A pH scale offers a measurement of how acidic or basic a solution is, which is determined by its concentration of hydrogen ions. The pH scale ranges from zero to 14. Solutions with a pH under seven are considered acidic, solutions with a pH over seven are considered basic (also referred to as alkaline), and solutions that have a pH of exactly seven are referred to as neutral. Pure water, for example, represents a neutral solution.
Robert Boyle, a seventeenth century Irish amateur chemist, was the first scientist to categorize solutions as either acids or bases. Although several scientists refined Boyle's definitions, the pH scale was not invented until 1909 when the Danish biochemist Sören Sörensen created a formula to measure acidity.
The formula for measuring pH is logarithmic, not linear, so each pH number under seven is ten times more acidic than the number above it, and each pH number above seven is ten times more basic than the number below it. For example, a pH of two is ten times more acidic than a pH of three; a pH of nine is ten times more basic than a pH of eight. This means that small changes in pH can have large effects. For example, acid rain, which generally has a pH of 4.2 to 4.4, is more than ten times more acidic than clean rain, which usually has a pH of 5.6.
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Acids have more hydrogen ions than bases or neutral solutions. Acids also have a sour taste and react very strongly to metals. Some common acids include bananas (pH of around five), orange juice (pH of three), lemons (pH of two) and sulfuric acid (pH of one).
Bases have fewer hydrogen ions than neutral solutions or acids. Bases tend to feel slippery, and they usually have a bitter taste. Some common bases include baking soda (pH of nine), soapy water (pH of 12) and bleach (pH of 13).
You can easily measure the pH of a solution using a special kind of paper referred to as a litmus strip. Litmus paper changes color when you place it in an acid or base. In acidic solutions, litmus paper turns red. It turns blue in basic solutions. Litmus paper represents an example of a pH indicator, or something that changes color according to the pH of a solution.