Chemistry offers two simple ways to identify acids and bases. Most learners meet color indicators first—litmus, phenolphthalein, and methyl orange—that switch colors with pH. An equally smart classroom tool is the olfactory indicator, where the smell of a substance changes in acidic or basic media. Together they make acid–base testing visual and sensory, quick, and memorable.
What are olfactory indicators?
Olfactory indicators are aromatic substances whose odour is retained in acidic medium and fades in basic medium. The smell decreases in bases because many fragrance molecules (often weak acids such as phenols) become ionized in alkali, forming less volatile ions that do not reach our nose easily. In acids, these molecules remain largely non-ionized and volatile, so the fragrance stays strong.
Everyday examples and observations
| Olfactory indicator | In Acid | In Base |
|---|---|---|
| Onion | Retains smell | Loses smell |
| Vanilla extract (vanillin) | Retains smell | Loses smell |
| Clove oil (eugenol) | Retains smell | Loses smell |
| Nilgiri/Eucalyptus oil | Retains smell | Loses smell |
Classroom test idea: Put a drop of dilute HCl on one strip of paper and a drop of soap solution/NaOH on another. Add a tiny smear or drop of the indicator to each strip and waft the air gently. The acid strip smells, the base strip does not.
Safety: Always waft, never inhale directly; avoid skin/eye contact; work with dilute solutions; wash hands after the activity.
Why does the smell change?
Fragrance molecules such as eugenol (clove) and vanillin (vanilla) contain weakly acidic –OH groups. In alkaline solution, they deprotonate to form phenolate ions, which are non-volatile and far less odorous. In acidic solution, they are neutral molecules—more volatile—and their characteristic smell is evident. Onion’s pungency arises from small sulfur compounds that are also suppressed in basic media through rapid reaction/ionization.
Color indicators at a glance
Color indicators complement smell-based tests and are useful for titrations and pH checks.
| Indicator | Acidic solution | Neutral solution | Basic solution |
|---|---|---|---|
| Blue litmus | Turns red | Blue if neutral | Blue |
| Red litmus | Red | Red if neutral | Turns blue |
| Phenolphthalein | Colorless | Colorless | Pink |
| Methyl orange | Red | Orange | Yellow |
Natural indicators: Litmus is a purple dye extracted from lichen; other plant materials behave similarly—turmeric, red cabbage, and some flower petals (hydrangea, petunia, geranium) show pH-dependent colors due to plant pigments (mainly anthocyanins and curcumin).
Quick applications
In kitchens and labs, these tools help confirm whether a solution is acidic (lemon juice, vinegar) or basic (soap, baking soda). Olfactory indicators are especially handy when colored solutions make visual indicators hard to read, while color indicators are preferred for quantitative work because their transition points are sharp and reproducible.
FAQs
What is an olfactory indicator?
A substance whose odour is retained in acid and fades in base due to ionization that reduces volatility.
Why doesn’t the smell disappear in acids?
Fragrance molecules remain neutral in acidic media, stay volatile, and reach our nose easily.
Which is better—olfactory or color indicators?
Use olfactory indicators for quick, demonstrative tests and colored samples; use color indicators for titrations and precise endpoints.
Can all perfumes act as olfactory indicators?
No. Only those with functional groups that ionize in alkali (e.g., phenolic fragrances) reliably show the effect.