1. Bleach 21 What is a bleach? What has caused this colour change?
Why is chlorine added to water in a swimming pool?

2. Bleach 21 21.1 What are household bleaches? 21.2 Production of bleach
21.3 Preparation and properties of chlorine
21.4 The bleaching action of chlorine
21.5 Sulphur dioxide - an alternative bleach
21.6 Comparing bleaches
21.7 Advantages and problems with the use of bleaches
21.8 Use of chlorine to sterilize water
21.9 Setting up a chlorine plant in the Hong Kong SAR

3. IT on the net
Summary
Misconception analysis

4. Bleach is used in two ways:
21.1
What are household
bleaches?
Bleach is used in two ways:
1 to clean and bleach fabrics
2 to kill germs
Class
Practice
Activity 21.1 B
Testing the bleach solution.

5. 21.2
Production of bleach
The bleach we use at home smells like chlorine gas.
Thus, we call it a ‘chlorine bleach’. It is made by dissolving chlorine gas in a solution of sodium hydroxide.
Cl2(g) + 2NaOH(aq)  NaOCl(aq) + NaCl(aq) + H2O(l)
bleaching solution

6. The laboratory preparation of bleach
Bleach is prepared during the electrolysis of brine (a concentrated solution of sodium chloride).
During electrolysis, three products are formed:
1 chlorine gas
2 hydrogen gas
3 sodium hydroxide solution
Activity 21.1 C
How can we prepare bleach solution?

7. The ionic equation for the reaction is:
The chlorine then reacts with the sodium hydroxide solution to form a dilute solution of bleach.
The ionic equation for the reaction is:
Cl2(g) + 2OH-(aq)  Cl-(aq) + OCl-(aq) + H2O(l)
Thus, the bleaching solution contains the hypochlorite ion OCl-.
IT ON THE NET
Class
Practice

8. Preparation and properties
21.3
Preparation and properties
of chlorine
Obtaining chlorine from bleach
Chlorine is conveniently obtained from a chlorine bleach by the action of a dilute acid.
For example:
Cl-(aq) + OCl-(aq) + 2H+(aq)  Cl2(g) + H2O(l)
from bleaching solution
from acid
Class
Practice
Activity 21.2 A
How can chlorine be prepared?

9. Properties of chlorine
is a greenish-yellow gas
is denser than most gases
has a choking smell
is poisonous
is slightly soluble in water
turns moist blue litmus paper red then bleaches it
IT ON THE NET

10. Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
Chlorine water
When chlorine dissolves in water, it reacts to form hydrochloric acid HCl and hypochlorous acid HOCl:
Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)
‘chlorine water’ Cl2(aq)
Activity 21.2 B
Investigationg chlorine water.

11. Chlorine water is a convenient source of chlorine.
Hypochlorous acid is unstable. It slowly decomposes into oxygen and hydrochloric acid:
2HOCl(aq)  2HCl(aq) + O2(g)
This decomposition is faster in sunlight.
Class
Practice

12. Chlorine as an oxidizing agent
When chlorine molecules react, they gain electrons and form stable chloride ions:
reduction -1
Cl e-  2Cl-
Therefore, chlorine is an oxidizing agent.
Activity 21.2 C
Is chlorine an oxidizing or a reducing agent?

13. For example, when chlorine water is added to a
solution of potassium bromide, the colourless solution changes to pale brown.
Cl2(aq) + 2e-  2Cl-(aq) reduction - gain of electrons
very pale green
colourless
2Br-(aq)  Br2(aq) + 2e- oxidation - loss of electrons
colourless
pale brown
The overall ionic change is:
Cl2(aq) + 2Br-(aq)  2Cl-(aq) + Br2(aq)

14. Extraction of bromine
Seawater contains a small percentage of sodium bromide NaBr.
Bromine is extracted from seawater using chlorine gas.

15. Chlorine turns moist blue litmus paper red and then bleaches it.
Test for chlorine
Chlorine turns moist blue litmus paper red and then
bleaches it.
Class
Practice

16. The bleaching action of chlorine
21.4
The bleaching action of chlorine
Both chlorine water and household bleach are
bleaching agents.
They are used to bleach dyes from plants and fabrics made from plant fibres.
Therefore they bleach:
flowers and litmus (a dye obtained from plants)

17. stains made by fruit juice, tea and wine
cotton, linen and other fabrics made from plant fibres

18. How chlorine bleach works
Chlorine water contains hypochlorous acid HOCl.
Household bleach contains sodium hypochlorite NaOCl.
Both solutions contain the hypochlorite ion OCl-.
The hypochlorite ion gives up its oxygen to the coloured dye, thus making it colourless as follows:

19. OCl-(aq) + dye(aq)  Cl-(aq) + (dye + O)
coloured
colourless
As oxygen is added to the dye, chlorine bleaches by oxidation.

20. 21.5
Sulphur dioxide -
an alternative bleach
An alternative to chlorine bleach is sulphur dioxide
bleach. This bleach is mainly for industrial use.
Bleaching delicate materials such as wool, silk and paper that are damaged by the more powerful chlorine bleaches.
Paper making
To give foods such as flour and some cheeses a white appearance.

