650 APPENDICES 



While tliere is such a marked difference between a true solution and 

 the colloid state, there is no clear line of demarcation between the two 

 conditions. As the size of the molecules or aggregates of molecules of the 

 dispersed phase of a colloid becomes smaller, the colloid comes to take on 

 the character of a solution, so that the particles or molecules of the colloid 

 show movement, and therefore kinetic energy. 



Substances that go into a true solution and pass through an animal 

 membrane {see Osmosis) have been called crystalloids to distinguish them 

 from colloids. There is, however, no fixed dividing line between 

 crystalloids and colloids. Some colloids, e.g. htemoglobin, can be 

 obtained in the crystalloid form. 



Ions. 



When salts are dissolved in water they become "dissociated." Thus 

 NaCl in a dilute solution becomes Na, carrying a positive charge of 

 electricity usually indicated by the sign + or by a dot, e.g. Na+ or Na-, 

 and CI carrying a negative charge usually indicated by a dash, e.g. Cl~" or 

 Cr. Na does not exist as an atom of sodium nor CI as an atom of chlorine. 

 Each is combined with a definite quantity of electricity. A monovalent 

 atom or group of atoms carries one unit charge, a divalent two and so on. 

 Thus, if a phosi^hate be dissociated, the PO4 part, being trivalent, carries 

 three unit charges and is written PO/". In an ammonium salt the NH^ is 

 monovalent. It carries one unit charge and is written NH4'. The atom 

 or group of atoms thus dissociated and carrying a quantity of electricity is 

 termed an Ion. It is the presence of ions that enables a salt solute to 

 conduct electricity. 



Substances that become dissociated into ions when in solution are 

 called Electrolytes, to distinguish them from substances such as sugar 

 that are not dissociated when dissolved. Nearly all salts, acids, and bases 

 are electrolytes. 



The Reaction of Watery Fluids. 



The Hydrogen Ion Concentration. 



Water between 22' and 23' C. is, to a slight extent, ionised. That is, 

 some of the water molecules are dissociated into hydrogen and hydroxyl 

 ions. According to the law of mass action, the ratio of the product of the 

 ions to the undissociated water is a constant. This may be written — 



This constant has been found experimentally for the temperature of 23° C. 



to 1)6 — 



1 



100,000,000,000,000 



or IQ-^* 



