Damp red litmus paper can be replaced with which indicator for pH testing?

• A. Universal indicator
• B. Blue litmus paper
• C. Methyl orange indicator only
• D. Phenolphthalein indicator only

Answer: (A)

pH indicators work by changing color according to how acidic or basic a solution is. A universal indicator is a mix of several indicators that covers the full pH range, so it gives a color that matches most pH values on a standard chart. This lets you estimate the pH across acidic, neutral, and basic solutions in one test. Damp red litmus paper only tells you if something is acidic or not (and it mainly shows acidity versus basicity by turning blue in bases); it doesn’t provide a full pH value. The other single-range indicators (methyl orange and phenolphthalein) only indicate acidity in narrow zones of the scale, so they don’t give a complete pH picture. Blue litmus paper mainly serves to show basicity, not a pH value. So the universal indicator is the best replacement for testing pH.