7 Living Things and Air

7 Living things and air

7 / Living things and air

Gases in the air

1 The Earth is surrounded by a layer of air called the atmosphere.

2 Air is a mixture of gases. It contains nitrogen, oxygen, carbon dioxide, noble gases (e.g. neon), water vapour and other gases.

3 Test for oxygen, carbon dioxide, nitrogen and water:

Gas / How to test? / Result
Oxygen / By using a glowing splint. / The glowing splint relights.
Carbon dioxide / By using hydrogencarbonate indicator or lime water. / The indicator turns yellow;
the lime water turns milky.
Water / By using dry cobalt chloride paper. / The paper changes from blue to pink.

4 The percentages of gases in unbreathed air and breathed air:

Gas in the air / Unbreathed air / Breathed air
Nitrogen / 78% / 78%
Oxygen / 21% / 16%
Carbon dioxide / 0.03% / 4%
Water vapour / Variable / Saturated
Noble gases / 0.9% / 0.9%
Other gases / Variable / Variable

5 The temperature of breathed air is higher than that of unbreathed air.

Air and burning

1 Burning is a chemical reaction between a fuel and oxygen.

2 The word equation of the process of burning:

3 The fire triangle shows the three conditions that are necessary for a fire to occur:

4 To put out a fire, we can remove the fuel, cut off the oxygen supply, and/or lower the temperature.

How humans obtain energy

1 Food stores chemical energy. We obtain energy from the food we eat.

2 The set-up for comparing the amounts of energy in different foods:

3 The energy value of food is measured in kilojoules (kJ) or kilocalories (kcal). It is usually shown on the food labels on food packages.

How green plants obtain energy

1 Green plants can make their own food by photosynthesis.

2 Green plant cells contain chloroplasts. The chlorophyll in the chloroplasts absorbs light

energy for carrying out photosynthesis.

3 The word equation of photosynthesis:

4 Test for starch in green leaves:

Step / Purpose
i Boil the leaf in water. / Destroy the cell membranes of the leaf cells.
ii Soak the leaf in hot alcohol. / Remove the chlorophyll from the leaf.
iii Wash the leaf in hot water. / Wash away the alcohol and soften the leaf.
iv Add iodine solution to the leaf. / Test for the presence of starch.

If the leaf becomes blue-black in the iodine test in step iv, starch is present in the leaf.

5 Chlorophyll, light, carbon dioxide and water are necessary for photosynthesis.

6 A food chain can represent the feeding relationship among living things.

7 An example of a food chain:

Grass (producer) Rabbit (consumer) Snake (consumer)

Gaseous exchange in animals and plants

1 The process in which living cells use oxygen to release energy from food is called respiration.

2 The word equation of respiration:

3 The exchange of oxygen and carbon dioxide between living things and the surroundings is known as gaseous exchange.

4 The major parts of the human respiratory system:

5 Gaseous exchange takes place at the surface of the air sac:

6 The breathing mechanism:

Breathing in / Breathing out
Intercostal muscles contract. Ribs move upwards and outwards. Diaphragm flattens / Intercostal muscles relax. Ribs move downwards and inwards. Diaphragm returns to dome shape.
The volume of the chest cavity
increases. The gas pressure inside the chest cavity decreases and becomes lower than the atmospheric pressure. / The volume of the chest cavity decreases. The gas pressure inside the chest cavity increases and becomes higher than the atmospheric pressure.
Air is drawn into the lungs. / Air is forced out of the lungs.

7 The rib cage model can show the action of the intercostal muscles during breathing.

/ Part in the model / Represents
strip A / backbone
strips B / ribs
strip C / chest bone
rubber band / intercostal muscles

p Represents the situation of breathing in /
p Represents the situation of breathing out

8 The balloon-bell jar model can show the action of the diaphragm during breathing.

/ Part in the model / Represents
tube A / trachea
tubes B / bronchi
balloons / lungs
rubber sheet / diaphragm

p Represents the situation of breathing in /
p Represents the situation of breathing out

9 Gaseous exchange in green plants:

Rates of photosynthesis
and respiration / Results
In the daytime / Rate of photosynthesis > Rate of respiration / Net release of oxygen;
net uptake of carbon dioxide
At night / Stop taking in carbon dioxide for photosynthesis; respiration continues / Net release of carbon dioxide; net uptake of oxygen

10 The balance of oxygen and carbon dioxide in nature:

11 Carbon dioxide can absorb heat energy from the sun. It helps keep the Earth warm. This natural phenomenon is called the greenhouse effect.

Air pollution and smoking

1 Burning fossil fuels in power stations, factories and motor cars gives out large amounts of
air pollutants.

2 The effects of some air pollutants on health:

Air pollutant / Effect on health
Sulphur dioxide and nitrogen oxides / Ø Irritate the eyes and throat.
Ø Irritate our lungs and cause bronchitis and asthma.
Carbon particles / Ø Irritate our lungs and cause bronchitis and asthma.
Lead particles / Ø Adversely affect the brain functions.
Carbon monoxide / Ø Lowers the amount of oxygen in the blood, causing dizziness and headaches.

3 The Air Pollution Index ranges from 0 to 500 and is divided into five levels. The higher the API, the poorer the air quality.

4 The effects of some harmful substances in cigarette smoke on health:

Harmful substance / Effect on health
Carbon monoxide / Ø Lowers the oxygen content in blood.
Tar / Ø Causes throat and lung cancer.
Ø Causes coughs and other respiratory diseases (e.g. bronchitis).
Ø Stains the teeth and finger nails.
Nicotine / Ø Causes addiction.
Ø Increases heart rate and blood pressure.
Ø Leads to heart diseases.
Hydrogen cyanide / Ø Irritates the eyes, nose and skin.
Ø Causes headaches and vomiting.

5 Non- smokers breathing in the smoke given out from other people’s cigarette is known as passive smoking.

Mastering Science Short notes for revision 2