what substance turns litmus paper blue

The substance that turns litmus paper blue is an alkaline substance, also known as a base. When red litmus paper comes in contact with a base, it changes color from red to blue due to the alkaline nature of the substance. Examples of substances that turn litmus paper blue include ammonia, baking soda, milk of magnesia, sodium hydroxide, and limewater. This color change indicates the presence of hydroxide ions (OH⁻) in the substance, which makes the solution basic or alkaline.