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3. 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.

4. Some common acids
Three acids are particularly common in the laboratory .
Acid
Formula
Salts formed
hydrochloric
HCl
chlorides
sulphuric
H2SO4
sulphates
nitric
HNO3
nitrates
These are strong acids that should be treated with the greatest respect.

5. 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.

6. Litmus Test TEACHER’S NOTES Results of the litmus test are : alkali
acid
water

7. 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 alkali
More acid

8. Indicators: the pH scale
This attaches a number called the pH value to each universal indicator colour.
This means we can quickly say how acid or alkali a substance is by quoting a single number.
pH7 is neutral
pH 1 is strongly acid
pH14 is strongly alkali 1 2 14 13 12 11 10 9 8 7 6 5 4 3
Strong acid
Neutral
Strong alkali
Weak acid
Weak alkali

9. Indicators: the pH scale

10. Indicators: the pH scale
Will it be acidic, basic or neutral and how strong?
Substance pH
Description of acid / alkali
Soda water 6
Car battery acid 1
Soap 8
Washing soda 10
Stomach acid 2
Oven cleaner 14
Vinegar 4
very weak acid
very strong acid
very weak alkali
weak alkali
strong acid
very strong alkali
weak acid 1 2 14 13 12 11 10 9 8 7 6 5 4 3
Alkali
Acid

11. Bases are substances that neutralise acids.
Bases are usually:
Metal hydroxides contain OH
Metal oxides contain O
Metal carbonates contain CO3
The following general word equation describes neutralisations:
acid +
base 
a salt
water
In the case of carbonates we also get carbon dioxide.

12. Alkalis are soluble bases.
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.

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

14. 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.

15. 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.

16. 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. 
nitric acid +
ammonium hydroxide
ammonium nitrate +
water
NH4OH +
HNO3 
NH4NO3
H2O

17. 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.

18. Acid rain damaged tress
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

19. 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

20. 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
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
Acid
Alkali
Carbonate
Chloride
Hydrogen
Indicator
Indigestion
Litmus
Metal
Neutralise
Nitrate
Nitrogen
Salt
Water

21. Multiple-Choice Questions
Acids and Alkalis
Multiple-Choice Questions

22. 1. Which of these is an indicator?
A Sodium bicarbonate
B Vinegar
C Litmus
D Salt

23. 2. Which of these is a salt?
A Potassium hydroxide
B Sodium chloride
C Sodium hydroxide
D Sodium

24. 3. Which of these is a weak alkali?
A substance with:
A pH 7
B pH 1
C pH 8
D pH 13

25. 4. Which of these might you put on a ant sting to neutralise the acid in the sting?
A Water - pH 7
B Lemon - pH 3
C Oven cleaner - pH 14
D Bicarbonate of Soda – pH 9

26. 5. Which of these do all acids contain?
A Hydrogen
B Sulphur
C Oxygen
D Nitrogen

27. 6. Which of these substances would not neutralise an acid?
A Silver nitrate
B Sodium hydroxide
C Calcium hydroxide
D Magnesium hydroxide