GO ELEMENTS OF GENERAL SCIENCE 



contain both hydrogen and oxygen atoms. When we decom- 

 posed water by the electric current, we secured twice as much 

 hydrogen as oxygen. That would suggest that in the molecule 

 there are probably twice as many atoms of hydrogen as of 

 oxygen. Further study by chemists has shown that in a mole- 

 cule of water there are two hydrogen atoms and one oxygen 

 atom, making three atoms in all. 



Chemists have a very convenient shorthand way of repre- 

 senting these facts by a symbol. They let a certain letter 

 stand for a certain kind of atom. Thus, H stands for a 

 hydrogen atom and O for an atom of oxygen. If there is 

 more than one atom, a small figure is written below and to 

 the right of the letter to tell how many atoms are meant. 

 Written in this way the symbol for a molecule of water is 

 H n O. We shall find it convenient to use this symbol. 



61. Explanation of some chemical terms. When a substance 

 is made up of molecules having two or more kinds of atoms 

 in them, as in water, it is clear that it is possible to separate 

 such a substance into two or more different substances, as we 

 did with water. Such a substance is called a compound sub- 

 stance. Water is a compound. A substance whose molecules 

 have in them only one kind of atom, as oxygen, cannot be 

 separated into simpler substances. It is called a simple sub- 

 stance (element). Hydrogen, oxygen, iron, silver, lead, copper, 

 and gold are examples of simple substances. 



Another chemical term that you will need to remember is 

 chemical decomposition. When a compound is separated into 

 its elements or into simpler compounds, the process is called 

 chemical decomposition. Have you seen an example of chemi- 

 cal decomposition ? It is sometimes said in that case that the 

 compound has " broken down." 



Any change in a substance in which the molecule is changed 

 by atoms being taken away from it, added to it, or ex- 

 changed for others is called a chemical change. So long as the 

 molecules remain the same, there is no chemical change. 



