SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 37 



ions in a crystal of common salt are too rigidly held to move, but if the 

 crystal is dissolved in water they are free. If into different parts of such 

 a solution wires from the two poles of a l^attery are placed, a current of 

 electricity is carried through the solution from one pole of the battery 

 to the other (Fig. 26). The positive ions (Na+) go toward the negative 

 pole and, by taking up electrons from it, becomes ordinary neutral 

 sodium (Xa). Removal of electrons from the negative pole reduces the 

 negative charge conferred upon it by the battery and sets up a current 

 in the wire. The negative ions (Cl~) pass in like manner to the positive 

 pole, where they deposit their surplus 

 electrons on that pole, forming neutral 

 chlorine (CI). Sodium is thus col- 

 lected about one pole of the battery, 

 where it reacts with the water; chlo- 

 rine collects about the other pole and 

 escapes as a gas. Decomposition of 



a substance in this manner is known t. „^ t.- ^ . , ■ . 



biG. 2b. — Diagram of electrolysis of 



as electrolysis. In the metal mdus- sodium chloride in solution. Chloride 



tries this process is used to separate \'^l\^^'''' *° *^^ "^^*' ^°'^'"'" '""'' *° 

 certain metals from their ores. Sub- 

 stances which, like sodium chloride, form ions in solution and are thus 

 capable of carrying a current are called electrolytes. Most of the salts 

 are good electrolytes. 



Energy. — Energy is the capacity to do work, that is, to produce 

 change. The arrangement of the electrons and protons in an atom 

 involves energy. Changing that arrangement either requires that 

 energy be expended upon the change or releases energy no longer needed 

 in the new arrangement. Both types of change are exceedingly common. 

 Of the common elements about us near the earth's surface, oxygen is by 

 far the most abundant, making up nearly half of the total. It is also 

 very common in living things. Since oxygen is a fairly active element, 

 some of the most frequent chemical reactions are the combinations of 

 oxygen with other substances. These changes are called oxidation. 

 The rusting of iron and the burning of wood or coal are examples. An 

 important feature of oxidation is that it releases energy. Use is made 

 of this fact in industry, when the energy of steam engines or electric 

 current is furnished by burning coal, and in plants and animals whose 

 activities depend on energy obtained by oxidizing food. The energy 

 which is tied up in the composition of chemical substances, whether foods 

 or any other, is called potential energy. When converted by a chemical 

 reaction into the energy of heat or of movement, it becomes kinetic energy. 



Applications to Biology. — The examples used in this chapter to illus- 

 trate chemical principles have been taken mostly from inorganic chemis- 



