New 21st Century Chemistry
Suggested answers to in-text activities and unit-end exercises
Topic 1 Unit 3
In-text activities
STSE Connections (page 44)
1 Advantages
• It can continue to deliver drinking water for consumption even if there is no rain.
• It provides an alternative source of supply to make the overall supply more robust and less vulnerable to interruption.
• It provides a superior quality product regardless of the source water quality.
• It is accomplished by means of pumps, filters and other pieces of equipment. This feature results in smaller size facilities when compared with other conventional water supply alternatives, such as surfacewater reservoirs.
Disadvantages
• The cost of plant construction is relatively high.
• Water desalination consumes a lot of energy. Approximately one third of the operational costs of a water desalination facility is the power consumption.
• High-energy consumption also leads to increases in greenhouse gas emissions.
2 Students may suggest the following points:
• Be economical with your washing machine and dishwasher. Save for a bigger wash and cut down the rinse cycle.
• Water can be used again. Save your bath slops to wash the floor or water the pot plants.
• A dripping tap can waste up to 70 litres a day. Get it fixed.
• By taking a shower instead of a bath, you may use only one fifth of the water.
• By installing a flow restrictor at water tap can effectively save water up to 75%.
• Never rinse your hands, clothes or vegetables under a running tap.
• Do not draw off more water than you can use.
• Using dual flush cistern can save 1/3 of the flushing water.
(Source: Water Supplies Department, http://www.wsd.gov.hk/en/html/pdf/savedollar-e.pdf)
Problem Solving (page 46)
Remove the oil using a dropper. Distil the common salt solution to obtain the common salt and water.
The oil and common salt solutions can be also separated using a separating funnel.
Checkpoint (page 49)
1 a) The colour of copper(II) sulphate crystals changes from blue to white upon heating.
b) Test the liquid obtained using dry cobalt(II) chloride paper.
The paper turns from blue to pink.
c) To prevent the condensed water from running back to the tube and cracking the hot glass.
2 Use flame test.
Potassium chloride gives a lilac flame
while sodium chloride gives a golden yellow flame.
Discussion (page 55)
1 a) Physical change
No new substance is formed.
b) Physical change
No new substance is formed.
c) Chemical change
Digesting food produces new substances. For example, starch is broken down into simple sugars.
d) Chemical change
A match produces new substances, such as carbon dioxide and water, when burnt.
2 • Dry ice can produce a very low temperature (–78 °C).
• Dry ice sublimes and no messy liquid (as in the case of ice) is produced.
Unit-end exercises (pages 59-63)
1
2
3 a) False
During the electrolysis of sea water, chlorine forms at the positive electrode.
b) True
c) False
The process of evaporating a liquid and condensing the vapour is called distillation.
d) False
Sodium chloride is the most abundant salt in sea water.
e) True
4 a) Evaporation
b) Distillation
c) Fractional distillation
5 a) Physical change
No new substance is formed.
b) Chemical change
Iron rusts because it reacts with air and moisture in the environment. Rust is an oxide formed in the process.
c) Physical change
No new substance is formed.
d) Physical change
No new substance is formed.
6 B
7 A
8 C
(Option 1) Calcium sulphate is insoluble in water.
The following table shows the salt composition of sea water:
Salt / %Sodium chloride
Magnesium chloride
Sodium sulphate
Calcium chloride
Other salts / 68
14.6
11.4
3.1
2.9
9 D
10 B
11 a) Filtration
b) Filter paper
c) Filter funnel
d) Filtrate
e) Residue
f) The particles of sodium chloride in sea water are much smaller than mud particles in muddy water.
Hence particles of sodium chloride in sea water can pass through the tiny holes on the filter paper while mud particles cannot.
12 a) Distillation
b) When sea water evaporates, water vapour escapes.
If the water vapour is passed into another container and cooled, it will turn back into water.
c) Condenser
d) To ensure even boiling.
13 a) i) A white precipitate was observed.
ii) Chloride
b) i)
ii) Use flame test.
The solid gives a golden yellow flame.
14 a) Electrolysis
b) • Manufacture of hydrochloric acid
• Sterilizing drinking water / swimming pool water
• Manufacture of PVC
• Manufacture of organic solvents
• Manufacture of bleach
c) • Manufacture of bleach
• Manufacture of soaps and detergents
Suggested answers to in-text activities and unit-end exercises 5 © Jing Kung. All rights reserved.
Topic 1 Unit 3