litmus

Everyday

1. (science) Litmus is a dye made from lichens that turns red in acid and blue in alkali, used to test liquids. tree →
2. A litmus test is a test with litmus paper; (figurative) it is also a simple test that clearly shows what someone thinks or what something is really like. tree →

In the lessons

In science lessons pupils dip litmus paper into lemon juice (turns red, acid), soap water (turns blue, alkali) and pure water (no change, neutral). In everyday and political talk, «a litmus test» means one key question that shows the truth. Phrases: «litmus paper», «red litmus», «blue litmus», «litmus test».

Examples

• Lemon juice turned the blue litmus paper red. tree →
• Soap turned the red litmus paper blue. tree →
• Use litmus to find out if the liquid is an acid. tree →
• Water does not change the colour of litmus. tree →
• The vote was a litmus test for the new leader. tree →
• How people treat waiters is a litmus test of their kindness. tree →

Note

Uncountable: «some litmus», «litmus paper». Rule to remember: acids turn blue litmus red; alkalis turn red litmus blue. Litmus shows acid or alkali but not how strong; universal indicator gives a range of colours (pH 1–14).