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INORGANIC CHEMISTRY

PART I

INTRODUCTORY OUTLINES

CHAPTER I

CONSTITUTION OF MATTER

THE science of chemistry may be described as the study of a certain class of changes which matter is capable of undergoing. Matter is susceptible of a variety of changes, some of which are regarded as physical and others as chemical. Thus, when a steel knitting-needle is rubbed upon a magnet, the needle undergoes a change, by virtue of which it becomes endowed with the power of attracting to itself iron filings or nails; and when an ordinary lucifer match is rubbed upon a match-box the match undergoes a change, resulting in the production of flame. In the first case the change is said to be a physical one, while the ignition and combustion of the match is a chemical change.

When a fragment of ice is gently warmed, it is changed from a hard, brittle solid to a mobile, transparent liquid; and when white of egg is gently heated, it changes from a transparent, colourless liquid to an opaque white solid. These changes, which appear at first sight to be of a similar order, are in reality essentially different in their nature: the transformation of solid ice into liquid water is a physical change, the coagulation of albumen is a chemical change.

Again, when certain substances (such as the materials which constitute the so-called luminous paint) are exposed to a bright light, they undergo a change whereby they become invested with

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the power to emit a feeble light when seen in the dark. A stick of phosphorus also emits a very similar light when seen in the dark. The glowing of these materials under these circumstances might readily be regarded as the result of the same kind of change in both cases; but in reality the luminosity of the phosphorus is due to a chemical change taking place upon the surface of that substance, while the emission of light from the luminous paint is a purely physical phenomenon.

The two sciences, chemistry and physics, are so closely related and interdependent upon each other, that no sharp distinction or line of separation between them is possible. Every chemical change that takes place is attended by some physical change, and it often happens that this accompanying physical change forms the only indication of the chemical change that has taken place. In certain important points, however, a chemical change is very different from one that is purely physical: in the latter case no material alteration in the essential nature of the substance takes place. This will be seen in the examples quoted. The steel needle remains unaltered in its essence, although by magnetisation it has acquired a new property--a property which it again loses, and which can be again and again imparted to it. The match, on the other hand, when ignited has undergone a material and permanent change: the combustible substance is now no longer combustible, neither will it ever return to its original state. The solid water, in being transformed to liquid water, has not undergone any vital change; in essence it is the same substance merely endowed with a new property of liquidity, a property which it loses again when cooled, and which can be again and again imparted to it. On the other hand, the coagulated albumen has undergone a complete and lasting change, and never returns to its original condition.

In the same way, the luminous paint gradually ceases to emit light, and returns to its original state; it may be exposed to the influence of light, when it once more acquires the property of phosphorescence, and this change may be brought about indefinitely, without altering the intrinsic nature of the substance. The glowing phosphorus, on the other hand, is gradually changed into a white substance, which escapes from it as a smoke or fume; in the act of glowing the phosphorus is undergoing a process of slow burning, and if allowed to remain will continue glowing and burning until the whole of it has disappeared in the form of smoke.

The Constitution of Matter. Molecules.-Matter is regarded by the chemist and physicist as being composed of aggregations of minute particles; every substance, whether it be solid, liquid, or gaseous, presents the appearance to his mind of a vast number of extremely minute particles. To these particles the name molecules ("little masses”) has been given. The particles or molecules of any particular substance are all alike: thus in sulphur the molecules are all of one kind, while in water they are all of another kind; the properties associated with sulphur are the properties of the individual sulphur molecules, while those belonging to water are the properties of the molecules of that substance. All matter, therefore, is to be conceived as having what may be called a grained structure. The actual sizes of molecules is a matter which has not yet been determined with exactness; they are orders of magnitude which are as difficult for the mind to grasp on account of their minuteness, as many astronomical measurements are by reason of their vastness. It is certain that their size is less than half a single wave-length of light,* and that therefore they are beyond the visual limits of the microscope. Some general idea of their order of magnitude may be gathered from Lord Kelvin's calculation, that if a single drop of water were magnified to the size of the earth, each molecule being proportionately enlarged, the grained appearance which the mass would present would probably be finer than that of a heap of cricket-balls, but coarser than a heap of small shot.

It will be evident, therefore, that in the strictest sense matter is not homogeneous: a fragment of ice or a drop of water consists of an aggregation of a certain number of molecules, between which there exist certain interspaces. When the fragment of ice is heated, the spaces between the molecules are enlarged, and the solid passes into the liquid state; and when water is still further heated, and converted into water vapour or steam, the molecules are still more thrust asunder, and the intermolecular spaces are still further increased.

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The forces which similar molecules exert upon each other are regarded as physical, in contradistinction to chemical. forces are either attractive in their nature, or repellent. the attractive forces are in the ascendency, the molecules are drawn more or less closely together, and the substance assumes The wave-length of the blue ray (G) 0.0004311 millimetre, or 0.0000169 inch.

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the solid state. If the repellent forces have the upper hand the material takes the gaseous condition, while the liquid state may be regarded as resulting from a certain balance of these opposite forces. Changes which matter undergoes by the action of these forces are physical changes; they do not affect the inherent nature and properties of the substance, which properties, as already stated, reside in the molecules themselves.

In each of the three states of matter, viz., solid, liquid, or gaseous, the molecules are conceived as being in a state of motion; they are regarded as executing some vibratory movement within the spaces that divide them. In the solid state this movement is the most restricted, for the reason that the intermolecular spaces are in this case the smallest. In the gaseous condition the amplitude of vibration of the molecules is very greatly increased; for the attractive forces being at a minimum, and the intermolecular spaces being greatest, the molecules have a further distance to travel before they strike one another.

Such changes in matter, which are merely the result of alterations in the motions of the molecules, are likewise purely physical changes.

Molecules may be defined as the smallest particles of matter which can exist in the free state; or as the smallest weight of matter in which the original properties of the matter are retained.

Atoms. It is the belief of chemists that most molecules are possessed of a structure. That is to say, they are not simple, single, indivisible masses, but themselves consist of aggregations of still smaller particles, which are held together by the operations of some other force. These particles of which molecules are composed are termed atoms, and the force which holds them together is called chemical affinity, or chemical attraction. To the mind of the chemist, such molecules are little systems, consisting of a number of atoms which are attracted to each other by this particular force; in the ordinary movements of the molecule, the system moves about as a whole. In this respect it bears some analogy, on an infinitely minute scale, to a solar system. The atoms of a molecule are regarded as in a state of motion as respects one another, possibly revolving about one another, while the entire system, or molecule, at the same time performs its independent movements, just as in a solar system the various members perform various movements towards each other, while at the same time the whole system travels upon its prescribed

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