21. How sulphur dioxide bleach works
Sulphur dioxide only bleaches in moist conditions.
The gas dissolves in the water to form sulphurous acid:
SO2(g) + H2O(l) H2SO3(aq)
sulphurous acid
The acid contains the sulphite ion SO32-(aq).
This ion removes oxygen from the coloured dye, leaving it colourless:
SO32-(aq) + dye  SO42-(aq) + (dye - O)
coloured
colourless
Activity 21.3 A
How does sulphur dioxide affect coloured substances?

22. As oxygen is removed from the dye, sulphur dioxide
bleaches by reduction.
The bleaching action of sulphur dioxide is not permanent. On exposure to air and light, the original colour slowly returns. This is because the air supplies the oxygen removed during bleaching.
Class
Practice

23. Sulphur dioxide as a reducing agent
Sulphur dioxide in the presence of water is a good reducing agent.
oxidation +4 +6
SO32- (aq)  SO42- (aq)
Activity 21.3 B
Investigating reactions of aqueous sulphur dioxide.

24. SO32- (aq) + H2O(l)  SO42- (aq) + 2H+(aq) +2e-
The half equation is written:
SO32- (aq) + H2O(l)  SO42- (aq) + 2H+(aq) +2e-
The oxidation number of sulphur increases from +4 to +6.
Here are three examples.

25. 1 Iron(III) ions
SO2(aq)
Fe3+(aq)
Fe2+(aq)
Reaction of aqueous sulphur dioxide with a solution of iron(III) ions.
Fe3+(aq) e-  Fe2+(aq)
reduction
pale green
yellow/brown

26. 2 Acidified potassium permanganate solution
SO2(aq)
MnO4-(aq)
Mn2+(aq)
Reaction of aqueous sulphur dioxide with a solution of acidified potassium permanganate.
MnO4-(aq) + 8H+(aq) + 5e-  Mn2+(aq) + 4H2O(l)
colourless
purple

27. 3 Bromine solution
SO2(aq)
Br-(aq)
Br2(aq)
Reaction of aqueous sulphur dioxide with bromine water.
Br2(aq) + 2e-  2Br-(aq)
brown
colourless

28. Tests for sulphur dioxide
Sulphur dioxide has a characteristic choking smell.
Filter paper moistened with acidified potassium dichromate solution changes from orange to green in the presence of sulphur dioxide.
Class
Practice

29. Sulphur dioxide bleach
21.6
Comparing bleaches
Sulphur dioxide bleach
Chlorine bleach
involves the sulphite ion SO32-(aq)
involves the hypochlorite ion OCl-(aq)
bleaches by reduction
bleaches by oxidation
bleaching action is slow and mild
bleaching action is faster and more vigorous
bleaching action is not permanent and the original colour will slowly be restored
bleaching action can last for a longer time
used with wool, silk, paper, some foods
used with cotton, linen, some food stains
A comparison of sulphur dioxide and chlorine bleaches.

30. Advantages and problems with the use of bleaches
21.7
Advantages and problems with the use of bleaches
Advantages
The use of chlorine in drinking water has prevented the spread of certain diseases.
Bleaches have improved hygiene in the home.
Bleaches have helped to improve the quality of food and clothes.

31. Chlorine and sulphur dioxide are toxic substances.
Some problems
Chlorine and sulphur dioxide are toxic substances.
Household bleaches can harm the eyes and skin.
Sulphur dioxide in food can cause severe breathing difficulties in some people.
Class
Practice

32. Chlorine in water can kill bacteria.
21.8
Use of chlorine to
sterilize water
Chlorine in water can kill bacteria.
In the Hong Kong SAR, chlorine gas is added to our drinking water at water treatment plants. It is also added to the water in swimming pools
IT ON THE NET
Class
Practice

33. Setting up a chlorine plant in the Hong Kong SAR
21.9
Setting up a chlorine plant in the Hong Kong SAR
Class
Practice

34. IT on the net 21.2 Production of bleach
More on sodium hypochlorite and its uses.
21.3 Preparation and properties of chlorine
Uses of chlorine

35. IT on the net 21.8 Use of chlorine to sterilize water
How does chlorine kill harmful micro-organisms
The use of sodium hypochlorite for the treatment of swimming pool water.
Basics of water purification - main steps

36. Summary
1 Make a written summary of this chapter by answering these questions:
What is household bleach used for?
How is household bleach manufactured?
How is chlorine prepared in the laboratory?
What are some properties of chlorine?

37. How do the bleaching action of sulphur dioxide bleach and chlorine bleach differ?
What are some properties of sulphur dioxide?
What are some industrial uses of bleaches?
2 Using the answers to your questions, prepare a table to compare chlorine and sulphur dioxide bleaches.

38. Misconception Analysis
Think carefully about the following statements and then mark them TRUE or FALSE. Check your answers at the back of the book to see whether or not you have any misconceptions.
1Q: Domestic bleach is usually prepared from chlorine and sodium hydroxide obtained from electrolysis of brine.
1A: True

39. 2Q: The active ingredient in household bleach is hypochlorous acid HOCl.
2A: False
3Q: A piece of red litmus paper will turn blue then white when dipped into a solution of sodium hypochlorite.
3A: False

40. 4Q: Chlorine water is a solution of chlorine molecules in water.
4A: False
5Q: Bleaches work by oxidation.
5A: False