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with a cork through which passes a tube narrowed at the end which is to go into the jar; A is filled with oxygen, B with hydrogen ; each stands in a little water whereby the

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Fig. 18. gas inside the jar is isolated from the air outside; the tube passing through the cork which fits jar A is connected with a gasholder containing hydrogen, the other tube is connected with a gasholder containing oxygen. Hydrogen is caused to pass slowly through one tube and oxygen slowly through the other; after a minute or so (when the air is all driven out of these tubes) the hydrogen jet is lighted, the stopper of A is withdrawn and the cork with its tube is quickly inserted; the hydrogen burns brilliantly; the stopper of B is withdrawn and a light is brought near the opening of B, the hydrogen in B burns; the cork is now very quickly pressed into its place, and the jet of oxygen is seen to burn in the atmosphere of hydrogen.

A little consideration shews that the chemical reaction 2H +0= H,0 must occur, for the most part, at or near the surface of that gas which is flowing into the other, which other is, comparatively, at rest.

If the inflowing gas is hydrogen, then, as the flame is visible along the surface of the inflowing gas, we say that the hydrogen burns in the oxygen ; that the hydrogen is burnt and the oxygen supports the combustion. If the inflowing gas is oxygen, the flame being as before visible along the surface of the inflowing gas, we say that the oxygen is burnt, and the hydrogen supports the combustion.

Oxygen combines directly with many elements; compounds 121 of oxygen

every other element, except bromine and fluorine, have been prepared, either by direct combination, or as the results of several chemical changes.


I. The elements sodium, potassium, lithium, thallium, phosphorus, and some others, combine with oxygen more or less rapidly at ordinary temperatures.

II. Antimony, arsenic, carbon, lead, sulphur, and many other elements, combine with oxygen at temperatures above the ordinary. III. Oxides of calcium, bismuth, chromium, copper, &c. &c. are usually prepared by (i) obtaining compounds of these metals with oxygen and hydrogen, and (ii) heating these hydroxides, and so decomposing them into oxides and water. IV. Oxides of lead, manganese, bismuth, and some other metals—composed of much oxygen relatively to the mass of lead &c.— are obtained by bringing these metals, or oxides of them composed of the metal united with relatively small masses of oxygen, in contact with two or more compounds which interact to produce oxygen.

V. Oxides of nitrogen, sulphur, tellurium, &c. are obtained by decomposing, by heat or otherwise, compounds of these elements with oxygen and some other element or elements.

The following equations present examples of each of the
foregoing methods of preparing oxides :-
I. 2Na+ 0 = Na,0; 2P+50=PO

at ordinary temps.
II. 2Sb + 30 = Sb,0%; C + 20 = CO,.


at higher temps.
III. CaO,H, = CaO + H,0; Bi 0 H. = Bi, O, + 3H,0.

by action of heat.
IV. KC1OAq + PbO (heated) = Pb0, +KCIAq;

Sb,02 + 2HNO, (heated) = Sb 0,+H,0+ 2NO,.
V. 2HNO, +PO, (heated)=N,+ 2HPO,;

H TO, (heated) = H 0 + Teoz.
Many elements form more than one compound with oxygen.
Thus, five oxides of nitrogen are known, viz. NO, NO, NO,
NO,, 1,0,; four oxides of lead have been prepared, viz. Pbê,
Pb ,, PbO3, PbO,

Hydrogen combines directly with a few elements ; the combination usually occurs at moderately high temperatures : thus, 2H+S (molten)=H.S; H+ Br (heated) = HBr; 2C+2H (by passing electric sparks) = C,H,; &c.

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Compounds of hydrogen with other elements are sometimes formed in chemical interactions between several elements or compounds; thus when phosphorus is heated with an aqueous solution of caustic potash, phosphorus hydride (PH) is one of the products of the reaction ; when a solution of arsenic oxide in water is brought into contact with dilute sulphuric acid and zinc, arsenic hydride (AsH), zinc sulphate, hydrogen, and water are formed.

Very many compounds of oxygen and hydrogen, each with two or more other elements have been prepared.

Compounds of oxygen and one other element are called 123 oxides ; compounds of hydrogen with one other element are called hydrides.

The physical properties of oxides and hydrides vary much ; some are solids, others are liquids, others are gases, at ordinary temperatures and pressures. The chemical properties of these compounds also vary much, but a great many oxides


be placed in one or other of two classes.

As representatives of these classes let us take the oxides of 124 sulphur and magnesium whose compositions are expressed by the formulae SO, and MgO, respectively (S = 32, Mg = 24, 0= 16).

