Acids and bases: the pH scale

0:00How do chemists describe how acidic or basic a solution is?

0:05They use the pH scale, which runs from 0 to 14 for most solutions.

0:11In the middle, pH 7 is neutral, and pure water has a pH of 7.

0:17Below 7 is acidic: vinegar is about pH 3, and lemon juice is about pH 2.

0:25Above 7 is basic: baking soda solution is about pH 8, and household ammonia is about pH 11.

0:34In water, acids release hydrogen ions, written H plus.

0:39A base that dissolves in water is called an alkali, and it produces hydroxide ions, written O H minus.

0:48Each step down the scale means ten times the concentration of hydrogen ions.

0:54So pH 3 has ten times ten, or 100 times, the hydrogen ion concentration of pH 5.

1:03An indicator is a substance that changes colour depending on the pH.

1:08Blue litmus turns red in an acid, and red litmus turns blue in an alkali.

1:15Bases are not harmless: strong bases, like the sodium hydroxide in drain cleaner at about pH 14, burn skin just as strong acids do.

1:27So on the pH scale, below 7 is acidic, 7 is neutral, above 7 is basic, and each step is a factor of ten.