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chlorite (formed by the interaction of the chlorine and potash) is simultaneously reduced. (8. par. 158.) When hydrogen is produced, by the interaction of zinc and dilute sulphuric acid, in contact with sodium sulphite (Na SO) in solution, the hydrogen is oxidised to water, and simultaneously the sodium sulphite is reduced to sodium oxide (Na,0) which reacts with the sulphuric acid present to form sodium sulphate (Na SO2), and hydrogen sulphide (H,S).

In these, and in very many other, cases, the processes of oxidation and reduction occur together as parts of a chemical change.

In the experiment with mercuric oxide and chlorine, the chlorine acted as the reducer or reducing agent, and the mercuric oxide as the oxidiser or oxidising agent. In the experiment with tellurium and nitric acid, the acid acted as the oxidiser and the tellurium as the reducer. When bismuthous oxide was oxidised by potassium hypochlorite, the latter was the oxidiser, the former the reducer: but the production of the hypochlorite was due to the interaction of chlorine with potassium hydroxide

(2KOHAq + 2Cl = KCIAq+KCIOAq), therefore it might be said that the chlorine was the primary oxidising agent. Similarly, if selenion is suspended in water and chlorine is passed into the liquid, selenic and hydrochloric acids are produced ; thus,

Se + 6Cl + 4H,0+ Aq=H SEO, Aq + 6HCIAq: the selenion is oxidised by the oxygen which before the change began was combined with hydrogen; therefore the water is the oxidiser, and the selenion is the reducing agent; but the interaction of chlorine is required to decompose the water,

therefore the primary oxidising agent is chlorine. 184 Many elements and compounds may be classified in accord

ance with their actions as oxidisers or reducers; or in accordance with the conditions under which they are oxidised or reduced. Hydrogen, carbon, sodium, carbon monoxide (CO), sulphur dioxide (SO2), nitrous acid (HNO,Aq), stannous chloride (SnCl), aldehyde (C,H,0), are some of the more commonly used reducing agents. Oxygen, ozone, chlorine, nitric acid, potassium chlorate (KCIOz), potassium permanganate (KMnO), are among the commonly used oxidising agents. The following equations present examples of the use of these reducers and oxidisers.

+1,0 + PbO, Lead monoxide suspended in concentrated warm potash solution

+2KCIO, =2KCÍ +3002 Carbon dropped into molten potassium chlorate. 3H,0,0,Aq + K,Mn, Aq=Kq0Aq+2Mn0,+ 3H,/ +6C0, Potassium permanganate solution dropped into a warm solution of

Action of reducing agents.



+ Zn
+ Cu
+ Be

+ 2Na

Original Reducer. Oxidised product. Reduced

Conditions. compound.


Passing hydrogen over heated ferric oxide.

Passing carbon monoxide over heated zinc oxide.

Heating carbon with copper oxide.
= 2NaCl

Sodium in contact with fused beryllium chloride.
Fe,350_Aq +SO, = 250Aq

+2FeSO.Aq Sulphur dioxide into warm solution of ferrio sulphate.

+H,0 Hydrogen peroxide solution in contact with solution of nitrous

acid. HgCl,aq +SnCl, Aq = SnCl Aq

+ Hg Solutions of mercuric chloride and stannous chloride mixed.
H,0+ 2AgNO3Aq+C,H,0 = 2HNO3Aq+C,H,02 + 2Ag Aldehyde added to warm solution of silver nitrate.


Action of oxidising agents
Original ele. Oxidiser. Reduced


Conditions. ment or com



H,0 Electric sparks through a mixture of hydrogen and oxygen.

= 20

+ HgO Ozone passed into mercury.

SbCl, Antimonious chloride heated in a stream of chlorine.
Pbo + 2KOH + 2Cl = 2KCI

and chlorine passed in.
Sn + 2HNO3 N,0

+H,0+Sn0, Tin heated with concentrated nitric acid. 3C

oxalic acid.



In all these instances of the action of reducing agents, the reduction of one substance is accompanied by the oxidation of another; in most of the instances of the action of oxidising agents, the oxidation of one substance is accompanied by the reduction of another.

In some cases, -e.g. reduction of beryllium chloride by sodium, reduction of mercuric chloride by stannous chloride, oxidation of antimonious chloride by chlorine-none of the substances taking part in the reactions is a compound of oxygen. It is customary to apply the term reduction to all chemical changes wherein the negative or non-metallic part of a compound is either wholly or partially removed, and the term oxidation to all chemical changes wherein the negative part of a compound is increased, or a negative element (or elements) is added to a more positive element (or elements).