Sulphur trioxide (SO) is a white, crystalline, solid ; it dissolves very easily in water forming a colourless solution. This solution has the following (among other) properties :(1) a sour taste ; (2) it turns blue litmus solution bright red; (3) it dissolves many metals-e.g. zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c.—with production of two or more substances, one of which is hydrogen, and another is a compound of sulphur, oxygen, and the metal used; (4) it interacts with oxides of many


oxide of zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c.—to produce two or more substances one of which is the same compound of sulphur, oxygen, and the metal of the oxide used, which was produced in reaction (3), and another of which is water. Further, if the solution of sulphur trioxide in water is evaporated considerably a thick oily liquid is obtained ; if this liquid is cooled below 0° crystals separate having the composition H.SO,. This compound is called sulphuric acid. If sulphuric acid is dissolved in water the solution exhibits the same properties as a solution in water of sulphur trioxide. It seems therefore fair to conclude that an aqueous

solution of this oxide contains the compound H SO,

Assuming that this is so, the chemical changes enumerated above may be represented in equations as follows ::

SO, + H2O + Aq=H, SO Aq.
(3) Zn + H SO, Aq=ZnSO, Aq + 2H ;

Fe + H SO Aq = FeSO Aq + 2H;
2 Al + 3H SO Aq=A1,380 Aq + 6H;
Mg +H SO Aq = MgSO, Aq + 2H ;
Cd + H SO Aq=CdSO Aq + 2H ;

Ba + H, SO Aq = BaSO4 + 2H+ Aq.
(4) ZnO + H SO Aq = ZnSO, Aq+H.0;

FeO + H SO, Aq = FeSO, Aq +H,0);
A1,02 + 3H SO Aq = Al 350, Aq + 3H,0 ;
MgO + H 50 Aq= MgSO, Aq + H,0;
Cdo +H SO, Aq=Cašo, Åq+H.O;

BaO + H SỬAq= BaSO4 + H20+ Aq. The compounds of zinc, iron, &c. produced in these reactions are called salts. If the composition of each of these salts is compared with that of the sulphuric acid, H,SO, by the interaction of an aqueous solution of which with a metal or the oxide of a metal the salt is produced, it is seen that the salt is composed of a metal together with all the sulphur and oxygen which were combined, before the chemical change began, with hydrogen, forming the compound H,SO, The compound H SO, is called an acid. The solution of this acid in water has a sour taste; turns blue litmus red; reacts with zinc, iron, aluminium, and many other metals, to produce salts, and hydrogen; and reacts with oxides of zinc, iron, aluminium, magnesium, and other metals, to produce salts and water.

The oxide So, is an acidic, or acid-forming, oxide; that is to say, it reacts with water to produce a compound which is characterised by the properties enumerated in the preceding sentence.

Many oxides resemble sulphur trioxide in that they react with water to produce compounds of oxygen, hydrogen, and the other element of the oxide, which compounds have a sour taste, turn blue litmus red, and interact with metals and metallic oxides to produce salts. These oxides are placed in one class and are called acid-forming, or acidic, oxides. Thus, the oxides whose compositions are represented by the formulae P,Os, Croy, N,O,, Seo,, respectively, are acidic oxides. The interactions of these oxides with water may be thus represented in equations :



Acidic oxide.

P,05 + 3H,0=2H PO,
Cro; + HO=H,Oro

Seo, + HO=H Seog. Let us now turn to the other oxide-Mg0. Magnesium 125 oxide is a white solid; it dissolves in a large quantity of water; this solution has not a sour taste; it turns red litmus blue. Magnesium oxide interacts with sulphuric acid, and with other acids, to produce salts and water (e. par. 124).

Many oxides resemble magnesium oxide in that they interact with acids to form salts; some of these oxides further resemble magnesium oxide in being more or less soluble in water and thus forming solutions which turn red litmus blue.

Those oxides which interact with acids to produce salts are placed in one class and are called basic oxides, or sometimes salt-forming oxides. Those basic oxides which easily dissolve in water producing liquids which turn red litmus blue are usually placed in a sub-class to which the name alkaline, or alkali-forming, oxides is given. Thus the oxides A1,0z, ZnO, Cdo, FeO, BaO are basic oxides (s. reactions represented in equations in par. 124).

The oxides of boron, chlorine, iodine, nitrogen, phosphorus, 126 selenion, sulphur, and several other elements, are acidic oxides.

The oxides of aluminium, barium, beryllium, cadmium, copper, iron, lithium, magnesium, mercury, nickel

, palladium, silver, sodium, and many other elements, are basic oxides.

Some of the oxides of chromium, molybdenum, tin, tungsten, uranium, vanadium, and a few other elements, are basic, while other oxides of the same elements are acidic.

Can we classify the hydrides by a method similar to that 127 by which we have roughly arranged the oxides in classes ?

The only element which forms many compounds with hydrogen is carbon. Some of the hydrides, other than those of carbon, interact with water to produce acids; among these are HBr, HCI, HF, HI, H S. One or two hydrides interact with water to produce compounds which again react with acids to form salts; the best marked hydride of this class is ammonia, NH . Several hydrides are either unchanged by water, or dissolve in it without producing either an acid or a salt-forming compound; e.g. H Ib, H As, H Cu,, HP, H Te.

Hydrides cannot therefore be wholly classified' by arranging them as acidic (or acid-forming), and basic (or salt-forming), M. E. C.


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