Answer: Acids turn blue litmus paper to red.
Litmus paper is a type of paper that changes colour in response to the acidity of the solution it is dipped in and can thus be used to measure acidity.
- Litmus is mainly used to assess whether a solution is acidic or basic.
- The colour transition occurs over the pH range 4.5–8.3 at 25 °C, with light blue litmus paper turning red under acidic conditions and red litmus paper turning blue under basic or alkaline conditions.
- Purple litmus paper is neutral.