Page:Text-book of Electrochemistry.djvu/142

 friction — the " mobility " of the ions must remain unchanged when the solution is diluted with wat^r. Let us now suppose that water is poured into the trough to the level nni, then if the number of zinc ions and sulphate ions were not altered by the dilution the conductivity would be the same as before, since the number and mobilities of the ions had remained constant

This, however, is not the case. If we start, for instance, with a 0*01 normal solution of zinc sulphate, which contains 1'61 grams of ZnS04 ^ ^ litre, then on dilution to double the volume, the resistance is reduced by 11"7 per cent., or

= TT^o)- If the solution be diluted to four times its

original volume, so that the level in the trough is ppu the conductivity is increased by no less than 26*3 per cent. To explain this phenomenon it must be assumed that the and 26*3 per cent, respectively by the dilutions. We must therefore conclude that the quantity of the ions, arid con- %equently also the degree of electrolytic dissociation^ increases with dilution.

Specific and Molecular Conductivity.— A large number of data has been collected on the subject of conductivity of solutions, and to express these some important units have been adopted, which will now be defined.

The specific electrical resistance of a conductor is that resistance offered by a column of it 1 metre long and of 1 sq. mm. cross section. Usually the resiatance is expressed in Siemens' units, and the specific resistance of mercury at 0** is then equal to 1. If it be desired to express the specific resistance in ohms, the value in S.U. has to be divided by 1"063. Eecent values of the specific resist- ance have been expressed as the resistance of a column 1 cm. long and of 1 sq. cm. cross section, measured in ohms. The = 10630 times smaller than in the previously described

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