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What is an acid?
Here are some facts about acids.
• Strong acids are corrosive “eating away” at things like
metal, stone and flesh!
• Weak acids, like lemon juice and vinegar, taste sour.
• Acids turn litmus red.
• Acids have a pH below 7.
• Acids contain hydrogen (but not all things that contain
hydrogen are acids!).
• Acids can be neutralised with alkalis.
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Some common acids
Three acids are particularly common in the laboratory .
Acid
hydrochloric
sulphuric
nitric
Formula
HCl
H2SO4
HNO3
Salts formed
chlorides
sulphates
nitrates
These are strong acids that should be treated with
the greatest respect.
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Indicators: acid or alkali?
An indicator is a dye which changes colour according to
whether it is in an acidic or alkaline solution.
For example, litmus is an indicator
that is red in acid and blue in alkali.
Litmus is available as a liquid that
can be added to the solution.
It is also available as strips of
paper so that you can add a drop
of the solution to the paper.
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Litmus Test
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Indicators: how acid or alkali?
Litmus will tell you whether a solution is acid or alkali:
what it won’t tell you is how acid or alkali.
Universal indicator has a whole range of colours that
tell us how strong an acid or alkali is.
Strong acid
Neutral
Strong alkali
Weak acid
Weak alkali
More acid
More alkali
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Indicators: the pH scale
This attaches a number called the pH value to each
universal indicator colour.
•pH7 is neutral
•pH 1 is strongly acid
•pH14 is strongly alkali
This means we can quickly say how acid or alkali a
substance is by quoting a single number.
1 2 3 4 5 6
7 8 9 10 11 12 13 14
Strong acid
Neutral
Strong alkali
Weak acid
Weak alkali
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Indicators: the pH scale
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Indicators: the pH scale
Will it be acidic, basic or neutral and how strong?
Substance
pH
Soda water
6
very weak acid
Car battery acid
1
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 2 3 4 5 6
Description of acid / alkali
7
8 9 10 11 12 13 14
Alkali
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Bases
Bases are substances that neutralise acids.
Bases are usually:
•Metal hydroxides
•Metal oxides
•Metal carbonates
contain OH
contain O
contain CO3
The following general word equation describes
neutralisations:
acid
+
base

a salt
+
water
In the case of carbonates we also get carbon dioxide.
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Alkalis
Bases are substances that neutralise acids.
Alkalis are soluble bases.
Although both can neutralise acids solubility is
important when it comes to the pH of
solutions. For example, adding sodium
hydroxide to water gives a solution with a pH
of about 14.
When calcium carbonate is added to water it
does not dissolve and so the pH remains
close to 7. Even so it can neutralise acid that
is added although more slowly than a soluble
base might.
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Neutralisation - Indigestion
If we have too much acid in our stomachs, we
get indigestion.
Acid can move up out of our stomach creating
a burning feeling in the chest.
We neutralise the excess acid by taking a tablet containing
a base.
This is usually a carbonate or an oxide.
Strong soluble bases (like sodium hydroxide) would create
too alkaline a solution and cannot be used.
Acid
1 2 3 4 5 6
7
8 9 10 11 12 13 14
Alkali
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Neutralisation - Stings
A bee sting is acidic.
A wasp sting is alkaline.
So one way to treat a an acidic bee sting is to
dab on a base: bicarbonate of soda more
properly known as sodium hydrogen
carbonate.
One way to treat a basic wasp’s sting is with
an acid : vinegar - ethanoic acid.
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Neutralisation – Soil pH
Plants remove compounds from the soil in a way
that tends to leave the soil acidic.
Many plants won’t grow well in acid soil and so
farmers have to regularly check the pH and
adjust it by adding a base.
Calcium carbonate or calcium
hydroxide are cheap and so are
often used for this purpose.
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Neutralisation – Soil fertilisers
Plants also remove nitrogen compounds from the soil and
this is often replaced using fertiliser.
Ammonia is a water soluble gas high in nitrogen and in
some countries it is injected directly into moist soil.
However, because it is a gas much of it is quickly lost.
In the UK ammonia is dissolved in water to give
ammonium hydroxide (an alkali) and this is neutralised
by reacting it with nitric acid to give a solid nitrogen rich
fertiliser.
ammonium
hydroxide
NH4OH
+
nitric acid
+
HNO3
 ammonium
nitrate

NH4NO3
+
water
+
H2O
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Neutralisation - acid gases
Many power stations burn coal containing sulphur.
When this burns it produces acidic sulphur oxides which
can cause acid rain.
The gases are “scrubbed”, as much as possible,
of these acidic oxides by reacting them with a
base before releasing them into the air.
Calcium oxide or calcium hydroxide are often
used for this purpose.
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Acid rain – living things
Steps have been taken to reduce emissions of acidic
sulfur oxides from power stations and nitrogen oxides
from cars.
Even so the atmosphere still contains enough of them to
make the rain from industrial areas quite acidic.
Trees and lakes are badly
affected in many parts of the
world including Northern
Germany and Scandinavia
which suffers from South-West
winds from the UK.
Acid rain damaged tress
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Acid rain – metals and stone
Acid rain increases the rate of corrosion of metals.
It also greatly accelerates the rate of chemical weathering
of certain stones used in building such as limestone and
marble. (These stones are carbonates. What gas will be
given off as they dissolve?)
CO2
The metal above the
wheel arch of this car
is rusting away
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• There are sixteen
“easy-find” words
in the word square.
• Write a clue for
each word drawing
upon what you
have studied in this
unit of work
Acid
Alkali
Carbonate
Chloride
Hydrogen
Indicator
Indigestion
Litmus
Metal
Neutralise
Nitrate
Nitrogen
Salt
Water
I Q W R A C I D Q P S S W K G
N E U T R A L I S E G A R M Y
D P L X S R Q N I T R O G E N
I M S U U B K D X O C G W Y I
G S C H L O R I D E B Z S R F
E W T Y F N S C M E T A L O J
S G K D A A U A L K A L I J O
T S A R T T L T C U W A T E R
I I P O E E F O S D H I M U W
O N K G E E U R A C O W U M D
N O B E B C R C L Q K D S U J
N V J N I T T A T E A J N B B
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Acids and Alkalis
Multiple-Choice Questions
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1.
Which of these is an indicator?
A
B
C
D
Sodium bicarbonate
Vinegar
Litmus
Salt
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2.
Which of these is a salt?
A
B
C
D
Potassium hydroxide
Sodium chloride
Sodium hydroxide
Sodium
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3. Which of these is a weak alkali?
A substance with:
A
B
C
D
pH 7
pH 1
pH 8
pH 13
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4. Which of these might you put on a ant
sting to neutralise the acid in the sting?
A
B
C
D
Water - pH 7
Lemon - pH 3
Oven cleaner - pH 14
Bicarbonate of Soda – pH 9
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5.
Which of these do all acids contain?
A
B
C
D
Hydrogen
Sulphur
Oxygen
Nitrogen
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6. Which of these substances would not
neutralise an acid?
A
B
C
D
Silver nitrate
Sodium hydroxide
Calcium hydroxide
Magnesium hydroxide
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