Acids, Bases and Salts
The pH scale
Read the pH scale and explain why pH matters in daily life.
Indicators tell you if something is acidic. The pH scale tells you how much. It runs from 0 to 14:
- below 7: acidic (the lower, the more acidic);
- 7: neutral, like pure water;
- above 7: basic (the higher, the more basic).
The 'p' comes from potenz, German for power. More H⁺ ions means a lower pH.
A universal indicator is a mixture of indicators. It shows a different colour at each pH, so one drop tells you roughly the pH.
Strong and weak. A strong acid such as hydrochloric acid gives a lot of H⁺ ions; a weak acid such as acetic acid (in vinegar) gives only a few. In the same way, strong bases give many OH⁻ ions.
pH matters in everyday life:
| Where | What pH does |
|---|---|
| Your body | works only between pH 7.0 and 7.8 |
| Rain | rain with pH below 5.6 is acid rain, which harms river life |
| Soil | plants grow well only in a certain pH range |
| Stomach | extra acid causes pain; an antacid like milk of magnesia neutralises it |
| Teeth | enamel is corroded below pH 5.5 |
Teeth: bacteria in your mouth break down sugar and food left after eating and make acids. When the pH falls below 5.5, tooth enamel starts to wear away. Toothpastes are basic, so brushing after meals neutralises the acid.
Stings: a bee sting leaves an acid that hurts. Rubbing a mild base such as baking soda on it gives relief. The stinging hairs of a nettle leaf inject methanoic acid; the traditional cure is rubbing the leaf of the dock plant, which often grows next to the nettle.
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