The two-colour trick

The idea

A substance that shows different colours in acidic and basic solutions is called an acid-base indicator; indicators may be natural or synthetic (made in laboratories).

Labels fell off!

Two test tubes look alike. One holds an acid (lemon juice). The other holds a base (soap solution). Their labels have fallen off.

How can you find out which is which?

• Taste both of them
• Add a liquid that shows different colours in an acid and in a base
• Look at them very closely

Drop it in both tubes

The left tube is an acid, the right tube is a base. Pick a liquid from the shelf and put it into both tubes. Then decide: is it an indicator?

What makes an indicator

Some liquids showed two different colours: one in the acid, another in the base. The others looked the same in both tubes.

A substance that shows different colours in acidic and basic solutions is called an acid-base indicator.

Indicators can be natural (from lichens, flowers, spices) or synthetic (made in laboratories). Salt solution and tap water are not indicators: they look the same in both tubes.

• Indicator | Colour in acid | Colour in base
• Litmus solution | red | blue
• Red rose extract | red | green
• Turmeric | yellow | red

Notes

A substance that shows different colours in acidic and basic solutions is an acid-base indicator. Indicators may be natural or synthetic (made in laboratories).

Check yourself

A liquid is pink in lemon juice and pink in soap solution. Is it an acid-base indicator?

Red rose extract is red in the acid tube and green in the base tube. What does this show?

Litmus comes from lichens. Which word describes litmus?

What is a synthetic indicator?

• Yes, because it is coloured. Colour alone is not enough. It must show a different colour in an acid and in a base.
• Yes, because it is pink. The colour pink is not the test. Compare the two tubes.
• No. It is the same colour in both. — correct. Right! It cannot tell an acid from a base, so it is not an indicator.
• Red rose extract is an acid. It is red in an acid and green in a base. It is neither itself; it is a tester.
• Red rose extract is an acid-base indicator — correct. Yes! Two different colours in the acid and the base: it is an indicator.
• Red rose extract is a base. It is green in a base and red in an acid. It is the tester, not the thing tested.
• A synthetic indicator. Synthetic indicators are made in laboratories. Litmus comes from lichens.
• A natural indicator — correct. Yes! It comes from living things, so it is a natural indicator.
• Not an indicator. It shows different colours in acid and base, so it is an indicator.
• One that comes from flowers. Flowers give natural indicators.
• One that never changes colour. An indicator must change colour between acid and base.
• One made in a laboratory — correct. Yes! Synthetic means made by people in a laboratory.