Litmus is a substance that is commonly used in chemistry to indicate whether a solution is an acid or a base. It can be prepared in an acidic form, which is red, and in a basic form, which is blue. An acidic solution will turn blue litmus red, but will not affect red litmus. A basic solution will turn red litmus blue, but will not affect blue litmus. A neutral solution, which is neither acidic nor basic, will not change the color of either litmus.