Indicators you can smell

So far every indicator was one you look at. Some indicators you smell. Substances whose smell changes in an acidic or a basic medium are called olfactory indicators (olfactory means to do with smell).

Activity 2.6 uses onions:

• Put finely chopped onions in a container with some strips of clean cotton cloth or filter paper.
• Close it tightly and leave it overnight.
• Next day, take out two strips and smell them.
• Put a few drops of tamarind water (acidic) on one strip and baking soda solution (basic) on the other.
• Smell them again.

Does the onion smell change after the drops? Note what you find, then try other acidic and basic substances the same way.

Why one acid and one base? An olfactory indicator should smell different in the two. Testing both side by side lets you compare.