Air Around Us

Air is present everywhere around us, though we cannot see it. We feel it when leaves rustle, a windmill turns or a kite flies. In this chapter you will perform simple activities to show that air occupies space, such as pushing an empty glass upside down into water, where the trapped air stops water from entering. The blanket of air surrounding the Earth is called the atmosphere. Air is a mixture of gases: nitrogen makes up about 78 percent and oxygen about 21 percent, while carbon dioxide, water vapour and other gases, along with dust and smoke, form the rest. You will see how water vapour condenses on a glass of ice-cold water, how a burning candle covered with a tumbler goes out when its oxygen is used up, and why lime water turns milky with carbon dioxide. The chapter also explains how animals in water and soil get oxygen, how plants and animals balance oxygen and carbon dioxide, and how moving air runs windmills and sailing boats.

Detailed Worksheet: Air Around Us Section A - Definitions (10 marks) 1. What is the atmosphere? Up to roughly what height does it extend above the Earth? (2 marks) 2. Name the two main gases present in air and give their approximate percentages by volume. (2 marks) 3. What is wind? Give two observations that show that air is moving. (2 marks) 4. Why is oxygen called a supporter of burning? Name the gas in air that does not support burning but dilutes it. (2 marks) 5. What is meant by the balance of oxygen and carbon dioxide in the atmosphere? (2 marks) Section B - Calculations and Applications (15 marks) 6. Air contains about 78 percent nitrogen and 21 percent oxygen by volume. In a room containing 500 litres of air, calculate (i) the volume of nitrogen (ii) the volume of oxygen (iii) the volume of all other gases together. (3 marks) 7. A candle is covered by a glass tumbler containing 200 mL of air. Assuming air has 21 percent oxygen, (i) find the volume of oxygen available. (ii) Why does the candle go out after some time? (iii) Would a candle under a 400 mL tumbler burn for a longer or shorter time? Explain. (3 marks) 8. A person breathes about 16 times per minute and takes in about 500 mL of air each time. (i) How much air does the person breathe in one minute? (ii) In one hour? (iii) About how much oxygen is breathed in per minute if air is 21 percent oxygen? (3 marks) 9. Give one application each of the following properties of air: (i) moving air can turn blades (ii) air contains oxygen (iii) air occupies space. (3 marks) 10. A boy pushes an empty glass tumbler, open end down, straight into a bucket of water and then tilts it slightly. (i) What does he observe when he tilts it? (ii) What does this show about air? (iii) Why does water not enter the tumbler when it is held straight? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity in which a burning candle is covered with an inverted glass tumbler placed in a plate of water. Label the candle, tumbler, water and the level of water. Explain why the water level rises inside the tumbler. (4 marks) 12. Draw a pie chart showing the composition of air: nitrogen 78 percent, oxygen 21 percent and other gases 1 percent. Label each sector. (3 marks) 13. Draw a labelled diagram of a glass of ice-cold water with water droplets on its outer surface. Explain where the droplets come from. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A boy puts a lump of dry soil into a beaker and slowly pours water over it. (i) What does he observe? (ii) What do the bubbles tell us about soil? (iii) How do earthworms and other soil animals use this? (iv) Why do earthworms come out of the soil during heavy rain? (5 marks) 15. Explain how plants and animals depend on each other for the exchange of gases in air. (i) Which gas do animals take in and give out during respiration? (ii) Which gas do plants take in and give out while making food? (iii) What would happen if all the trees in a city were cut down? (iv) Suggest two ways to keep the air clean. (5 marks) 16. People living in cities near factories often suffer from coughing and burning eyes. (i) What harmful substances may be present in the air near factories? (ii) Why is smoke harmful? (iii) Why do traffic policemen sometimes wear masks? (iv) Why is the air in a forest fresher than in a busy market? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Show the working for all calculations.