Acid-test paper is a type of LITMUS paper used to test the acidity or alkalinity of a solution. LITMUS paper is a type of paper that has been treated with a natural water-soluble dye obtained from lichens. It turns red in acidic solutions and blue in alkaline solutions, allowing for a simple and quick test of the pH of a solution. Acid-test paper is a special type of LITMUS paper that is more sensitive to acids than to bases, making it ideal for testing the acidity of a solution.