Changes that bring danger
Do you know you can make an acid detector? All you need is a red cabbage, a kitchen and an adult to help you. First the red cabbage needs chopping up. Then the pieces of leaves need to be put in a pan of water. Next let the adult boil the cabbage for five minutes and leave it to cool. When you look in the pan you should find that the cabbage water has turned blue. Pour the cabbage water through a tea strainer into a jar. This acts as a filter and removes any bits of cabbage leaves from the water. You now have your acid detector ready for use.

When you take some of the cabbage water and add vinegar, the cabbage water turns red. This is the test for acid. All acids will turn cabbage water red. If you tested the cabbage water with grapefruit juice or orange juice it would turn red because both juices contain an acid called citric acid. When you add water to cabbage water there is no change. This shows that water is not an acid. If you try your cabbage water with salt it will remain unchanged. Salt water is not an acid.

If you test your cabbage water with bicarbonate of soda something different happens. The cabbage water turns green. Toothpaste may produce the same result. Ask an adult to help you test the cabbage water with washing soda: the cabbage water turns green. It seems that your acid detector has identified another group of substances.

These substances are called alkalis. This name is an Arabic word and means plant ash because the first alkalis were discovered in the ash or wood and in other plants.

You may think that alkalis are the opposite of acids and in a way they are. If you sting yourself on a nettle it is methanoic acid that gives you pain. You can relieve the pain by finding a dock leaf and rubbing it on your skin. The dock leaf contains an alkali, which takes part in a change with the acid and destroys it. When an acid has been destroyed in this way, we say that the acid has been neutralised.

You may think that because acids can be unsafe and harmful alkalis are safe and harmless. This is very dangerous thinking. Just as some acids, like the acid in vinegar, are weak acids and some acids, such as sulphuric acid, are strong acids, there are also weak and strong alkalis. Bicarbonate of soda is a weak alkali and is harmless; washing soda is a stronger alkali and must be used with care. For example, you must not let it splash in your eyes. Some alkalis, such as caustic soda, are very strong and must only be handled carefully by adults.

Acids and alkalis are two substances which can take part in changes. Weak acids and alkalis are harmless but strong ones can cause burns to the skin and damage flesh. Fortunately there is a way to recognise harmful chemicals without using cabbage water. Someone has tested them for you and all containers of harmful chemicals have hazard warning symbols on them. A cross means the substance is an irritant or is harmful. A pair of dripping test tubes means the substance is corrosive and will burn skin and flesh. A skull and crossbones, as you may have made on a pirate's flag, means the substance is poisonous. Leave these containers alone. Let adults handle them.

Isn't there some paper that changes different colours when you use it to test for acids and alkalis?
Yes, there is. It is called universal indicator paper. The cabbage water contained just one dye, which changed when a substance was either an acid or an alkali. The universal indicator paper contains a number of dyes. Each one changes colour when it meets an acid or an alkali of a particular strength. For example, hydrochloric acid is very strong and turns the paper red, while vinegar is a weaker acid and turns the paper orange.

What colour does the indicator paper turn with water?
It turns green. Water is neither an acid nor an alkali. It is neutral. Universal indicator paper detects neutral substances by turning green.

What colour does the indicator paper change with alkalis?
If the alkali is weak, like the alkali in toothpaste, the indicator paper turns a dark green. If it is a stronger alkali, such as washing soda, the paper turns purple.

Can other kinds of stings beside nettle sting be cured by neutralisation?
Yes, they can. A bee sting is caused by an acid. It can be neutralised by soap because soap is an alkali. A wasp sting is made by an alkali. It may be neutralised with vinegar.

What is an antacid tablet?
It is a tablet containing an alkali. It is taken to cure some kinds of indigestion. When you are eating a meal the stomach produces hydrochloric acid to help you digest your food. If your stomach produces too much you get a pain called indigestion. The antacid neutralises the stomach acid and the indigestion goes away.

How can you have a strong acid in your stomach and not be harmed?
The stomach produces slime which coats its walls. The acid cannot get through the barrier of slime to harm the stomach wall.

Baking soda contains an alkali. Is it safe?
Yes. The alkali is sodium bicarbonate. Baking powder also contains a weak safe acid. When it is used in baking the acid and alkali neutralise each other and carbon dioxide gas is made. This gas forms bubbles in a cake mixture and makes the cake rise.

Why are dangerous alkalis used in the home?
They are used because they can help to keep the home clean. Grease is one form of dirt and a strong alkali, such as caustic soda, destroys it quickly. Alkalis can also destroy germs in drains.

If the alkalis are handled with care, by a responsible adult, the danger is reduced.

Are alkalis used for anything else?
Yes. They are used to make soap, paper, and clothing and to get aluminium out of its ore. You may see tankers on the road carrying alkalis to factories. Next time you see one look for the hazard warning labels on the side. These substances are very strong and are handled with great care by the people at the factories.