphate.

Solutions of barium chloride, barium nitrate, and barium

acetate.

Mix a little of each chloride solution with a little of each silver solution (10 experiments in all) and compare the precipitates produced. Also mix dilute hydrochloric acid with a little of each of the silver solutions.

Mix a little of each sulphate solution with a little of each barium solution (9 experiments) and compare the precipitates. Also mix dilute sulphuric acid with a little of each of the barium solutions.

These and other peculiarities of electrolyte solutions are explained by assuming that when an electrolyte dissolves in water, some of its molecules immediately undergo a reversible decomposition into what are called ions.

The mode of ionization of a few acids, bases, and salts is illustrated by the following equations:

There

The double arrows, used in place of the usual equality sign, signify that the reaction is a reversible one. are always present in the solution some un-ionized molecules of the acid, base, or salt. In some instances, e.g. acetic acid, most of the molecules are un-ionized; in others, e.g. sodium chloride, there are only a few un-ionized molecules. When a solution is diluted (by adding more water), more of the molecules ionize. When the solution is concentrated (by evaporating off some of the water), some of the ions recombine into molecules; when it is evaporated to dryness, all of the ions recombine into molecules.

All acids yield the hydrogen ion, H+, as their positive ion (cation). The effects of acids on litmus may, therefore, be regarded as an action of the hydrogen ion. When an acid acts on a metal, the hydrogen ions are converted into molecules of hydrogen gas and the molecules of metal are converted into metal ions.

All bases yield the hydroxyl ion, —OH, as their negative ion (anion). The action of bases on litmus or other indicators is, accordingly, a reaction of the hydroxyl ion. The neutralization of an acid by a base involves the combining of the hydrogen ions with the hydroxyl ions to form water molecules. Thus, when hydrochloric acid and sodium hydroxide

solutions are mixed, the hydrogen and hydroxyl ions disappear, but the sodium and chloride ions remain:

Na+ + OH + H+ + Cl ̄ = Na+ + Cl ̄ + H2O

When the insoluble base magnesium hydroxide, Mg(OH)2 is acted upon by hydrochloric acid, the magnesium is converted into ions:

Mg(OH)2 + 2 H++ 2 Cl = Mg++ + 2 Cl ̄ + 2 H2O

CHAPTER XVIII

METAL TARNISHES

Most metals, when heated in oxygen or air, readily combine with the oxygen. The following are examples of oxides formed from metals in this way:

Magnesium oxide or magnesia, MgO. (See Expt. 51, p. 96.)
Calcium oxide or quicklime, CaO.
Aluminium oxide or alumina, Al2O3.

Cupric oxide (black), CuO.

Magnetic iron oxide (black), Fe3O4.
Sodium peroxide, Na2O2.

In the presence of moisture such combination of metal with oxygen occurs also at ordinary temperatures, though at a much slower rate than when the metal is heated. In case the oxide formed upon the surface adheres closely to the metal, the oxidation soon comes to a stop, because the film of oxide prevents the air coming in contact with any more of the metal. Magnesium, zinc, and aluminium form lightcolored tarnishes of this kind, and consequently retain their whitish color, although losing something of their metallic luster. Lead also tarnishes rapidly by oxidation, but the oxide formed is darker in color than the metal itself.

Experiment 58.

Materials:

Magnesium ribbon.

Zinc sheet or rod.

Aluminium.

Lead.

Emery cloth.

Polish a little of each of the metals and compare the freshly polished with the tarnished surface.