The pH scale was introduced by a man named S.P.L. Sorensen (1868-1939). He made the scale to measure the concentration of hydrogen ions in solutions. (Hydrogen ions are hydrogen atoms missing an electron.) The more hydrogen ions in a solution the stronger the acid.
The amount of hydrogen ions in solution can affect the color of dyes in nature. These dyes can be used as indicators to test for acids and bases (alkalis). An example of this is the hydrangea. Hydrangeas are a type of plant whose floweres are different colors depending on what kind of soil they're growing in. In acidic soil the floweres are blue and in alkaline soil the flowers are pink. Another example is litmus, which is obtained from lichen. It turns red in acid. If a base is slowly added, the litmus will turn blue when the acid has been neutralized. On the pH scale neutral is about 6-7. Other indicators will change color at different pHs.
A combination of indicators is used to make a universal indicator. Universal indicators are used with a scale of either 2-11 or 0-14.
When using Litmus paper the pH scale starts with pink for strong acids below pH 2, and goes through various colors before green at neutral, (pH 6 or 7), to end at dark purple for strong bases or alkalis at pH 11 and above.
Here's a link to Max's page.
Here's a link to David K's page.
Here's a link to Sammy's page.
Here's a link to the Acids Bases and Salts page.
Here's a link to the Team C page