Question
What colour changes do phenolphthalein, methyl orange, and litmus show in acidic, neutral, and basic solutions? Which indicator should we use for which type of titration?
(CBSE 10 Board — frequently asked 2-3 mark question)
Solution — Step by Step
| Indicator | In acid | In neutral | In base | pH range of change |
|---|---|---|---|---|
| Litmus | Red | Purple | Blue | 5.0 - 8.0 |
| Phenolphthalein | Colourless | Colourless | Pink | 8.2 - 10.0 |
| Methyl orange | Red | Orange | Yellow | 3.1 - 4.4 |
| Universal indicator | Red/orange | Green | Blue/violet | Full pH scale |
| Titration type | Best indicator | Why |
|---|---|---|
| Strong acid + Strong base | Any indicator works | Steep pH change at equivalence |
| Strong acid + Weak base | Methyl orange | Equivalence point is acidic (pH 4-6) |
| Weak acid + Strong base | Phenolphthalein | Equivalence point is basic (pH 8-10) |
| Weak acid + Weak base | None of the above | No sharp equivalence point — use pH meter |
Indicators are weak acids or bases themselves. In acidic solution, they exist in one form (one colour). In basic solution, they exist in another form (another colour). The transition happens over a narrow pH range.
Phenolphthalein:
In acid, equilibrium shifts left — colourless. In base, equilibrium shifts right — pink.
flowchart TD
A["Need to choose an indicator"] --> B{"Type of titration?"}
B -- "Strong acid + Strong base" --> C["Any: litmus, phenolphthalein, methyl orange"]
B -- "Strong acid + Weak base" --> D["Methyl orange - changes in acidic range"]
B -- "Weak acid + Strong base" --> E["Phenolphthalein - changes in basic range"]
B -- "Weak acid + Weak base" --> F["No suitable indicator - use pH meter"]
D --> G["End point: red to yellow/orange"]
E --> H["End point: colourless to pink"]Why This Works
The indicator must change colour at or near the equivalence point of the titration. Strong acid + weak base has an acidic equivalence point, so we need an indicator that changes in the acidic range (methyl orange, pH 3-4). Weak acid + strong base has a basic equivalence point, so we need one that changes in the basic range (phenolphthalein, pH 8-10).
Universal indicator is useful for finding approximate pH but is too gradual for precise titrations — it does not give a sharp colour change at a single pH.
Alternative Method
For quick CBSE revision, remember this table of litmus results:
| Solution | Red litmus | Blue litmus |
|---|---|---|
| Acidic | Stays red | Turns red |
| Basic | Turns blue | Stays blue |
| Neutral | Stays red | Stays blue |
For CBSE 10, the most commonly asked indicator question is about phenolphthalein. Remember: it is colourless in acid, colourless in neutral, and pink in base. The colour appears only when pH crosses about 8.2. If the question says “colourless to pink,” it is phenolphthalein in a basic solution.
Common Mistake
Students write that phenolphthalein turns “red” in base. The correct colour is PINK (not red). Red in acid is methyl orange. Pink in base is phenolphthalein. Mixing these colours up costs marks in board exams. Also, phenolphthalein is colourless (not white) in acid — “colourless” means transparent, “white” means opaque.