3. _Freezing points of solutions._ A solution freezes at a lower temperature than the pure solvent. The lowering of the freezing point obeys the same law which holds for the raising of the boiling point: the extent of lowering is proportional to the weight of dissolved substance, that is, to the concentration of the solution. ~Electrolysis of solutions.~ Pure water does not appreciably conduct the electric current. If, however, certain substances such as common salt are dissolved in the water, the resulting solutions are found to be conductors of electricity. Such solutions are called _electrolytes_. When the current passes through an electrolyte some chemical change always takes place. This change is called _electrolysis_. [Illustration: Fig. 31] The general method used in the electrolysis of a solution is illustrated in Fig. 31. The vessel D contains the electrolyte. Two plates or rods, A and B, made of suitable material, are connected with the wires from a battery (or dynamo) and dipped into the electrolyte, as shown in the figure. These plates or rods are called _electrodes_. The electrode connected with the zinc plate of the battery is the negative electrode or _cathode_, while that connected with the carbon plate is the positive electrode or _anode_. ~Theory of electrolytic dissociation.~ The facts which have just been described in connection with solutions, together with many others, have led chemists to adopt a theory of solutions called _the theory of electrolytic dissociation_. The main assumptions in this theory are the following. 1. _Formation of ions._ Many compounds when dissolved in water undergo an important change. A portion of their molecules fall apart, or _dissociate_, into two or more parts, called _ions_. Thus sodium nitrate (NaNO_{3}) dissociates into the ions Na and NO_{3}; sodium chloride, into the ions Na and Cl. These ions are free to move about in the solution independently of each other like independent molecules, and for this reason were given the name ion, which signifies a wanderer. 2. _The electrical charge of ions._ Each ion carries a heavy electrical charge, and in this respect differs from an atom or molecule. It is evident that the sodium in the form of an ion must differ in some important way from ordinary sodium, for sodium ions, formed from sodium nitrate, give no visible evidence of their presence in water, whereas metallic sodium at once decomposes the water. The electrical charge, therefore, greatly modifies the usual chemical properties of the element.