Answer: acids turn blue litmus to red.
Litmus turns blue when it comes into contact with acids. When blue litmus paper is placed in an acidic solution, it becomes red.
However, when placed in a basic or neutral substance, it will remain blue. The litmus paper can be used to test a solution’s acidity or alkalinity.
Under acidic conditions, the blue paper turns red when the pH falls below 4.5. In alkaline conditions, red paper, on the other hand, does not change colour (pH more than 8.3).
In alkaline conditions, the colour of blue litmus does not change, whereas the colour of red litmus does not change in acidic conditions.