As already stated, when the proper proportions by weight of oxygen and hydrogen are mixed and a spark passed through, water is formed. This change, often called a reaction, may be written as follows:

2H

+

O

=

H1O

2 atoms of hydrogen

combined with

1 atom of oxygen

forms

1 molecule of water

Abbreviating a reaction in this manner is called writing a chemical equation. In a very concise form it shows: on the left-hand side of the equation the substances (called factors) which enter the reaction, on the right-hand side the products, and also the exact amount of each that must be taken or formed. Once the products are determined (usually by experiments), the equation may be written and balanced by having the same number of atoms of the elements on each side of the equation. Forming compounds by combining elements is called synthesis.