MELTING POINTS OF ISOMERIC COMPOUNDS

117

CH, between the carboxyl and hydrogen raises the melting point, whereas the introduction of the second group lowers the melting point; consequently those members in this series of acids which contain an uneven number of carbon atoms melt at a lower temperature than either of their neighbours containing an even number of carbon atoms. As the molecular weight The melting

increases this difference gradually disappears. point of the dibasic acids of the formula,

CnH2n_,O,=HO–CO—(CH,)m CO—OH,

2n-2

consisting of oxalic, malonic, and succinic acids, &c., exhibit similar relationships.

The melting points of many hydro-carbons, e.g. of benzene, as shown by Jungfleisch, is alternately raised and lowered by the replacement of the hydrogen by chlorine.

Still, it is only when the chief products of the action of chlorine upon benzene are compared with one another that such regularities are observed. In addition to these, several isomeric compounds are formed, but in much smaller quantities, and these again have different melting points. In fact, it is found that the melting point of a compound is influenced by the positions which the chlorine atoms occupy relatively to one another.

As a general rule, it may be stated that of the three isomeric di-substitution products which may be obtained by replacing two atoms of hydrogen in benzene by two other atoms or radicals, the para- compound has a melting point much higher than the ortho- and the meta-. Which of the latter has the higher melting point depends upon the nature of the atom or radical replacing the hydrogen. If one of these is the carboxyl group, COOH, then the meta- compound has a higher melting point than the ortho-, otherwise the ortho- compound will melt at

the higher temperature. Still these rules are not without exceptions; in the presence of the nitro- group, NO,, it sometimes happens that the ortho- is more easily fusible than the meta-, and in some cases the reverse obtains. The following examples will serve to illustrate these points :

When a third atom of hydrogen in benzene is replaced, then the melting point is altered still more; as a rule, the melting point of a para- compound is lowered, and indeed often very considerably; whilst those of the other isomeric di-substitution products are raised. Still, even in this case the change in the melting point is determined, not only by the nature of the replacing radical, but also by their relative positions. In the most symmetrical arrangements of these several groups in the position 1. 3. 5 (vide § 54) the melting point is found to attain the maximum.

§ 68. Melting Points of Mixtures.-Heterogeneous solid bodies melt either in such a way that only one portion is liquefied, whilst the other remains solid or all the several constituents become liquid simultaneously. In the last case the fusion always takes place at a fixed temperature, which may be below the melting point of the most difficultly fusible constituent, and

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119

is frequently found to be lower than the melting point of the most easily fusible component. In this we have an explanation of the observations so frequently made in laboratory practice that even very small impurities suffice to effect a considerable reduction in the melting point of a substance. Such a mixture can frequently be distinguished from a pure homogeneous substance by the fact that the temperature does not remain stationary during the fusion. As a rule, the constituent with a lower melting point melts first, and with it only a part of the higher melting constituent, the remainder of the latter continuing in the solid state, and not melting until a higher temperature has been reached. If one If one were to separate the liquid portion from the solid before this had occurred, then each portion when separately examined would be found to possess a higher melting point, because it contains a smaller portion of impurity. An excellent method for the purification of solids is based upon this difference.

§ 69. Homogeneous Liquids, Cohesion, Capillarity, Friction. As has already been pointed out in § 60, the liquid state of aggregation is distinguished by the fact that the particles, although held together, can move easily over one another. In consequence of this, liquids under the influence of the force of gravity assume the form of the vessel containing them; whilst the surface assumes a direction perpendicular to the line of the action of gravity, provided that other forces-e.g. the centrifugal force-do not tend to change this position. The space occupied by a liquid can only be reduced to a very small extent by great pressure; liquids are therefore only slightly compressible fluids.

The mobility of the particles is very different in different liquids. On the one hand we have liquids possessing a so-called syrupy consistence; on the other hand, those possessing a mobility approaching very nearly to that characteristic of gases. The resistance which they offer to movement is what is usually styled the internal friction or the viscosity of the liquids. This property may be determined from the velocity with which the liquid flows through a narrow tube (transpiration according to Graham), or by the retardation, which a body rotating round its axis, experiences when set in motion in such liquids. The friction is dependent upon the nature and the composition of the

liquid; still, too little is known of the connection between these properties to allow of any general statement being made.

Nor is our knowledge of the manner in which the particles. of a liquid are held together in a much more advanced state, the cohesion of liquids, which is especially exhibited in the phenomena of capillarity, i.e. the manner in which liquids rise in very narrow tubes, the walls of which are moistened by them, and is likewise shown in the formation of drops. The weight or volume of the liquid raised by capillarity is dependent upon the chemical nature and composition of the liquid; still, of this inter-dependence so little is known that it would not be advisable to discuss it further.

§ 70. Density of Liquids.—The subject of the density or the specific gravity of liquids, i.e. the weight of a unit volume, is one which has been exhaustively investigated. Usually, however, it is not the density, but rather its reciprocal, the so-called specific volume-that is, the volume of the unit of weight-which is dealt with in these investigations. The product of these values into the atomic weights of elements and into the molecular weights of compounds gives the atomic and molecular volumes. Relationships have been recognised amongst these values similar to those found to obtain in the case of solids. As the majority of elements are only to be obtained in the liquid state, at either inconveniently low or high temperatures, their atomic volumes in the liquid state have been but little studied.

Inasmuch as for an equal rise in temperature liquids expand much more easily than solids, it is of much importance in the case of liquids that comparisons should be instituted at corresponding temperatures. Hermann Kopp proposed that this comparison should in the case of liquids be made at a temperature at which their vapour pressures are the same, viz. at the boiling point under the same pressure. The pressure usually taken as normal is the mean atmospheric pressure, viz. 760 millimetres, although in the light of the more recent investigations it would appear more desirable to choose a much smaller pressure. But even the molecular volumes of compounds compared at their boiling points under the atmospheric pressure, more especially those of organic compounds, exhibit numerous relationships, which, although they cannot be regarded as fixed

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natural laws, may at any rate be taken to represent approximations to such laws.

The fundamental law of atomic volumes is that every atom in a compound at its boiling point occupies a given space which is chiefly determined by its nature, and only to a limited extent by the manner in which it is combined; so that the volumes occupied by the molecular weights of different compounds may be taken to be represented by the sum of the volumes of all the atoms contained in them.

Thus if V be this volume, then in the case of alcohol we have the following:

V(C2H ̧O) = 2V(C)+6V(H)+1V(0);

and similarly in other compounds.

The unit of volume in this case is the space which the unit weight of water at its maximum density occupies, and the unit of weight the weight of an atom of hydrogen. The value of this latter unit is unknown, but that does not signify, as in this case it is, as in all determinations of density, only a question of relative values. In fact, the same values for the molecular volumes are obtained if, instead of an atom, one gramme of hydrogen is taken as the unit of weight and one cubic centimetre as the unit of volume.

Expressed in these terms, according to Kopp's determinations, the volumes of the atoms of the following elements in their compounds at their boiling points would be approximately the following:

V(H) = 5.5, V(C) = 11, V(0) = 7.8.

Accordingly in the case of alcohol, already cited, the following value must be obtained :

2V(C)+6V(H)+1V(0)

=

22+33 +7.8 = 62.8;

whilst the actual determination at the boiling point 78° C. shows the molecular volume of alcohol to be 62.2.

From Kopp's law it follows, then, that a fixed difference in the composition must always be associated with a similar difference in the molecular volumes; thus, for example, the difference CH, in a homologous series of compounds must give rise to a difference in volume. This difference in volume for every addition

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