. Stoichiometry . point of maximum or minimum pressure onthe curve. It is obvious, also, that the smaller the difference between/a and /b, the smaller will be the deviation required to give a maxi-mum or minimum pressure. ^ Loc. cit. 250 STOICIIfOMIiTRY The influence of chemical relationship is well seen by the be-haviour of the monohydric, aliphatic alcohols towards water. Thesealcohols may be regarded as derivatives of water, formed by thereplacement of a hydrogen atom by the group C^Hg,, + j, thusHO—H -fe. HO—C„H2„ + 1; or as derivatives of the paraffins, formedby replacement of a hydrogen
. Stoichiometry . point of maximum or minimum pressure onthe curve. It is obvious, also, that the smaller the difference between/a and /b, the smaller will be the deviation required to give a maxi-mum or minimum pressure. ^ Loc. cit. 250 STOICIIfOMIiTRY The influence of chemical relationship is well seen by the be-haviour of the monohydric, aliphatic alcohols towards water. Thesealcohols may be regarded as derivatives of water, formed by thereplacement of a hydrogen atom by the group C^Hg,, + j, thusHO—H -fe. HO—C„H2„ + 1; or as derivatives of the paraffins, formedby replacement of a hydrogen atom by a hydroxyl group, thusQt^iu + 1 H —> C„H^,„ + 1 OH. The smaller the alkyl group, the closer is the relationship towater; the larger the alkyl group, the smaller is the influence of thehydroxyl radical, and the closer is the relationship to the correspond-ing paraffin. At the ordinary temperature methyl alcohol is miscible withwater in all proportions, as also are ethyl and propyl ° MOLECUUR PERCENTAGE OF B °° Fig. 63. Normal butyl alcohol dissolves in about 12 parts of water, and isobutylalcohol in about io5 parts, and the solubility of the higher alcoholsin water diminishes as the molecular weight rises, cetyl alcohol, forexample, being practically insoluble in water. The volume andtemperature changes (p. 239) similarly indicate that the mutual at-traction of the unlike molecules diminishes as the molecular weightof the alcohols increases. Lastly, the deviation of the vapour-|)ressure curves from straightness increases,^ as may be seen fromFig. 64, in which the vapour pressure-molecular composition curvesfor four alcohols are given. The temperatures are such that thevapour pressure of each pure alcohol is 400 mm. The maximumdifferences between the pressures represented by the actual curvesand the theoretical straight lines are roughly as follows :— 1 Konowalow, Wicd. Ami., I/|, 3 ( (1881). MIXTURES 251 Methyl alcohol and water . Ethyl n
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