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What is an acid?

Acids are a group of soluble chemicals.

What do you know about acids?

Are all acids dangerous?


Acid or alkali?


What is a weak acid?

Some acids are found in everyday items such as food and drink, and skincare products.

It is safe to handle these acids, and in some cases even to taste them! These acids are known as weak acids.


What is a strong acid?

Some acids, like those found in the laboratory or a car battery, are too dangerous to taste or touch.

These acids are said to be corrosive as they can damage other materials by wearing them away.

What safety precautions should you take when handling a strong acid?

These are strong acids.


The Acid Bath Murderer

Concentrated acids can completely dissolve skin and bones.

In the 1940s, an Englishman called John Haigh murdered six people, and disposed of each of the bodies by dissolving them in sulfuric acid for two days.

Without the bodies as evidence Haigh thought he would get away with the crimes, but a pathologist found human fat, pieces of bone, gall stones and false teeth, which had resisted the acid.

John Haigh was found guilty of the murders and sentenced to death by hanging in 1949.


What is an alkali?

Alkalis are substances that are chemically the opposite of acids.

Weak alkalis are found in soaps and other cleaning materials. They are also used in antacids to treat indigestion.

It is safe to handle these alkalis, which often feel soapy.

These alkalis are weak alkalis.


What is a strong alkali?

Some alkalis, like those used in laboratories, or in cleaning materials such as bleach, are too dangerous to touch.

What safety precautions should you take when handling a strong alkali?

These alkalis are said to be caustic because they can burn skin, and damage other materials.

These are strong alkalis.


What effect do alkalis have on our skin?

Many people suffer from eczema, a condition which can make the skin itchy and sore.

Sometimes this is caused by an allergic reaction to soap or other cleaning materials.

Which chemicals in soap do you think might cause the allergic reaction?

How could you find out what is causing the allergic reaction?


What is a neutral substance?

Neutral substances are neither acidic nor alkaline.

Pure water is a neutral substance.

What safety precautions should you take when handling a neutral substance?

Saltwater, milk, saliva and blood may actually be slightly acidic or alkaline, but are all very close to neutral.


Acids, alkalis and plants

Some flowers change colour depending on whether they are growing in acidic or alkaline soil. Hydrangeas grow blue flowers in an acidic soil, but pink flowers in an alkaline soil.

The flowers contain compounds called anthocyanins, which act as indicators.


What is an indicator?

An indicator is a chemical that turns a different colour depending on whether it is added to an acid or an alkali.

Litmus is a commonly used indicator. It can be added to filter paper, or used as a liquid. Litmus is made from lichen, which grows on stone and rock.

Litmus can be blue or red. Blue litmus turns red under acidic conditions (< pH 4.5), and red litmus turns blue under alkaline conditions (> pH 8.3).

What colour do you think litmus is under neutral conditions?

acid

alka

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neu

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The litmus test


Making indicators from plants


What is universal indicator?

Universal indicator has a range of colours that show how weak or strong the acid or alkali is.

What is the colour for a strong alkali?

What is the colour for a weak acid?

Universal indicator is made from a mixture of indicators.Why do you think it is more useful than litmus?

weak alkaliweak acidstrong acid neutral strong alkali


What is the pH scale?

The strength of an acid or alkali is measured by the pH scale.

Each universal indicator colour is given a pH value.

Universal indicator can tell you the pH of a solution.

What is the pH of a strong alkali?

What is the pH of a weak acid?

1 2 3 4 5 6 7 8 9 10 11 12 13 14

strong acid neutral strong alkali


Using universal indicator


What is the pH?


Soil pH

Different soils have different pH levels. This is because they are made up of different rocks, and so contain different chemicals.

The pH of soil affects the availability of nutrients, and plants prefer to grow in soils of a certain pH.

How could you check the pH of the soil in your garden?

For example, strawberries grow best in acidic soil, and plums grow best in alkaline soil.


Testing the pH of soils


Changing soil pH

If soil is too acidic to grow certain crops, lime can be added.

Lime is an alkali called calcium oxide.

What effect does lime have on the pH of soil?


Acid or alkali: true or false?


Acid or alkali: summary


Glossary


Anagrams


Multiple-choice quiz

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