litmus

US /ˈlɪt.məs/
UK /ˈlɪt.məs/
"litmus" picture
1.

a coloring matter obtained from certain lichens, that turns red in acid solutions and blue in alkaline solutions, and is used as an indicator in chemistry

:
The scientist used litmus paper to test the pH of the solution.
The color change of litmus indicates the acidity or alkalinity.
2.

a decisive test or indicator of something

:
The election results will be a litmus test for the public's trust in the government.
His reaction was a litmus of his true feelings.