Mastering Science
Answers to Student’s Book 2A
Unit 7
Unit 7
Living things and air
Laboratory Activity 7.1 (p.4)
12
Oxygen
Carbon
dioxide
Nitrogen
What is the colour of it?
colourless
colourless
colourless
What is the smell of it?
odourless
odourless
odourless
What happens to the burning
splint?
burns more
brightly
goes out
goes out
What happens to the glowing
splint?
relights
goes out
goes out
What happens to the
hydrogencarbonate indicator?
no change
turns yellow
no change
What happens to the lime water?
no change
turns milky
no change
Discussion
1 a
Oxygen relights a glowing splint.
b Carbon dioxide turns hydrogencarbonate indicator yellow. / Carbon dioxide
turns lime water milky.
2
No. Nitrogen is an unreactive gas.
Laboratory Activity 7.2 (p.8)
1 Blue.
2
3
It changes from blue to pink.
Dry cobalt chloride paper changes from blue to pink in the presence of water.
Laboratory Activity 7.3 (p.9)
6 a
In the jar with unbreathed air.
b Breathed air contains less oxygen than unbreathed air.
7 It changes from red to yellow.
8 a
There is no colour change.
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Mastering Science
Answers to Student’s Book 2A
b
Unit 7
Breathed air contains more carbon dioxide than unbreathed air.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Laboratory Activity 7.4 (p.11)
1 It remains blue in colour.
2 a
It changes from blue to pink.
b Breathed air contains water vapour.
Laboratory Activity 7.5 (p.12)
1 (Depends on the experimental results.)
2
The temperature of breathed air is (Depends on the experimental results.).
The temperature of breathed air is higher than that of unbreathed air.
Section quiz 1 (p.13)
1 B
2 B
Laboratory Activity 7.6 (p.16)
2 It burns more brightly for some time and then goes out.
3
4
5
6
a
The wall of the gas jar is warm.
b Heat energy is given out from the burning candle.
A film of moisture/liquid is found on the inner surface of the gas jar.
a
It changes from blue to pink.
b Water is produced during burning.
a
It changes from red to yellow.
b Carbon dioxide is produced during burning.
(p.18)

For a fire to occur, fuel, oxygen and high temperature are required. These three
conditions form the fire triangle.
Laboratory Activity 7.7 (p.19)
1 Some gas bubbles come out from the liquid.
3 It goes out.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Classroom Activity 7.1 (p.20)
Working principle
1
Spray the buildings on fire with
water.
Removing the high temperature (and
oxygen) from the fire triangle.
2
Cover the fire with sand.
Removing the oxygen from the fire triangle.
3
Cover the fire with a fire
blanket.
Removing the oxygen from the fire triangle.
4
Hit the fire with a fire beater
(山火拍).
Removing the oxygen from the fire triangle.
5
Clear a strip of vegetation to
make a fire break (防火帶).
Removing the fuel from the fire triangle.
Classroom Activity 7.2 (p.22)
1 Hill fires may cause death and injury. Hill fires may also destroy the habitats of
living things and make living things homeless.
Classroom Activity 7.3 (p.23)
2 Do not overload sockets.
3 Do not dry wet clothing on an electric heater.
4 Do not let children play with matches, lighters or any flammable materials.
5 Extinguish cigarette ends and dispose of them in the ashtray.
Section quiz 2 (p.25)
1 Carbon dioxide cuts off the oxygen supply by displacing the air surrounding the
flame.
2
a
oxygen
b
c
fuel
high temperature (and oxygen)
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Laboratory Activity 7.8 (p.26)
2 (Depends on the experimental results.)
5 (Depends on the experimental results.)
Discussion
1 The temperature of the water rises.
2 When food is burnt, heat energy is released.
Laboratory Activity 7.9 (p.28)
Identifying variables
a The temperature rise of water
b The type of snack
c
Weight of snack, volume of water in the boiling tube, the distance between the
flame and the boiling tube, etc.
Carrying out the experiment
Suggested steps:
1 Put 20 cm3 of water in a boiling tube and fix it on the stand with a clamp.
2 Weigh and record the mass of snack with an electronic balance.
3 Put the snack into a burning spoon and heat it over a Bunsen flame. Move the
burning spoon near the bottom of the boiling tube when the snack starts burning.
Record the temperature of the water when the snack is completely burnt.
4
Repeat the steps above for other snacks.
Recording the results
Type of
snack
Mass of
snack (g)
Initial water
temperature
(°C)
Water temperature
after the snack is
burnt (°C)
Rise in temperature
per gram of the
snack (°C)
(Depend on the experimental results.)
Drawing a conclusion
(Snack X) contains the largest amount of energy per gram, while (snack Y) contains
the least energy per gram.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Section quiz 3 (p.30)
1 food
2 heat, light
3 kJ, kcal
(p.32)

Green plants make their own food by a process called photosynthesis.
Laboratory Activity 7.10 (p.32)

