Air can be found everywhere on Earth and it is essential for living things to survive. Among the components of air, oxygen and carbon dioxide are both involved in three of the essential life processes of living things, including photosynthesis, respiration and gas exchange. Through these life processes, the proportions of oxygen and carbon dioxide in the atmosphere is balanced. However, some human activities may disrupt such balance in Nature and living things may be adversely affected. Hence, it is essential to maintain the air quality in the environment.

In this Unit, students will learn about the different components of air, the above mentioned essential life processes as well as the importance of the natural balance of oxygen and carbon dioxide in the atmosphere. Through performing different practical works and investigations, e.g. testing for oxygen and carbon dioxide, testing for the presence of starch in green leaves, burning of food to release energy for heating water and investigating the factors necessary for photosynthesis, students will practise different science process skills, like handling apparatus, observing, designing investigation and inferring.

Students will realise the unifying concept “change and constancy” through the learning of photosynthesis, respiration and the balance of carbon dioxide and oxygen in Nature. Simple molecules like carbon dioxide and water are converted into food in photosynthesis while these food can be converted back into carbon dioxide and water in respiration. On the other hand, light energy is changed to chemical energy in food by photosynthesis in green plants while the chemical energy stored in food is changed to other useful forms of energy for living things in respiration. Through learning the balance of carbon dioxide and oxygen in Nature, students will realise the “constancy” of the composition of different gases
in air.