Acids
An acid is a substance that tastes sour and has a pH value below 7. Common acids include the citric acid in lemons, the acetic acid in vinegar and the hydrochloric acid in our stomach. Acids turn blue litmus paper red.
Alkalis
An alkali is a substance that tastes bitter, feels soapy or slippery and has a pH value above 7. Examples include soap, toothpaste and baking soda. Alkalis turn red litmus paper blue.
Key idea
The pH scale runs from 0 to 14. A pH below 7 is acidic, a pH of exactly 7 is neutral (like pure water), and a pH above 7 is alkaline. The further from 7, the stronger the acid or alkali.
Indicators
An indicator is a substance that changes colour to show whether something is an acid or an alkali. The most common is litmus paper. Universal indicator gives a range of colours matching the pH value.
| Substance | Blue litmus | Red litmus |
|---|---|---|
| Acid | Turns red | Stays red |
| Alkali | Stays blue | Turns blue |
Remember
- Acids: sour, pH below 7, turn blue litmus red.
- Alkalis: bitter and soapy, pH above 7, turn red litmus blue.
- Never taste laboratory chemicals to test them.