Final answer:
Red litmus paper turns blue when placed in an alkali, indicating the presence of a basic substance with a pH greater than 7.
Step-by-step explanation:
Red litmus paper is used as a pH indicator to test the acidity or basicity (alkalinity) of a solution. When red litmus paper is placed in an alkali (basic solution), the color changes from red to blue. This color change is a result of a chemical reaction that occurs between the dye in the litmus paper and the base. Bases have a pH value greater than 7 and include common substances like soap, toothpaste, and cleaning agents. It's important to note that this reaction is specific to litmus paper; the pH indicator phenolphthalein, for instance, turns pink in a basic solution but does not affect the color of red or blue litmus paper.
Understanding the color changes in indicators like litmus paper is crucial for students learning about acids, bases, and the pH scale in Chemistry. In summary, when red litmus paper is placed in a basic solution, it turns blue, indicating the presence of an alkali.