Which type of paper is most commonly used as an indicator?

Litmus paper is the most commonly used type of indicator because it is designed specifically to test the acidity or alkalinity of a solution. When immersed in a solution, litmus paper changes color depending on the pH level: it turns red in acidic conditions (pH less than 7) and blue in alkaline conditions (pH greater than 7). This color change allows for a straightforward and immediate visual assessment of a solution's pH, making it an invaluable tool in various scientific, educational, and medical applications.

In contrast, filter paper is primarily used for separating solids from liquids, and parchment paper is often utilized in cooking or as a writing surface, not as an indicator. Construction paper is a heavier, colored paper mainly used in arts and crafts, lacking any properties related to pH indication. Thus, litmus paper stands out as the go-to for pH testing due to its specific design and practical application.