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are considering occur, and some of the more important physical changes which accompany the chemical changes.

Properties of water. Here we must distinguish the physical from the chemical properties (v. ante; pars. 2 to 13, and 35 to 41). The physical properties are those which characterise the substance water considered as a definite kind of matter apart from other kinds of matter. The chemical properties are those which are exhibited by water when it interacts with other kinds of matter. If this definition is accepted, then, strictly speaking, water can have no chemical properties; the properties we shall have to examine are properties exhibited by a changing system composed of water plus something else; the properties called chemical only come into play when water interacts with other bodies, hence they are the properties neither of the water per se, nor of the other body or bodies per se, but of the system of which water forms a part. The simplest chemical occurrence is at least two-sided.

One of the chemical properties of hydrogen is that 1 part by weight of this gas combines with 8 parts by weight of oxygen to produce 9 parts by weight of water. But this chemical occurrence may be stated in terms of oxygen, hydrogen, or water. A chemical property of oxygen is that 8 parts by weight of oxygen combine with 1 part of hydrogen to produce 9 parts of water. A chemical property of water is that 9 parts by weight of it are produced by the union of 1 part of hydrogen with 8 parts of oxygen.

We can here only enumerate a few of the more important physical properties of water. At temperatures below 0° C. water is a solid, from 0° to 100° it is a liquid, and above 100° it is a gas. The change from solid water to liquid water is accompanied by a slight decrease of volume; the change from liquid water to water-gas is accompanied by an increase of volume; each change is accompanied by the disappearance of a considerable quantity of heat. The reverse change from water-gas to liquid water is accompanied by a large condensation of volume, and the change from liquid water to ice is accompanied by a slight increase of volume; during both changes much heat is produced. The temperature at which each change occurs depends upon the pressure of the atmosphere on the surface of the water; the temperature at which the change from liquid to gaseous water occurs freely varies very considerably with variations of pressure.

To trace the relations of volume, pressure, and temperature,

for a given mass of water, is a typical physical inquiry. Under ordinary conditions of pressure, solid water melts at 0°, and liquid water boils at 100o.

Water dissolves very many and very different substances. 102 The solution is sometimes unattended with any chemical change; e.g. common salt or sugar dissolves in water, on evaporating off the water the salt or sugar remains (v. ante, par. 10). In some cases solution in water is accompanied by chemical change; e.g. sodium dissolves in water, but on evaporating off the water, not sodium, but a different substance, caustic soda, is obtained (v. ante, par. 86). The phenomena presented during solution of bodies in water are extremely complex; they cannot be classed wholly as physical or wholly as chemical. We are not yet in a position to examine these phenomena with any prospect of approximately understanding them. Let us rather turn to some of the more important phenomena presented by the interactions of water with other kinds of matter, that is, to the chemical properties of water.

Water combines with many compounds, and with one or two 103 elements.

When the gaseous element chlorine is passed into ice-cold water, after a time the whole becomes semi-solid; by filtering off the liquid water at a temperature under 0o, crystals having the composition Cl. 5H2O (CÎ= 35.5) are obtained. Heated to a little above 0° chlorine hydrate is wholly decomposed into

chlorine and water.

The compound cobalt chloride is a blue solid; its composition is expressed by the formula CoCl, (Co = 59, Cl=35.5). When this salt is dissolved in water, a reddish liquid is produced from which, after partial evaporation, red crystals separate having the composition CoCl,. 6H,O. When these red crystals are heated they separate into water, which passes off as steam, and blue cobalt chloride, which remains. When a little water is added to the blue solid, the red compound is reproduced. The equations

2

(1) CoCl,+6H,O= CoCl,. 6H2O, occurring at the ordinary temperature;

(2) CoCl,. 6H2O = CoCl2+ 6H2O, occurring at about 100°; represent the two chemical changes.

When water is poured on to the compound calcium oxide, or lime, the water disappears, the lime swells up, much heat is produced, and the new compound calcium hydroxide, or slaked

104

lime, is formed. The equation CaO + H2O = CaO. H2O represents the change of composition (Ca = 40). When the solid compound calcium hydroxide is strongly heated-say to 400° or 500°—it is decomposed into calcium oxide and water; thus CaO. H2O = CaO+H2O; the water passes away and the calcium oxide remains as a solid.

Copper sulphate-CuSO, (Cu 63.2, S = 32)—is a white solid; when it is exposed to moist air it begins to turn blue; this change is more quickly effected by pouring a little water on to the solid. The blue substance thus produced is a compound of copper sulphate and water, called hydrated copper sulphate; its composition is CuSO4.5H,O. When this compound is heated to about 220° water passes off as steam, and CuSO, remains as a white solid.

