S3.6.7 Chapter 7

Acids and alkalis

Lemon juice is sour and soap feels slippery. Can you sort them without tasting them?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A No. The only way to tell is to taste them.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. A plant dye can show which is an acid and which is an alkali.
You picked B. Read on. At the end, the chapter comes back to your guess.
C No. All liquids at home are the same, so you cannot sort them.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Make an indicator from red cabbage, turmeric or hibiscus flowers
Sort lemon juice, vinegar, soap and baking soda into acids and alkalis
Place common substances on the pH scale from 0 to 14

Some liquids are sour, like lemon juice. Some feel slippery, like soapy water. Scientists sort these into three groups. They are acids, neutral liquids and alkalis.

An acid tastes sour. Lemon juice, vinegar and the juice in your stomach are acids. An alkali is the opposite of an acid. Soap and baking soda are alkalis. Strong ones feel slippery and burn.

Tasting is not a safe test. Instead we use an indicator. This is a dye that has one colour in an acid and another in an alkali. A liquid that is neither is neutral. Pure water is neutral.

Draw it first

Draw the pH scale

1 Draw a strip and number it from 0 to 14. Mark 7 in the middle as neutral. Write acid on the left and alkali on the right.
2 Add the acids. Lemon juice goes at 2 and vinegar at 3.
3 Add pure water at 7, the neutral place, and soap at 10 on the alkali side.

The pH scale runs from 0 to 14. Below 7 is acid, and the lower the number, the stronger the acid. 7 is neutral. Above 7 is alkali, and the higher the number, the stronger the alkali.

The pH of some things at home. These values are about, and real ones vary. Stomach acid is about 1 and lemon juice about 2.

Baking soda in water is a weak alkali, at about 8. Soap is stronger, at about 10.

Red cabbage makes a good indicator. Its juice is red or pink in an acid. It is purple in a neutral liquid. It turns blue, then green, in an alkali, and yellow in a very strong one.

Red cabbage indicator in three liquids. The word under each cup says the colour.

Turmeric is a yellow spice. It is yellow in acids and neutral liquids. It turns red-brown in an alkali. Hibiscus flowers also work, as red or pink in an acid and green or blue in an alkali. Colours vary a little with the plant and the water.

To make a turmeric indicator, stir a pinch of turmeric into a cup of water. To make a hibiscus indicator, a grown up pours hot water over a few dried petals. Wait until the water is coloured.

Turmeric stays yellow in an acid. It changes only in an alkali.

See it three ways

With things

Each liquid gives a pair of colours. Together they say what the liquid is. You can check one indicator with the other.

As a picture

In words

An acid turns cabbage pink and leaves turmeric yellow. A neutral liquid turns cabbage purple. An alkali turns cabbage blue or green, and turmeric red-brown.

Four things from the kitchen, sorted into acids and alkalis.

Try it

Make a red cabbage indicator

You need: 3 leaves of red cabbage, a bowl, hot water, poured by a grown up, a spoon, 4 clear cups, vinegar, baking soda, a little bar soap or laundry soap, scraped into flakes, water.

Be safe: A grown up pours the hot water. Use only these kitchen liquids. Never use bleach or toilet cleaner. Do not taste the liquids. Wash your hands when you finish.

Tear the cabbage into small pieces and put them in the bowl. A grown up pours hot water over them.
Wait about 15 minutes until the water is deep purple. Let it cool. Pour the purple liquid into a cup and throw the leaves away.
Set out 4 clear cups. Put vinegar in the first. Put plain water in the second. Stir a spoon of baking soda into water in the third. Stir a little scraped bar soap into water in the fourth. Liquid hand soap and dish liquid are often close to neutral, so they may show no change.
Add 2 spoons of the purple liquid to each cup. Watch the colours. Write them down.

What you will see: The vinegar turns pink. The plain water stays purple. The baking soda turns blue. The soap turns green. Your colours may be a little different.

If you cannot try it, this is what happens.

A wrong idea many people have

Acids are always strong and dangerous.

Why it feels right: We hear about acids that burn through metal, and we learn to keep away from them.

The test that breaks it: You eat acids every day. Lemon juice and vinegar are acids. They are weak enough to eat. Your stomach makes a strong acid to digest food. Mucus on its lining protects it. Soap and baking soda are alkalis you handle at home too.

What is true: Strength varies. A strong acid or strong alkali can burn. The pH scale tells you how strong, as well as which kind.

Key words

acid: a substance with a pH below 7. Many taste sour.
alkali: a substance with a pH above 7. Soap is one.
indicator: a dye that changes colour in acids and alkalis.
neutral: neither an acid nor an alkali, with a pH of 7.
pH scale: a scale from 0 to 14 for how acid or alkali something is.

Back to your guess

The answer is B: Yes. A plant dye can show which is an acid and which is an alkali.

An indicator is a dye that changes colour. It tells acids, neutral liquids and alkalis apart, with no tasting at all.

Keep it

Copy this into your exercise book.

Acids are below 7 on the pH scale, neutral is 7 and alkalis are above 7. Cabbage is pink in acid, purple in neutral and blue or green in alkali.

Draw: Draw a strip numbered 0 to 14. Write acid, neutral and alkali. Mark lemon juice, water and soap.