The colour swap

The idea

Litmus, a natural substance obtained from lichens, comes as blue and red litmus paper; acidic substances turn blue litmus red, and basic substances turn red litmus blue.

A paper that changes colour

Litmus is a natural dye. It is made from lichens (a fungus and an alga living together on rocks and trees). It is sold as blue and red paper strips.

Imagine you dip a blue litmus strip into lemon juice.

What do you think happens to the strip?

• It stays blue
• It turns red
• It turns green

Dip both strips

Pick a liquid. Then dip the blue strip and the red strip into it, one after the other. Watch which strip changes, and which does not.

The colour swap

Look at your table. Each strip changes colour for only one kind of liquid.

Acidic substances turn blue litmus red. Basic substances turn red litmus blue.

Think of it as a colour swap: the strip swaps to the other strip's colour, but only for its own kind of liquid.

• Strip | In an acidic liquid | In a basic liquid
• Blue litmus | turns red | stays blue
• Red litmus | stays red | turns blue

Notes

Litmus comes from lichens. Acidic substances turn blue litmus red. Basic substances turn red litmus blue.

Check yourself

A red litmus strip is dipped in soap solution and turns blue. What kind of liquid is soap solution?

Vinegar is acidic. What happens when you dip a blue strip in vinegar?

A blue litmus strip stays blue in a liquid. Does this prove the liquid is basic?

Litmus is a natural substance. It is obtained from...

• Acidic. Acids turn blue litmus red. This strip was red, and it turned blue.
• Basic — correct. A strip that changes from red to blue shows a base.
• It cannot be known. It can! Red turning blue is the sign of a base.
• It turns green. Litmus strips only change between blue and red.
• It stays blue. That would happen in a base. An acid changes the blue strip.
• It turns red — correct. Yes! Acids turn blue litmus red.
• It becomes colourless. The paper does not lose its colour. It swaps to red.
• Yes. Blue means basic.. The strip was already blue. Blue litmus stays blue in a base, and in other liquids too.
• No. Test with red litmus: a base turns it blue. — correct. Right! Blue litmus changes only for acids, so you need the red strip to find a base.
• Yes. It must be acidic.. An acid would have turned the blue strip red.
• red roses. Red rose extract is a different indicator. You will meet it soon.
• turmeric. Turmeric is another natural indicator, but it is not litmus.
• lichens — correct. Yes! Litmus comes from lichens.