ELEMENTARY SCIENCE 



The fact that molecules of water contain two atoms of 

 hydrogen to one of oxygen is indicated by the fact that 

 when they are decomposed into these two elements (which, 

 when liberated, take the form of gases) the volume of hydro- 

 gen obtained is just twice as great as the volume of oxygen. 

 Water may be so decomposed by passing through it an 

 electric current; the process is 

 called electrolysis. Study Fig. 58. 



FIG. 58. Diagram illustrat- 

 ing how water is decomposed 

 by electrolysis; the hydro- 

 gen collects in top of tube H 

 and the oxygen collects in 

 top of tube O After Mn.- 

 LIKAN and GALE. 



Chemical and Physical Changes. 

 Any change in the nature of the 

 molecule of a substance is a chem- 

 ical change. All other kinds of 

 changes are physical changes. The 

 change of water to ice or steam is a 

 physical change. The change of 

 water to hydrogen and oxygen is a 

 chemical change. Chemistry is the study of chemical 

 changes. Physics is the study of physical changes. Both 

 physical and chemical changes are constantly going on in 

 living as well as in non-living bodies. Life itself is a con- 

 stant procession of changes. Evidently, to understand liv- 

 ing things, it is quite necessary first to understand some- 

 thing of physics and chemistry. 



You have just noted that molecules are composed 

 of atoms; thus the water molecule is composed of two atoms 

 of hydrogen and one of oxygen. Molecules composed of 

 few atoms (few both in number and kind], like the water 

 molecule or the carbon-dioxide molecule, are said to be 

 simple. But molecules composed of many atoms are said 

 to be complex. The molecules of some substances are very 

 complex, containing even hundreds of atoms. 



