Sniff the difference
Some smells change in an acid or a base.
A smelly cloth
A cloth strip kept with chopped onion smells strongly of onion. A teacher dips it into a base (dilute sodium hydroxide) and you sniff it.
What happens to the onion smell?
What this lesson covers
The idea
Substances whose odour changes in acidic or basic media are called olfactory indicators.
A smelly cloth
A cloth strip kept with chopped onion smells strongly of onion. A teacher dips it into a base (dilute sodium hydroxide) and you sniff it.
What happens to the onion smell?
- It goes away
- It becomes stronger
- Nothing changes
Sniff test
Each cloth strip smells of onion, vanilla or clove oil. Dip each one in an acid (dilute HCl) and in a base (dilute NaOH), then sniff. Tap a box. (In real life a teacher does this in the school lab.)
Smell can be an indicator too
Substances whose odour changes in acidic or basic media are called olfactory indicators.
Litmus tells you with a colour. An olfactory indicator tells you with your nose.
- Smell | In acid | In base
- Onion | smell stays | smell is gone
- Vanilla | smell stays | smell is gone
- Clove oil | smell stays | smell is gone
Notes
Substances whose odour changes in acidic or basic media are called olfactory indicators.
Check yourself
A cloth strip with onion smell is dipped in dilute NaOH (a base). What happens to the smell?
A vanilla strip loses its smell in a liquid. The liquid is …
Why are onion, vanilla and clove oil called olfactory indicators?
Which one tells acid from base with your nose, not your eyes?
- The smell is lost — correct. Yes! Onion loses its smell in a base.
- The smell stays. It stays in an acid, but in a base it is lost.
- The smell turns into a colour. There is no colour change here. The smell just disappears.
- an acid. In an acid, vanilla keeps its smell.
- a base — correct. Yes! Vanilla loses its smell in a base.
- neither an acid nor a base. If it were neither, the smell would stay. Losing the smell points to a base.
- Because they all smell strongly. Many things smell strongly. What matters is that the smell changes in an acid or a base.
- Because they are all acids. They are not acids. They only show a change of smell.
- Their odour changes in acidic or basic media — correct. Yes! That is what an olfactory indicator is.
- An onion-smelling cloth strip — correct. Yes! It is an olfactory indicator.
- Blue litmus paper. Litmus tells with a colour change.
- Phenolphthalein. Phenolphthalein tells with a colour change.