An acid-base indicator is a substance that shows a different colour in acidic and in alkaline solutions, used to tell whether a solution is acidic or alkaline and to signal the end point of a titration.
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An acid-base indicator is a substance that shows a different colour in acidic and in alkaline solutions. That colour change is what lets you tell an acid from an alkali by sight, and, more precisely, it marks the moment a reaction between them is complete. An indicator does not measure how acidic a solution is; it flags which side of neutral the solution is on.
A worked example lists the ones you must know. Litmus is red in acid and blue in alkali. Phenolphthalein is colourless in acid and pink in alkali. Methyl orange is red in acid and yellow in alkali. Universal indicator is different in kind: instead of a single switch it shows a range of colours across the whole pH scale, red through orange and green to blue and purple, so it gives an approximate pH rather than a simple acid-or-alkali answer.
The confusion to avoid is mixing up an ordinary two-colour indicator with universal indicator. A two-colour indicator such as phenolphthalein answers “acid or alkali” and is used in titration to catch a sharp end point; universal indicator answers “roughly what pH” and is not used for a precise titration because its gradual colour change gives no sharp end point. A second slip is to forget that the colour depends on the solution, not the indicator alone, so quoting “phenolphthalein is pink” without the condition loses the mark.
Within this chapter indicators connect the pH scale to neutralisation and titration, since the colour change at the end point is how titration knows to stop. Our teachers have you memorise the colours as a small, reliable table you can reproduce under exam pressure.
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