Colour changers

The idea

Acid-base indicators change colour to show whether a substance is acidic or basic: acids turn blue litmus red, bases turn red litmus blue, and litmus is purple when neither; indicators may be natural (litmus, turmeric) or synthetic (methyl orange, phenolphthalein).

The curry stain

A curry stain on a white cloth is yellow. Scrub it with soap (soap is basic) and it turns reddish-brown. Wash the cloth with plenty of water and it turns yellow again.

Why did the stain change colour?

• Turmeric in the curry changes colour when a base (soap) touches it
• The soap painted the cloth red
• The cloth got hot

Dip and watch

First, litmus. Dip each paper or drop into an acid, a base and pure water. Tap a box to dip.

Now three more indicators. Test each in an acid and in a base.

Indicators are colour changers

Acid-base indicators change colour to show whether a substance is acidic or basic. Acids turn blue litmus red; bases turn red litmus blue; litmus is purple when it is neither.

Natural indicators: litmus, turmeric. Synthetic indicators: methyl orange, phenolphthalein.

• Indicator | In acid | In base
• Litmus (natural) | red | blue
• Turmeric (natural) | yellow | red-brown
• Phenolphthalein (synthetic) | colourless | pink
• Methyl orange (synthetic) | red | yellow

Notes

Acid-base indicators change colour to show whether a substance is acidic or basic: acids turn blue litmus red, bases turn red litmus blue, and litmus is purple when neither; indicators may be natural (litmus, turmeric) or synthetic (methyl orange, phenolphthalein).

Check yourself

Blue litmus paper is dipped in lemon juice. What colour does it turn?

Which liquid turns red litmus paper blue?

Litmus solution turns purple in a liquid. The liquid is …

Which pair has only natural indicators?

• It stays blue. Blue litmus stays blue in a base or in water. Lemon juice is an acid, so it turns red.
• Red — correct. Yes! Acids turn blue litmus red.
• Purple. Purple is the colour of litmus solution when it is neither acidic nor basic.
• Lemon juice. Lemon juice is an acid. It does not change red litmus.
• Pure water. Pure water is neither acid nor base. Red litmus stays red.
• Soap solution — correct. Yes! Soap solution is a base, and bases turn red litmus blue.
• neither acidic nor basic — correct. Yes! Litmus is purple when it is neither.
• acidic. An acid turns litmus red.
• basic. A base turns litmus blue.
• Methyl orange and phenolphthalein. These two are synthetic (made in a factory).
• Litmus and turmeric — correct. Yes! Both come from plants.
• Litmus and methyl orange. Litmus is natural but methyl orange is synthetic.