OSMOTIC PRESSURE. 445 



or do not conduct electricity when in solution. To the former 

 class belong strong" acids and most salts and also soluble bases 

 — to the latter, bodies such as sugars. 



Conduction is carried on in liquids by a process very different 

 from that which goes on in the case of a solid metal. In 

 solution the current is actually carried by the ions, a stream of 

 + ions going one way and a stream of — ions going the other. 

 We must have the two streams, so that only those substances 

 which on going into solution furnish both kinds of ions can 

 act as electrolytes. If we do not get the two kinds of ions, the 

 substance is " unionised," and does not conduct electricity. 



There is an intimate relation between the degree of ionisa- 

 tion and the change in the boiling or freezing point of a solu- 

 tion, so that equimolecular masses of different substances in 

 solution do not always give equal change in boiling point or 

 freezing point, the actual change being proportional to the 

 number of ions formed by the substance when going into solu- 

 tion. 



Sugar does not ionise, as far as we know at present, the 

 molecule going ito solution without breaking up. But this is 

 not the case with a molecule of common salt or similar binary 

 compound, for this on dissolving splits up into a positive 

 sodium ion and a negative chlorine ion. We have thus two 

 ions for every one obtained in the case of sugar, and if calcium 

 chloride is dissolved, we then get three ions, one positive of 

 calcium and two negative of chlorine. 



The rise in boiling point will be proportional to the number 

 of ions, so that if we take quantities of sugar, common salt 

 and calcium chloride, proportional to their respective molecular 

 weights, and dissolve them in equal amounts of water, the rise 

 in the boiling point of the solutions will be very nearly in the 

 proportions of i, 2, 3. 



It has several times been mentioned that the solutions are 

 supposed to be dilute, in order that the laws may be obeyed, 

 and the reason for this restriction is that ionisation is complete 

 only in dilute solutions ; and as the change in boiling point and 

 freezing point and the conductivity are proportional to the 

 ionisation, it follows that the experimental work must be done 

 in dilute solutions. The drawback to experiments on dilute 

 solutions is that the change in temperature will be small, but 

 with modern methods of measuring temperatures, this is not 

 of such great moment, and Mr. G. Bedford, in carrying out a 

 series of experiments, dealt with depressions of the freezing 

 point of water which ranged from o'ooi^C. to 0'i4°C., using 

 platinum resistance thermometers which would indicate 

 o"oooo6°C. 



What is the order of the depression of the freezing point 

 which should be obtained when definite weights of salts are 

 dissolved in water? If the gram molecular weight of the solu- 

 tions is dissolved in 1,000 cc. of water, and no ionisation 

 occurs, the depression should be i*85°C; if ionisation occurs, 

 the foregoing numbers must be multiplied by 1,2, 3, etc. 



If molecular weights of sugar, potassium chloride, barium 

 chloride, potassium ferricyanide could be dissolved in 1,000 cc. 



