📚 Air Composition and the “721” Mnemonic | 空气成分与”721″记忆法
Air is all around us, but have you ever wondered what it is made of? In IGCSE Science, understanding the composition of air is essential for topics such as respiration, combustion, and environmental chemistry. A simple way to memorise the main percentages is the code “721”.
空气无处不在，但你想过它由什么组成吗？在IGCSE科学中，理解空气成分对于呼吸、燃烧和环境化学等主题至关重要。掌握主要气体比例的一个简单方法就是代码”721″。
1. The Air We Breathe | 我们呼吸的空气
Air is not a single substance; it is a mixture of different gases. Each gas retains its own chemical properties, and the mixture is roughly constant near the Earth’s surface. The most abundant gas is nitrogen, followed by oxygen, with smaller amounts of argon, carbon dioxide, water vapour and other trace gases.
空气不是单一物质，而是一种由不同气体组成的混合物。每种气体都保留自身化学性质，并且在地球表面附近的混合比例大致恒定。含量最多的气体是氮气，其次是氧气，还有少量的氩气、二氧化碳、水蒸气和其他微量气体。
In school science, we often simplify the composition to three main groups: nitrogen, oxygen, and “other gases”. This simplification gives rise to the famous “721” mnemonic.
在学校科学中，我们常把空气成分简化为三类：氮气、氧气和”其他气体”。正是这一简化引出了著名的”721″记忆法。
2. The “721” Mnemonic | “721”记忆法
What does “721” stand for? Look at the approximate volume percentages of the main gases in dry air:
“721”代表什么呢？请看干燥空气中主要气体的近似体积分数：
- Nitrogen (N₂): about 78% — take the tens digit 7
- Oxygen (O₂): about 21% — take the tens digit 2
- Other gases (argon, CO₂, H₂O, etc.): about 1% — take the integer 1
- 氮气（N₂）：约占78%——取十位数字7
- 氧气（O₂）：约占21%——取十位数字2
- 其他气体（氩气、二氧化碳、水蒸气等）：约占1%——取整数1
Therefore, dry air can be quickly remembered as “721”. This is not a precise formula, but a handy exam-revision shortcut. The exact values are shown in the table below.
因此，干燥空气可以被快速记作”721″。这并非精确公式，而是实用的复习捷径。精确数值见下表。
Air Composition | 空气组成
| Gas | 气体 | Volume Percentage | 体积分数 | Role / Use | 作用/用途 |
| Nitrogen (N₂) | 氮气 | ~78% | Inert propellant; fertiliser manufacture | 惰性填充剂；制造化肥 |
| Oxygen (O₂) | 氧气 | ~21% | Respiration; combustion | 呼吸；燃烧 |
| Argon (Ar) | 氩气 | ~0.93% | Welding shield; light bulbs | 焊接保护气；灯泡 |
| Carbon dioxide (CO₂) | 二氧化碳 | ~0.04% | Photosynthesis; greenhouse gas | 光合作用；温室气体 |
| Water vapour (H₂O) | 水蒸气 | 0–4% (variable) | 0–4%（可变） | Weather; humidity | 天气；湿度 |
3. Nitrogen — The Dominant Gas | 氮气——最主要的气体
Nitrogen makes up roughly four-fifths of the atmosphere. It is a diatomic molecule (N₂) with a very strong triple bond, making it chemically unreactive at room temperature. This inertness is useful: nitrogen is pumped into food packaging to prevent spoilage, and it is used as a safe filler in tyres.
氮气约占大气的五分之四。它是双原子分子（N₂），具有极强的三键，因此在室温下化学性质不活泼。这种惰性很有用：氮气被充入食品包装以防止变质，也可用作轮胎的安全填充气。
However, nitrogen is essential for life because it is a key element in proteins and nucleic acids. Through the nitrogen cycle, bacteria convert atmospheric nitrogen into compounds that plants can absorb.
然而，氮气对生命至关重要，因为它是蛋白质和核酸中的关键元素。通过氮循环，细菌将空气中的氮转化为植物能够吸收的化合物。
In IGCSE questions, you may be asked why nitrogen is used in food bags: the answer is that nitrogen is unreactive, preventing oxidation and the growth of microbes.
