SLOWNESS OF DIFFUSION. 83 



distribution of the dissolved compounds, a deviation from uniformity in the 

 temperature of the solution will disturb the equilibrium and result in 

 unequal distribution of the dissolved substances.® 



The values of the coefficient of diffusion (D) of certain substances in 

 water solutions at various temperatures are given by the following table 

 from Nernst: 6 (The table gives the number of grams of the dissolved 

 substance which will pass in one day through a section of 1 sq. cm. when 

 the difference in concentration of the cross section 1 cm. apart amounts to 

 1 gram in a cubic centimeter.) 



Rates of diffusion of certain substances in water solutions at various temperatures. 



Temp. D. 



Hydrochloric acid 0. 1. 4 



Do 11.0 1.84 



Nitric acid 9. 1. 75 



Sulphuric acid 7.5 1. 04 



Acetic acid • 14. .81 



Potassium hydroxide 13. 5 1. 66 



Sodium hydroxide 8. 1. 96 



Ammonium hydroxide 4. 5 1. 06 



Sodium chloride 6. .75 



Ammonium chloride . . 17. 5 1. 31 



Potassium chloride 9. .66 



Barium chloride 8. .65 



Potassium nitrate 7. .92 



Sodium nitrate 13.0 .90 



Silver nitrate 7. 5 .90 



Lead nitrate 12. .70 



Urea 7.5 .81 



Chloral hydrate 9.0 .55 



Mannite 10. .38 



This table shows that diffusion is extremely slow. The slowness 

 with which diffusion occurs is due, according to Nernst, to "the resistant 

 friction experienced by the dissolved substance in its movement through 

 the solvent."" This friction is very great, because the molecules themselves 

 are exceedingly small. 



Later it will be seen that the process of diffusion is of very considerable 

 importance in the migration of compounds in ground water. (See pp. 

 636-639.) This is illustrated by the very important process of solution 

 and deposition or recrystallization. » 



«Ostwald, Solutions, cit, pp. 150-151. 



6 Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895. 

 p. 144. 



c Nernst, op. cit., p. 145. 



