Some substances are acids and some are alkalis.
Acids taste sour — like lemon and vinegar. Alkalis feel soapy — like soap and baking soda. We test them safely with litmus paper: an acid turns blue litmus red; an alkali turns red litmus blue. (Never taste or touch unknown substances.)
Remember
- Acid → sour → turns blue litmus red.
- Alkali → soapy → turns red litmus blue.