在IGCSE考题中，你可能会被问到为什么食品袋中使用氮气：答案是氮气不活泼，可以防止氧化和微生物生长。
4. Oxygen — Essential for Life and Combustion | 氧气——生命与燃烧所需
Oxygen is the second most abundant gas in air. It is also a diatomic molecule (O₂) and is much more reactive than nitrogen. Oxygen supports respiration, the process by which living organisms release energy from glucose.
氧气是空气中含量第二多的气体。它同样是双原子分子（O₂），反应活性远高于氮气。氧气支持呼吸作用，即生物体从葡萄糖中释放能量的过程。
Oxygen also supports combustion. A burning object will continue to burn only if oxygen is present; if the oxygen is removed, the flame goes out. This is why covering a fire with a blanket or carbon dioxide fire extinguisher puts it out.
氧气还支持燃烧。燃烧的物体只有在氧气存在时才能继续燃烧；如果去除氧气，火焰就会熄灭。这就是为什么用毯子或二氧化碳灭火器扑灭火焰。
To test for oxygen in the lab, place a glowing splint into a test tube of gas. If oxygen is present, the splint relights.
在实验室中检验氧气的方法：将带火星的木条伸入装有气体的试管中。如果有氧气，木条会复燃。
5. Other Gases: Argon, CO₂ and Water Vapour | 其他气体：氩气、二氧化碳和水蒸气
The remaining “1% pocket” of air includes several important gases. Argon is a noble gas with a very low reactivity; it is used in welding and in double-glazed windows. Carbon dioxide is a trace gas, but it is vital for photosynthesis and is a major greenhouse gas.
空气剩下的”1%小口袋”中含有几种重要气体。氩气是一种低反应性的稀有气体，用于焊接和中空玻璃窗。二氧化碳是微量气体，但它是光合作用的关键物质，也是主要的温室气体。
Water vapour is variable in amount. On a humid day, it can be more than 3% of the air; on a dry day, it is negligible. Water vapour is responsible for clouds, rain and dew.
水蒸气的含量是变化的。在潮湿的日子里，它可能占空气的3%以上；在干燥的日子里，它几乎可以忽略不计。水蒸气是云、雨和露形成的原因。
Other noble gases, such as neon and helium, are also present in tiny amounts. Neon is used in advertising signs, and helium is used to fill party balloons and weather balloons.
其他稀有气体，如氖和氦，也以微量存在。氖用于广告灯牌，氦用于填充派对气球和气象气球。
6. How to Find the “721” Experimentally | 如何通过实验得到”721″
You can measure the proportion of oxygen in air using a simple experiment with a burning substance. For example, a candle burning in a sealed container uses up oxygen until it goes out, and the water level rises by about one-fifth of the container. More accurately, using phosphorus can remove almost all the oxygen from a known volume of air.
用量燃物质进行简单实验可以测量空气中氧气的比例。例如，在密封容器中点燃蜡烛，耗尽氧气后蜡烛熄灭，水位上升约容器体积的五分之一。更精确的方法是使用白磷几乎完全除去已知体积空气中的氧气。
In a classic school experiment, some air is trapped in a cylinder over water. The phosphorus is ignited with a hot wire and reacts with oxygen to form white fumes (phosphorus oxide), which dissolve in water. After cooling, the water level rises, showing that about 21% of the air has been removed.
在一项经典的学校实验中，一些空气被封闭在倒置的量筒内，用热铁丝点燃白磷，白磷与氧气反应生成白色烟雾（五氧化二磷），烟雾溶于水。冷却后，水位上升，显示约21%的空气被除去。
This experiment gives a value close to the “2” in “721” for oxygen; the remaining gas is mostly nitrogen with traces of other gases.
这个实验得到的数值接近”721″中的”2″所代表的氧气；剩余气体主要为氮气及微量其他气体。
7. Testing Gases: Oxygen and Carbon Dioxide | 气体的检验：氧气和二氧化碳
In IGCSE practical exams, you need to know how to test for oxygen and carbon dioxide. These tests are quick but must be described clearly.
在IGCSE实验考试中，你需要知道如何检验氧气和二氧化碳。这些检验很快，但必须描述清晰。
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