Those elements which are easily oxidised might be placed in one class, and those which are oxidised with difficulty in another class; compounds which are easily and completely reduced might be classed apart from those which are only reduced at high temperatures and by indirect methods. Such a system of classification would be based, primarily at any rate, on the occurrence or non-occurrence under specified conditions of a certain chemical change; it would be based on a certain power of doing, rather than on the compositions, of the bodies classified. To make this classification fairly satisfactory it would be necessary to examine the compositions of the members of each class, and then to connect these compositions with the performance or non-performance of that chemical reaction which had been made the mark of each class.

We have had examples of classification founded on reactions rather than on composition. Oxides were divided into basic and acidic ; a great many compounds were placed in one class, and called acids, because they all interacted with metals and with basic oxides to produce salts.

Can we connect the composition of those oxides which are called basic, and the composition of those which are called acidic, with the facts that the former interact with acids to produce salts, and the latter interact with water to produce acids ? An answer to these questions will carry with it an answer to this; can we state in general terms the connexion between the composition of acids and the properties connoted by the term acid ?


Let us begin the inquiry by learning a little more about 188 the interactions of acids with metals, basic oxides, and alkalis, to produce salts. Let aqueous solutions of the three acids, hydrochloric (HCI), sulphuric (H SO.), and phosphoric (H PO,), be prepared, each containing a known mass of the acid in a specified volume ; let an aqueous solution of the alkali potassium hydroxide (KOH) be prepared containing a known mass of the compound in a specified volume. Let definite quantities of each acid solution be added to definite quantities of the alkali solution, and let all the products of each reaction be collected, examined, and analysed. The results may be represented as follows.

Reactions between aqueous solutions of hydrochloric acid (HCl) and potassium hydroxide (KOH). (1 gram HCl used in each case.) Grams KOH Grams salt Grams Grams KOH used. formed ; and

water remaining composition formed. unchanged.

of salt.

ei ei

1:54 1.54 x 2 1.54 x 3 1.54 x 4

2.04 KCI
2.04 KC
2.04 KCI
2.04 KCI


1:54 x 2
1:54 x 3

Reactions between aqueous solutions of sulphuric
acid (H,SO,) and potassium hydroxide. (1 gram H SO.
used in each case.)
Grams KOH Grams salt Grams Grams KOH

formed; and water remaining
composition formed. unchanged.

of salt.


1:39 KHSO,

•57 x 2
1.77 K SO,

.18 x 2

none •57 x 3 1.77 K SO,

.18 x 2

57 •57 x 4 1.77 K SOM

.18 x 2

•57 x 2 Reactions between a queous solutions of phosphoric acid (H PO,) and potassium hydroxide. (1 gram H PO, used in each case.)






Grams KOH Grams salt Grams Grams KOH used.

formed; and water remaining composition formed. unchanged.

of salt.

1.39 KH POL •18
•57 x 2 1.77 K HPO •18 x 2
•57 x 3 2.13 K PO

.18 x 3
•57 x 4
2.13 K POL .18 x 3

•57 x 5 2:13 K PO,

.18 x 3

•57 x 2 We see then (1) that hydrochloric acid and potassium hydroxide interact to produce one salt (KCI), and that the potash over and above that which interacts to produce this salt remains unchanged; (2) that two salts (KHSO, and K so.) are produced by the interaction of sulphuric acid and potash, and that the production of one or other salt depends upon the relative masses of the alkali and acid, but that if a greater mass of potash is added than is required for the production of the salt K so, the excess of potash remains unchanged; (3) that three salts (KH PO,, KHPO,, and K PO.) are produced by the interaction of phosphoric acid and potash, and that the production of one or other salt depends upon the relative masses of acid and alkali, but that any excess of potash beyond that which interacts to produce K PO, remains unchanged.

Similarly, if the metal potassium had been employed in place of its hydroxide we should have obtained one salt (KCI) in the case of hydrochloric acid, two salts (KHSO, and K SO) in the case of sulphuric acid, and three salts (KH,PO, KHIPO and K PO.) in the case of phosphoric acid.

If the interactions between potassium (or sodium) hy-
droxide and many acids are examined we find that the acids
may be classified as follows :-
I. Monobasic acids;

sacids which interact with
potash or soda to produce

only one salt. II. Dibasic acids;

two salts. III. Tribasic acids;

three salts, IV. Tetrabasic acids;

four salts. V. Pentabasic acids;

five salts. VI. Hexabasic acids;

six salts. An n-basic acid may also be defined as an acid from the reacting weight of which n combining weights of hydrogen can be displaced by sodium or potassium, when the acid interacts


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