Olfactory indicators are substances that change their smell (odour) in acidic and basic solutions. Unlike visual indicators (litmus, phenolphthalein) that change colour, olfactory indicators are detected by smell. Common examples include onion, clove oil, and vanilla essence. They are especially useful for coloured solutions where colour change cannot be observed. This is a Class 10 NCERT Chemistry topic.
Olfactory indicators detect acids and bases through change in smell, not colour.
Examples: onion, clove oil, vanilla essence.
All three lose smell in base; retain smell in acid.
Useful when solutions are coloured and colour-change indicators cannot be seen.
'Olfactory' relates to the sense of smell.
NCERT Class 10 Science, Chapter 2 — Acids, Bases and Salts.
Definition: Olfactory indicators are substances that detect acids and bases through change in smell (odour) rather than colour. 'Olfactory' comes from the Latin word for smell.
Why Use Olfactory Indicators? • Some solutions are already coloured → colour change cannot be seen • Olfactory indicators detect acid/base through smell instead • Also useful as a cross-verification method
Common Olfactory Indicators:
Onion: • Has a strong smell • Smell disappears in basic solution (loses odour in base) • Retains smell in acidic solution • Preparation: crush onion, soak in alcohol
Clove Oil: • Has a strong, distinct smell • Smell disappears or reduces in basic solution • Retains smell in acidic solution
Vanilla Essence: • Pleasant sweet smell • Smell disappears in basic solution • Retains smell in acidic solution
Summary Table:
| Olfactory Indicator | In Acid | In Base |
|---|---|---|
| Onion | Smell retained | Smell lost |
| Clove oil | Smell retained | Smell lost |
| Vanilla essence | Smell retained | Smell lost |
Pattern: • All three lose their smell in basic solutions • All three retain their smell in acidic solutions
Comparison — Visual vs Olfactory Indicators:
| Type | Detection Method | Examples |
|---|---|---|
| Visual | Colour change | Litmus, phenolphthalein, methyl orange |
| Olfactory | Smell change | Onion, clove, vanilla |
| Natural | From plants | Litmus, turmeric, China rose |
| Synthetic | Lab-made | Phenolphthalein, methyl orange |
NCERT Reference: Class 10 Science Chapter 2 — Acids, Bases and Salts Activity: Soak strips of cloth/paper in onion or vanilla juice, dry them → use as olfactory indicator strips
Olfactory indicators are substances that detect acids and bases through a change in their smell (odour) rather than colour change. Examples: onion, clove oil, and vanilla essence. All three lose their smell in basic solutions but retain it in acidic solutions. They are used when the solution is coloured and colour-change indicators cannot be easily observed. This is from NCERT Class 10 Science, Chapter 2.
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