الأسبوع 9 · الوحدة الثانية «الغلاف الجوي» (افتتاحية) والموضوع ٢-١ «مكونات وطبقات الغلاف الجوي» (١): مكونات الهواء وأهمية الغلاف الجوي والتروبوسفير والستراتوسفير وطبقة الأوزون والميزوسفير · 3 دروس
1
The gases of the air and why they matter
Integrated Science Sec 1 Term 1, Unit 2: pp. 67–72
GoalName the gases of the atmosphere with their shares, say why they change, and say why the atmosphere matters.
New words
atmosphere · the layer of gases around Earthinert gas · a gas that hardly reacts with other substances, like nitrogen and argongreenhouse gas · a gas, such as carbon dioxide, that holds heat in the atmosphere
Explanation
The atmosphere is the envelope of gases that surrounds Earth. Dry air is about 78% nitrogen, about 21% oxygen, about 0.93% argon (a noble gas) and about 0.04% carbon dioxide (a greenhouse gas), plus small amounts of water vapour (it varies), sulphur dioxide, ozone, carbon monoxide and nitrogen dioxide. Nitrogen is the main gas, not oxygen. Nitrogen is almost inert, because its two atoms are joined by a very strong triple bond, so its oxides are rare and need lightning or very high temperatures to form. Oxygen comes mostly from photosynthesis and is needed for respiration and burning. The shares change all the time: respiration takes in O₂ and gives out CO₂, photosynthesis does the opposite, and volcanoes add gases. The atmosphere shields Earth from harmful radiation and from objects from space (most meteors burn up), traps part of the Sun’s radiation so that the surface stays at a moderate temperature, and drives the water cycle. CO₂ is only 0.04%, but it is important.
Worked examples
Gas
Share
Litres in 10 000 L
nitrogen
78%
7800
oxygen
21%
2100
argon
0.93%
93
carbon dioxide
0.04%
4
Find the volume of each gas in 10 000 litres of dry air. Nitrogen: 78% of 10 000 = 7800 L. Oxygen: 21% = 2100 L. Argon: 0.93% = 93 L. Carbon dioxide: 0.04% = 4 L. Total 7800 + 2100 + 93 + 4 = 9997 L; the last 3 L are traces of other gases.
Why do the shares of gases change? When a forest grows by day, photosynthesis takes in CO₂ and releases O₂. At night and in all living things, respiration takes in O₂ and gives out CO₂. A volcano can add gases such as sulphur dioxide. The changes are small compared with the whole atmosphere.
A pupil says that oxygen is the main gas of the air. Is that right? No. Oxygen is the second gas with about 21%; nitrogen with about 78% is the main one. Nitrogen mostly stays unchanged in the air because it is almost inert.
Class activity
Air has weight, with a grown-up. Tape a ruler or a light stick at its middle to a pencil or a string so that it balances. Tie one empty balloon to each end so it still balances. An adult blows up one balloon and ties it, and you swap it in. Describe which end goes down and what this suggests about the air inside. Do not share balloons by mouth. Then draw a 10 × 10 grid of 100 squares and colour 78 for nitrogen, 21 for oxygen and 1 for the rest.
Practice
1
Name the two most abundant gases in dry air and their shares.
Nitrogen (about 78%) and oxygen (about 21%).
2
How many litres of oxygen are in 5000 L of air?
21% of 5000 = 1050 L.
3
Why is nitrogen called almost inert?
Its two atoms are held by a very strong triple bond, so it hardly reacts.
4
Give two ways the atmosphere helps life.
For example: it keeps the surface temperature moderate, shields Earth from harmful radiation and meteors, and drives the water cycle.
