KS3 Chemistry
7E Acids and Alkalis
1 of 20
34
© Boardworks Ltd 2004
Contents
7E Acids and Alkalis
What are acids and alkalis?
Hazard symbols
Indicators and the pH scale
Neutralization
Summary activities
1
2 of 20
34
© Boardworks Ltd 2004
What is an acid?
Acids are a group of chemicals.
What do you know about acids? Are all acids dangerous?
1
3 of 20
34
© Boardworks Ltd 2004
What is an acid?
Acids are a group of chemicals.
Which of these items contains an acid?
everyday acids
laboratory acids
Can you think of other examples of acids?
1
4 of 20
34
© Boardworks Ltd 2004
What is a weak acid?
Acids are found in household items such as food, drink
and beauty/skincare products.
It is safe to handle these acids and even taste them, if they
are for eating! These acids are known weak acids.
1
5 of 20
34
© Boardworks Ltd 2004
What is a strong acid?
Some acids, like those found in the laboratory or a car
battery, are very dangerous (too dangerous to taste or touch).
These acids are said to be corrosive as they can damage
other materials by wearing them away! They are strong acids.
Why must strong acids always be handled very carefully?
1
6 of 20
34
© Boardworks Ltd 2004
What is an alkali?
Alkalis are another group of chemicals, the opposite of acids.
Which of these items contains an alkali?
everyday alkalis
laboratory alkalis
Can you think of other examples of alkalis?
1
7 of 20
34
© Boardworks Ltd 2004
What is a weak alkali?
Alkalis are found in soaps and other materials used for
cleaning.
It is safe to handle these alkalis, which can feel soapy.
These alkalis are known as weak alkalis.
1
8 of 20
34
© Boardworks Ltd 2004
What is a strong alkali?
Some alkalis, like those in the laboratory or in cleaning
materials such as toilet cleaner, are too dangerous to touch.
These alkalis are said to be caustic because they can burn
skin and damage other materials! They are strong alkalis.
Why must strong alkalis always be handled very carefully?
1
9 of 20
34
© Boardworks Ltd 2004
Contents
7E Acids and Alkalis
What are acids and alkalis?
Hazard symbols
Indicators and the pH scale
Neutralization
Summary activities
1
10ofof20
34
© Boardworks Ltd 2004
What are hazard symbols?
How can you tell which chemicals are safe and which are
dangerous?
Special symbols are used on bottles and vehicles that
contain dangerous chemicals.
i
toxic
irritant
h
harmful corrosive
highly
flammable
These hazard symbols show why the chemical is dangerous.
Why is it important that these symbols can be recognised by
people from other countries?
1
11ofof20
34
© Boardworks Ltd 2004
What do hazard symbols mean?
1
12ofof20
34
© Boardworks Ltd 2004
Contents
7E Acids and Alkalis
What are acids and alkalis?
Hazard symbols
Indicators and the pH scale
Neutralization
Summary activities
1
13ofof20
34
© Boardworks Ltd 2004
What is an indicator?
How can you test a liquid to find if it is an acid or an alkali?
An indicator is a special chemical that changes
to a different colour in an acid or an alkali.
Litmus paper and litmus solution are
examples of indicators.
Litmus is
red in acid.
1
14ofof20
34
Litmus is
blue in alkali.
© Boardworks Ltd 2004
Litmus test experiment
1
15ofof20
34
© Boardworks Ltd 2004
What is universal indicator?
Litmus is used to show if a solution is acid or alkali.
Litmus does not show if the acid or alkali is weak or strong.
Universal indicator has a range of colours that show how
weak or strong the acid or alkali is.
stronger acid
strong
acid
stronger alkali
weak
weak
acid neutral alkali
strong
alkali
What is the colour for a weak acid?
What is the colour for a strong alkali?
1
16ofof20
34
© Boardworks Ltd 2004
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 number called
the pH value.
Universal indicator can tell you the pH of a solution.
stronger acid
stronger alkali
1 2 3 4 5 6
7 8 9 10 11 12 13 14
strong
acid
weak
weak
acid neutral alkali
strong
alkali
What is the pH of a weak acid?
What is the pH of strong alkali?
1
17ofof20
34
© Boardworks Ltd 2004
What is the pH?
1
18ofof20
34
© Boardworks Ltd 2004
What type of substance?