Iodine solution changes from brown to blue-black when starch is present. This
is a test for starch.
8
a
The leaf becomes pale yellow in colour. The alcohol becomes green in
colour.
11
b
a
b
To remove the chlorophyll from the leaf.
It becomes blue-black in colour.
Starch is present in the leaf.
Laboratory Activity 7.11 (p.34)
4 Gas bubbles are formed on the leaves.
5 a
It relights.
b Oxygen is released by the plant.
Laboratory Activity 7.12 (p.36)
Identifying variables
a starch
b chlorophyll
 Supply of carbon dioxide
c
Presence of chlorophyll
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 Supply of oxygen
 Supply of water
 Presence of light
 Temperature
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Recording the result
(Drawing should show the green parts of the leaf become blue-black, while the
non-green parts are brown in colour.)
Drawing a conclusion
1 The green parts of the leaf become blue-black in colour. The non-green parts are
brown in colour.
2
The green parts of the leaf contain starch while the non-green parts do not. This
shows that chlorophyll is needed for photosynthesis.
Laboratory Activity 7.13 (p.38)
Identifying variables
a Presence of starch in the leaf
b
c
Presence of light
 Supply of carbon dioxide
 Presence of chlorophyll
 Supply of oxygen
Presence of light
 Supply of water
 Temperature
Carrying out the experiment
Suggested steps:
1 Destarch a potted plant for 24 hours.
2 Wrap part of a leaf on the potted plant with a piece of aluminium foil.
3 Put the potted plant under sunlight for at least 4 hours.
4 Test the leaf for starch.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Recording the result
(Diagram should show that in the iodine test, the parts of the leaf exposed to sunlight
turn blue-black, while the parts covered with the foil remain brown.)
Drawing a conclusion
Light is needed for photosynthesis.
Laboratory Activity 7.14 (p.39)
Identifying variables
a Presence of starch in the leaf
b Supply of carbon dioxide
c
Supply of carbon dioxide
 Presence of chlorophyll
 Supply of oxygen
 Supply of water
 Presence of light
 Temperature
Carrying out the experiment
Suggested steps:
1 Destarch a potted plant for 24 hours.
2 Set up the apparatus as shown and put it under sunlight for at least 4 hours.
3 Test both leaves for starch.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Recording the result
(Diagram should show that leaf A remains brown while leaf B turns blue-black in the
iodine test.)
Drawing a conclusion
Carbon dioxide is needed for photosynthesis.
Section quiz 4 (p.42)
c → b → a
1
2
→
d
a
phytoplankton → shrimp → penguin → killer whale
b
i
ii
Phytoplankton.
Shrimp, penguin and killer whale.
Laboratory Activity 7.15 (p.44)
1 a
There is no mealworm in tube B.
b To eliminate the changes in the set-ups caused by changes in environmental
factors, such as air temperature and pressure.
2
Tube
Distance between the colour marker and the stopper (cm)
At the beginning
20 minutes later
A
B
(Depend on the experimental results. In general, the colour marker in tube A
moves further towards the boiling tube than that in tube B.)
Discussion
1 Tube A.
2 The mealworms take in oxygen and give out carbon dioxide. The carbon dioxide
is absorbed by soda lime. The gas pressure inside tube A therefore decreases and
becomes lower than the atmospheric pressure. As a result, the colour marker is
pushed towards the boiling tube.
3
The heat energy from our hands will heat up the air inside the tubes and cause
the air to expand. This affects the experimental results.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Laboratory Activity 7.16 (p.46)
a nasal cavity
b trachea
c
bronchus
d bronchiole
e
air sac
f
g
h
i
intercostal muscle
rib
lung
diaphragm
Classroom Activity 7.4 (p.48)
a They move upwards and outwards.
b
They move downwards and inwards.
Laboratory Activity 7.17 (p.50)
2 (Depends on the experimental results.)
3 (Depends on the experimental results.)
4 NO.
5 Position Y.
6 JKLM.
7 Position Y.
8
shortened, contracting, upwards, increases, in
Laboratory Activity 7.18 (p.52)
2
Rubber sheet pulled down
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Rubber sheet pushed up
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Mastering Science
Answers to Student’s Book 2A
3
4
Unit 7
bigger, increases, lower, into, in
The balloons become smaller.
When the rubber sheet is pushed up, the volume of the bell jar decreases and the
gas pressure inside the bell jar becomes higher. Therefore air flows out of the
balloons.
This is similar to the situation when we are breathing out.
(p.53)

In humans, gaseous exchange takes place in the air sacs.

In humans, breathing is brought about by the actions of the intercostal muscles
between the ribs, and the diaphragm that is located below the rib cage.
Section quiz 5 (p.54)
1 contract, upwards, outwards,
flattened, increase, decreases
2
Photo B, because the diaphragm is dome-shaped.
Laboratory Activity 7.19 (p.55)
1 a
To make sure that the result in tube A is caused by the presence of the green
leaf.
b
3
4
To make sure that the result in tube C is caused by the presence of the green
leaf.
Tube
a
b
Colour of the hydrogencarbonate indicator
At the beginning
After one hour
A
Red
Purple
B
Red
Red
C
Red
Yellow
D
Red
Red
The green leaf takes in carbon dioxide from the air in the presence of light.
The green leaf gives out carbon dioxide in the dark.
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Mastering Science
Answers to Student’s Book 2A
Unit 7
Revision exercise
A Multiple-choice questions (p.77)
1
A
2
A
3
B
4
B
5
A
6
D
7
B
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Mastering Science
Answers to Student’s Book 2A
Unit 7
B
1
2
3
4
5
6
7
8
True or false questions (p.78)
F
F
T
F
T
F
T
T
C
a
b
Question (p.78)
Jar B.
Jar B is smaller and contains less oxygen to support the burning of the candle
flame.
c
d
Nitrogen.
The burning splint goes out.
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