Two olfactory indicators are onion and vanilla extract. But how they are used to test whether the given substance in acidic or basic?

Onions have a characteristic odour but when chopped onions are kept in a dilute solution of NaOH, they lose their characteristic odour. However on keeping chopped onions in a dilute acidic solution, the characteristic odour of onions is not lost. This can help in identifying acidic or alkaline solution.
Similarly, vanilla extract has a sweet pleasant smell. This smell is due to a compound vanillin present in vanilla extract. When vanilla extract is added to a dilute solution of NaOH, the sweet pleasant odour vanishes. However vanilla extract added to a dilute solution of HCl still retains the sweet pleasant smell.
Hope this helps.

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