Indicators
Drop a single drop of colourless liquid into a test tube of clear liquid and watch it turn hot pink — no beaker of fumes, no pH meter, just colour. That is the entire trick of an indicator: a dye that answers "acidic, basic, or neutral?" by changing colour (or, for a few substances, by changing smell) depending on what it is dropped into. Chemists sort indicators into three families. Natural indicators are extracted straight from plants — litmus from lichen, turmeric from a spice jar, red cabbage or china rose petals crushed into juice. Synthetic indicators are laboratory dyes engineered for sharp, unmistakable colour jumps — methyl orange and phenolphthalein are the two you must know. Olfactory indicators are the odd ones out: their smell, not their colour, tells the story — onion and vanilla essence lose their characteristic odour in a basic medium but keep it in acid or neutral conditions. This section fixes the exact colour (and smell) behaviour of each indicator in acid, base, and neutral solutions, and drills the classic identification problems CBSE Class 10 builds around them.
An indicator is a dye or scent that takes on a different, characteristic colour or odour in acidic versus basic solutions, letting you classify an unknown liquid instantly without any calculation.
The core idea
Why colour works as a test at all. Every indicator molecule exists in (at least) two chemically distinct forms — one favoured in acid, one favoured in base — and the two forms happen to absorb light differently, so they look like different colours to the eye. CBSE Class 10 does not ask you to explain why the two forms differ (that equilibrium chemistry is Class 11 territory); you are only expected to know which colour goes with which medium, cold.
The three families, by origin. Natural indicators are extracted from living material: litmus (a purple dye from lichen), turmeric (yellow, from the spice), and red cabbage or china rose (Gudhal) petal extract. Synthetic indicators are man-made dyes: methyl orange and phenolphthalein. Olfactory indicators work through smell rather than colour: onion, vanilla essence, and clove oil all lose their familiar odour when dropped into a basic solution, while the odour survives in acid or neutral solution.
The colour table to memorise.
| Indicator | Acidic | Basic | Neutral |
|---|---|---|---|
| Blue litmus | turns red | stays blue | stays blue |
| Red litmus | stays red | turns blue | stays red |
| Methyl orange | red / pink | yellow | orange |
| Phenolphthalein | colourless | pink (magenta) | colourless |
| Turmeric | stays yellow | turns red-brown | stays yellow |
Two features of this table cause almost every exam mistake, so they are worth stating as plain rules rather than leaving them buried in the table. First: blue litmus cannot separate basic from neutral — it stays blue in both, so it only ever tells you "acid" versus "not acid." Second: phenolphthalein is colourless in two different situations (acidic and neutral) and only turns pink for base — so a colourless result never by itself proves neutrality; it proves "not basic."
Why it works
There is no computed formula behind indicators at this level — the syllabus asks for recognition and application, not the underlying equilibrium chemistry. What still needs to be reasoned through, and what examiners test hardest, is the logic of choosing and combining indicators to fully identify an unknown solution.
Step 1. Start from what a single indicator can and cannot distinguish. Blue litmus splits the world into two groups only: "turns red" (acidic) versus "stays blue" (basic or neutral, indistinguishable). It cannot separate the second group further, because both basic and neutral solutions leave blue litmus unchanged.
Step 2. To break the tie inside "stays blue," bring in a second indicator whose blind spot is different. Phenolphthalein is exactly complementary here: it is colourless in both acidic and neutral solutions, but pink only in basic solution — so applying it to the "stays blue" group (already known to be non-acidic) cleanly separates basic (turns pink) from neutral (stays colourless).
Step 3. This is why real laboratories and exam questions almost always chain two indicators rather than trusting one: each indicator collapses two of the three categories (acid/base/neutral) into one colour, so a single test only ever answers a yes/no question, never a three-way classification. Choosing indicators whose "blind spots" do not overlap is the entire skill being tested.