Are these substances acidic or alkaline?
Are they weak or strong?
Substance
soda water
car battery acid
1
Description of acid/alkali
very weak acid
very strong acid
soap
8
very weak alkali
washing soda
10
weak alkali
stomach acid
2
strong acid
oven cleaner
14
very strong alkali
vinegar
4
weak acid
acid
1
19ofof20
34
pH
6
1 2 3 4 5 6
7
8 9 10 11 12 13 14
alkali
© Boardworks Ltd 2004
Contents
7E Acids and Alkalis
What are acids and alkalis?
Hazard symbols
Indicators and the pH scale
Neutralization
Summary activities
1
20ofof20
34
© Boardworks Ltd 2004
Mixing an acid and an alkali
What happens when an acid and an alkali are mixed?
?
Mixing an acid and an alkali causes a chemical change.
This chemical change is called a chemical reaction.
How do you know this chemical reaction has taken place?
1
21ofof20
34
© Boardworks Ltd 2004
What is neutralization?
The chemical reaction between an acid and an alkali
is called neutralization.
acid
alkali
a salt
water
What happens to the pH value of the reaction mixture during
neutralization?
The pH value of the reaction mixture becomes closer to 7.
1
22ofof20
34
© Boardworks Ltd 2004
What is neutralization?
The chemical reaction between an an acid and an alkali
can be written as:
acid
alkali
a salt
water
What are the two new substances produced by a
neutralization reaction?
1
23ofof20
34
© Boardworks Ltd 2004
Neutralization and indigestion
1
24ofof20
34
© Boardworks Ltd 2004
Neutralizing stings
1
25ofof20
34
A bee sting
is acidic.
A wasp sting
is alkaline.
Why can bicarbonate
of soda (pH 9) be used
to treat a bee sting?
Why can vinegar (pH 3)
be used to treat
a wasp sting?
© Boardworks Ltd 2004
Neutralizing soil
Soils in different places can
be acidic, alkaline or neutral.
Many plants do not grow
well in acidic soil.
Why do farmers test soil
to check the pH?
Farmers often add an alkali to acidic soil to make their
plants grow better.
How does the alkali help to treat the acidic soil?
1
26ofof20
34
© Boardworks Ltd 2004
Neutralizing acid rain
Gases emitted by cars, power
stations and factories mix with
rainwater in the atmosphere.
This rainwater has a pH <5.5.
Why is it called acid rain?
Acid rain can have a harmful
effect on the environment
killing trees, polluting lakes
and damaging buildings.
How can the effects of
acid rain be prevented?
1
27ofof20
34
© Boardworks Ltd 2004
Contents
7E Acids and Alkalis
What are acids and alkalis?
Hazard symbols
Indicators and the pH scale
Neutralization
Summary activities
1
28ofof20
34
© Boardworks Ltd 2004
Glossary
 acid – A chemical that has a pH less than 7.
 alkali – A chemical that has a pH greater than 7 and
dissolves in water.
 indicator – A special chemical that changes into different




1
29ofof20
34
colours in acid and alkali solutions.
neutral – A chemical with a pH of 7 that is not an acid or
an alkali.
neutralization – The chemical reaction between an acid
and an alkali.
pH scale – The range of values that shows how strong or
weak an acid or alkali is.
universal indicator – The indicator that shows the pH
value of a chemical.
© Boardworks Ltd 2004
Anagrams
1
30ofof20
34
© Boardworks Ltd 2004
Word search
1
31ofof20
34
© Boardworks Ltd 2004
Facts about acids
 Weak acids, like lemon juice and vinegar, taste sour.
 Strong acids are corrosive and can “eat away” at metal,
stone and flesh!
 Acids turns blue litmus paper red.
 Acids have a pH of less than 7.
 Acids can be neutralized with alkalis.
1
32ofof20
34
© Boardworks Ltd 2004
Facts about alkalis
 Alkalis feel soapy and are used in household cleaning
materials.
 Strong alkalis are caustic.
 Alkalis turn red litmus paper blue.
 Alkalis have a pH of more than 7.
 Alkalis can be neutralized with acids.
1
33ofof20
34
© Boardworks Ltd 2004
Multiple-choice quiz
1
34ofof20
34
© Boardworks Ltd 2004