Smells that vanish

The idea

Substances whose odours change in an acidic or basic medium are called olfactory indicators.

Follow your nose

A cut onion has a strong smell. Vanilla essence smells sweet. A teacher dips cloth strips with these smells into a dilute acid and into a dilute base.

What do you think happens to the smell?

• It stays the same in both
• It goes away in the base, but stays in the acid
• It goes away in the acid, but stays in the base

A teacher demonstration

Choose a smell. Waft the dry strip first. Then test a strip dipped in dilute acid and one dipped in dilute base.

Smell as an indicator

Substances whose odours change in an acidic or basic medium are called olfactory indicators.

"Olfactory" means "to do with smell". An olfactory indicator tells acid from base by smell instead of colour. Do this only as a teacher's demonstration with dilute liquids.

• Smell | Dry strip | In dilute acid | In dilute base
• Onion | strong | strong | gone
• Vanilla essence | strong | strong | gone

Notes

Substances whose odours change in an acidic or basic medium are olfactory indicators. Onion and vanilla keep their smell in an acid and lose it in a base.

Check yourself

What changes in an olfactory indicator when it meets an acid or a base?

A strip with vanilla smell is dipped in a liquid and its smell vanishes. What kind of liquid is it likely to be?

An onion strip keeps its smell in a liquid. What can you say?

Who could use an olfactory indicator when colours are hard to tell apart?

• Its colour. Colour changes belong to the indicators like litmus. Olfactory is about smell.
• Its odour (smell) — correct. Yes! Olfactory indicators change their odour.
• Its taste. We do not test by tasting. Olfactory means smell.
• Acidic. Vanilla keeps its smell in an acid.
• Basic — correct. Yes! The smell of vanilla and onion is lost in a base.
• There is no way to know. There is: the smell vanishes in a base.
• The liquid is not basic — correct. Yes! A base would take the smell away.
• The liquid is basic. In a base the onion smell would be gone.
• The onion is spoilt. The onion is fine. The smell stays when no base is there.
• Nobody; only colours work. Smell works too, that is the point of olfactory indicators.
• Anyone, because it tells acid from base by smell — correct. Yes! The result is a smell, not a colour.
• Only a person who cannot smell. A person who cannot smell cannot use it.