ELECTROLYTES AND NON-ELECTROLYTES ii 



volume of many substances is very small, and for a few it is even negative, 

 i.e. there is a shrinkage in volume when the solute is added to the solvent. 

 On the other hand the solution volume of some compounds, especially the 

 sugars, is considerable. When a mol of sucrose is added to looo g. of water 

 the resulting solution will have a volume of 1207 cc. at 0° C. Hence the 

 solution volume of sucrose is 207 cc. The solution volume of a mol of sodium 

 chloride, on the other hand, is only about 18 cc. Since every solute has a 

 different solution volume, it follows that equal volumes of weight molar solu- 

 tions do not contain the same number of either solvent or solute molecules. 

 Neither will the dilution of a given volume of a weight molar solution with 

 an equal volume of water result in a 0.5 molal solution. In other words the 

 concentration of a weight molar solution does not change in proportion to the 

 amount by which it has been diluted. This is a fact which is sometimes over- 

 looked in experimental work. 



In physiological work it is often convenient to make up solutions on a per- 

 centage basis. Such solutions are made up either on the basis of percentage 

 by weight, or percentage by volume. Solutions of solids in water or other 

 solvents are made up on a weight percentage basis. A 10 per cent sodium 

 chloride solution, for example, is made by dissolving 10 g. of sodium chloride 

 in 90 g. of water. Solutions of liquids in water or other solvents can also 

 be prepared on a weight percentage basis. It is simpler, however, to make 

 up such solutions on a volume percentage basis. On this basis a 10 per cent 

 solution of alcohol is prepared by adding 10 cc. of alcohol to 90 cc. of water. 

 The percentage system of solutions has no direct relation to either the volume 

 molar or weight molar systems. 



Electrolytes and Non-Electrolytes. — Some aqueous solutions readily 

 conduct an electric current; others do not. The former are called electrolytes; 

 the latter non-electrolytes.^ The solutions of all acids, bases, and salts are 

 electrolytes. Solutions of most organic compounds such as the sugars, alcohols, 

 ketones, and ethers are non-electrolytes. 



Passage of an electric current through an electrolyte results in its decom- 

 position. This process is called electrolysis. If hydrochloric acid is the 

 electrolyte, for example, hydrogen gas will be liberated at the negative pole 

 (cathode) and chlorine gas will be liberated at the positive pole (anode). 

 If electrolysis is continued long enough eventually all of the hydrochloric acid 

 present in the system will be decomposed into hydrogen gas and chlorine gas. 



^ Strictly speaking the term electrolyte refers only to a solution of an ionized 

 substance, but it is also often applied to any compound which, when dissolved in 

 water, produces ions. A similar dual usage of the term non-electrolyte also 

 prevails. 



