litmus
US /ˈlɪt.məs/
UK /ˈlɪt.məs/

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.