Step 4. The same reasoning extends to turmeric and olfactory indicators — turmeric is essentially a natural analogue of phenolphthalein's logic (stays yellow for acid and neutral, changes only for base), while an olfactory indicator like onion swaps "colour" for "odour" but keeps the same two-state logic: odour present in acid/neutral, odour masked in base.
Worked example
Question: A student has three unlabelled test tubes, A, B and C, containing dilute HCl, distilled water, and dilute NaOH solution (not necessarily in that order). Blue litmus paper is dipped in each: tube A turns the paper red, tubes B and C cause no colour change. The student then wants to identify B and C using a second indicator, and also wants to predict what turmeric paper would show for all three tubes.
Step 1. Tube A turning blue litmus red confirms A is acidic, so A is dilute HCl — litmus alone settles this one instantly.
Step 2. Tubes B and C give no change with blue litmus because blue litmus stays blue in both basic and neutral solutions; it cannot distinguish base from neutral, so a second indicator is required.
Step 3. Use phenolphthalein on B and C. Phenolphthalein is colourless in a neutral solution but turns pink in a basic solution. If B turns pink, B is dilute NaOH (basic); if C stays colourless, C is distilled water (neutral).
Step 4. For turmeric paper on all three: turmeric stays yellow in both acidic and neutral solutions and turns red-brown only in basic solution. So tube A (acidic) keeps the paper yellow, tube B (basic, NaOH) turns it red-brown, and tube C (neutral, water) keeps it yellow.
Final answer: A = dilute HCl (turns blue litmus red), B = dilute NaOH (colourless-to-pink with phenolphthalein, yellow-to-red-brown with turmeric), C = distilled water (no change with phenolphthalein or turmeric).
Competitive stretch
If only turmeric paper (no litmus, no phenolphthalein) were available for the same three tubes, could the student still fully identify all three? Turmeric alone stays yellow for both A (acid) and C (neutral) and only turns red-brown for B (base) — so turmeric alone identifies B, but cannot separate A from C, exactly mirroring the blind spot blue litmus has for base versus neutral. A second indicator is still unavoidable.
Common traps
- Believing blue litmus can tell base from neutral — it cannot; it stays blue in both, so a "no change" result only rules out acid.
- Reading "colourless" as "phenolphthalein did nothing" — colourless IS its correct response in acid and neutral solution; only pink signals base.
- Assuming olfactory indicators change colour — they change smell only, and are a separate category from litmus, methyl orange, and phenolphthalein.
- Forgetting red litmus stays red in a neutral solution — students often expect every indicator to visibly react to every solution, but "no change" is itself a valid, informative result.
Ask the tutor: want a rapid-fire quiz mixing all four indicators across acid, base, and neutral cases, or a walkthrough of another unknown-solution identification problem?
Why do we need two indicators to identify a solution, instead of just one really good one
Because every common indicator has a "blind spot" where it gives the same response to two different categories. Blue litmus cannot separate basic from neutral; phenolphthalein cannot separate acidic from neutral. Pairing two indicators whose blind spots do not overlap lets you triangulate the answer — this is exactly why real labs stock more than one indicator, and why CBSE questions almost always ask you to pick a second indicator after the first one hits its limit.
Is turmeric basically the same as phenolphthalein, just natural instead of synthetic
Their colour logic is very similar — both stay one colour across acid and neutral, and only change for base — but they are chemically unrelated substances, and turmeric's colour shift (yellow to red-brown) is much less sharp and less commonly used for precise endpoint detection than phenolphthalein's dramatic colourless-to-pink change. Turmeric is mainly remembered here as the classic household demonstration, such as a soap stain on turmeric cloth.
Do olfactory indicators actually appear in Class 10 exams, or are they just extra information
They are examinable. CBSE explicitly lists olfactory indicators (onion, vanilla essence, clove oil) alongside natural and synthetic indicators, and expects you to know that their odour is masked in basic solution while it persists in acidic or neutral solution. Do not treat them as a footnote — a direct question naming an olfactory indicator and asking you to predict its behaviour is a realistic exam pattern.