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When solid potassium oxide, K,O (K = 39), is placed in water it instantly dissolves; when the solution is boiled to dryness a white solid remains having the composition K ̧0. H2O, called potassium hydroxide, or hydrated potassium oxide, or (more commonly) caustic potash. This solid is not chemically changed by heat; at a high temperature it melts, and at a very high temperature it is volatilised; but the melted substance, or that which is volatilised, has the same composition, KO. H2O, as the solid.

If an aqueous solution of caustic potash is added to an aqueous solution of copper sulphate, a greenish blue solid compound is formed; when this is collected, washed, and dried at about 40° to 50°, it has the composition CuO. HO (Cu = 63.2). When this greenish blue solid is heated to 100° or 150° water passes off as steam and black copper oxide, CuO, remains. If a little water is now added to this copper oxide no chemical change occurs; the water and the copper oxide remain mixed.

If we glance back over the statements made regarding the compounds of water with chlorine, cobalt chloride, &c. we see that the compounds may be divided into three classes, as follows :— (i) Compounds formed by bringing together water and the other constituent of the compound, and also decomposed by heat into water and the other constituent;

Cl. 5H2O; CoCl ̧.6H ̧O; CaO.H2O; CuSO̟. 5H2O.

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2

(ii) Compound formed by bringing together water and the other constituent, but not decomposed by heat;

KO. H2O.

(iii) Compound decomposed by heat into water and another compound, but not formed by bringing together water and that other compound;

CuO. H2O.

We also notice that the compounds under (i) are decomposed by heat at different temperatures, varying from a little above 0° in the case of Cl. 5H,O, to 400° or 500° in the case of CaO.H,O.

Inasmuch as each substance we have been considering is either formed by the combination of water with another substance, or is resolved into water and another substance, we are justified in calling them all compounds of water with other elements or compounds.

But in many of its interactions with elements and com- 105 pounds water is decomposed, and new bodies are formed which cannot be regarded as compounds of water.

We have learned that when sodium and water interact the products are hydrogen and sodium hydroxide; the equation Na + H2O+ Aq= NaOHAq+ H expresses the composition of the system before and after this interaction.

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A similar change occurs when potassium is thrown into water; K+ H2O + Aq = KOHAq + H. In each of these reactions much heat is produced; in the case of potassium the reaction proceeds very rapidly, and the temperature of the hydrogen produced is raised so much that this gas takes fire.

Fig. 15.

M. E. C.

6

When water-gas is passed over hot magnesium, or hot finely divided iron, in an apparatus as represented by fig. 15, hydrogen is obtained, and oxide of magnesium or iron is formed and remains in the tube in which the magnesium or iron was heated. Quantitative experiments have proved that for every 18 parts by weight of water decomposed 2 parts by weight of hydrogen are obtained, and an oxide of magnesium or iron is formed by the union of the 16 parts by weight of oxygen, formerly combined with the 2 parts of hydrogen, with 24 parts of magnesium or 42 parts of iron. The combining weights of magnesium and iron are 24 and 56, respectively; the reacting weights of the oxides formed in the process just described are 40 and 232, respectively, and the compositions of these oxides are represented by the formulae MgO and Fe,O,. Knowing that the combining weight of oxygen is 16, and the reacting weight of water is 18 (HO), we can summarise the changes of composition which occur in the reactions between steam and heated magnesium or iron in these equations (Mg = 24, Fe = 56, O=16);— (1) Mg+ H2O = MgO + 2H.

24 +18=40 + 2.

(2) Fe, + 4H,0 = Fe ̧0, + 8H.

168+ 72 = 232

4

+ 8.

When the gaseous element chlorine is passed into boiling water and the mixture of steam and chlorine thus obtained is passed through a porcelain tube, loosely packed with pieces of porcelain, and heated to bright redness, oxygen and hydrogen chloride are produced. The apparatus represented in fig. 16 may be used. The exit end of the porcelain tube is connected with a vessel containing caustic potash solution (A). The gases coming from the tube bubble through this solution. Hydrogen chloride is absorbed by caustic potash, but oxygen is not. The gas which is not absorbed by the caustic potash is collected and proved to be oxygen. Quantitative experiments shew that the compositions of the interacting substances and of the products of the interaction are expressed by the equation (CI = 35·5, O = 16) ;—

H2O+2C1 = 2HCl + 0.

18 +71 = 73 + 16.

For every 18 parts by weight of water decomposed, 71 parts of chlorine are used, and 73 parts of hydrogen chloride and 16 parts of oxygen are produced. As all the substances taking part in this reaction are gases under the conditions