📚 Tests for Gases | 气体检验
In IGCSE Edexcel Science (Double Award or Separate Chemistry), you need to be able to recall the specific confirmatory tests for five common gases: hydrogen, oxygen, carbon dioxide, ammonia and chlorine. These tests are straightforward but must be described with precision, including the observation you make and the chemical identity of any substance used (e.g. limewater). Understanding the underlying chemistry will also help you interpret results in exam-style tables and plan simple gas collection in the laboratory.
在IGCSE Edexcel 科学（双奖或单独化学）中，你需要记住五种常见气体的专属确认检验：氢气、氧气、二氧化碳、氨气和氯气。这些检验本身不复杂，但描述必须精确，包括你所观察到的现象以及所用试剂（如石灰水）的化学身份。理解背后的化学原理还能帮助你解读考试中表格给出的检验结果，并能合理设计实验室里简单气体的收集方法。
1. Why Gas Tests Matter | 为什么气体检验如此重要
Many chemical reactions produce gases, and being able to identify which gas has been formed is a fundamental practical skill. In the IGCSE specification, you may be asked to describe the test, predict the result when a particular reaction occurs, or suggest how you could safely test for an unknown gas. The tests themselves are quick, cheap and give clear visual or audible outcomes – they are ideal for the school laboratory.
很多化学反应都会生成气体，能够识别生成了哪种气体是一项基本的实验技能。在IGCSE 考纲中，你可能会被要求描述检验方法、预测某个反应发生时会出现什么现象，或者建议如何安全地检验一种未知气体。这些检验本身快速、廉价，并能给出明确的视觉或听觉结果——非常适合学校实验室。
You will also see these gas tests linked to topics such as acids and bases (CO₂ from carbonates), electrolysis (H₂ at the cathode, O₂ at the anode), the Haber process (NH₃), and redox reactions (Cl₂ from displacement). Therefore, mastering the tests supports your wider understanding of the syllabus.
你还会看到这些气体检验与诸如酸和碱（碳酸盐产生 CO₂）、电解（阴极产生 H₂，阳极产生 O₂）、哈伯法（NH₃）以及氧化还原反应（置换反应产生 Cl₂）等主题关联起来。因此，掌握这些检验有助于你对整个考纲形成更全面的理解。
2. Test for Hydrogen (H₂) | 氢气的检验
uth of a test tube containing the gas. If hydrogen is present, you will hear a distinctive ‘ on of hydrogen with oxygen from the air, producing water vapour: 2H₂(g) + O₂(g) → 2H₂O(g).
氢气在空气中燃烧迅速并生成水。标准检验方法是将一根点燃的木条放在装有该气体的试管口。如果有氢气，你会听到一声独特的“爆鸣声”，因为氢气被点燃并发生了一次微小的爆炸。声音是由氢气与空气中的氧气快速燃烧生成水蒸气而产生的： 2H₂(g) + O₂(g) → 2H₂O(g)。
It is important to hold the burning splint at the mouth of the tube, not deep inside. If the gas mixture is diluted with air, the pop will be faint or absent. For a reliable result, the test tube should be inverted (hydrogen is less dense than air) and the gas collected by downward delivery or over water.
关键是要把点燃的木条放在试管口，而不是伸进试管深处。如果气体混合物中了混入空气被稀释，爆鸣声会很小或完全听不到。为了得到可靠的结果，试管应该倒置（氢气密度小于空气），并且采用向下排空气法或排水集气法收集气体。
Do not confuse hydrogen's pop with the crackling sound of some other flammable gases; in IGCSE you only encounter hydrogen giving a squeaky pop.
不要把氢气的爆鸣声和其他易燃气体的噼啪声搞混；在 IGCSE 中，你遇到的能发出爆鸣声的气体就是氢气。
3. Test for Oxygen (O₂) | 氧气的检验
Oxygen relights a glowing splint. Take a wooden splint that has been lit and blow out the flame so that it is only glowing red at the tip. Immediately place the glowing splint into a test tube of the gas. If oxygen is present, the splint will relight, often with a small flame. This happens because oxygen supports combustion and the concentration of O₂ inside the tube is high enough to re-ignite the glowing wood.
氧气能使带火星的木条复燃。取一根已经点燃的木条，把明火吹灭，只让顶端留有红色的火星。立刻将带火星的木条伸进装有所测气体的试管中。如果氧气存在，木条会复燃，通常会燃起一小团火焰。这是因为氧气支持燃烧，试管内氧气的浓度足够高，能重新点燃带有火星的木条。
You should not confuse this with the test for other gases; only oxygen gives a positive relighting result. The glowing splint must still be visibly glowing, otherwise the test is invalid. Often O₂ is produced from the decomposition of hydrogen peroxide (with manganese(IV) oxide) or from electrolysis of water; these practicals are commonly examined alongside the test.
不要把它和其他气体的检验搞混；只有氧气能使带火星的木条复燃。木条必须仍然是可见的带火星状态，否则检验无效。氧气通常由过氧化氢（用二氧化锰催化）分解或电解水产生；这些实验常和气体检验一同出现在考题里。
When describing the test in an exam, use the phrase 'relights a glowing splint' and avoid saying 're-ignites a burning splint', which would be incorrect.
在考试中描述这个检验时，要用“复燃带火星的木条”这样的表述，而不要说“重新点燃燃烧的木条”，后者是错误的。
4. Test for Carbon Dioxide (CO₂) | 二氧化碳的检验
Carbon dioxide is tested with limewater – fine suspension of insoluble calcium carbonate: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
二氧化碳用石灰水检验——石灰水是氢氧化钙 Ca(OH)₂(aq) 的饱和水溶液。当把 CO₂ 通入石灰水或在石灰水中振荡时，溶液会变得像牛奶一样浑浊。这种浑浊是因为生成了不溶于水的碳酸钙细颗粒悬浮物： Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)。
If the gas is not CO₂, limewater remains clear and colourless. The test is extremely sensitive and is the definitive chemical test for carbon dioxide required at IGCSE. In some textbooks you may see that prolonged bubbling can make the cloudiness disappear because soluble calcium hydrogencarbonate forms, but this is beyond the core requirement.
如果该气体不是 CO₂，石灰水会保持澄清无色。这个检验非常灵敏，是 IGCSE 所要求的二氧化碳的确定性化学检验。有些教材里你会看到长时间通入气体会使浑浊消失，因为生成了可溶性的碳酸氢钙，但这超出核心要求。
Remember that CO₂ is also a product of respiration, thermal decomposition of carbonates and the reaction of acids with carbonates. You should be able to link the gas test to any of these contexts.
记住，CO₂ 也是呼吸作用、碳酸盐热分解以及酸和碳酸盐反应的产物。你应该能够把这一气体检验与这些情境联系起来。
5. Test for Ammonia (NH₃) | 氨气的检验
Ammonia is a colourless gas with a strong, pungent smell, but smell alone is not an acceptable identification test in IGCSE. The correct chemical test uses a piece of damp red litmus paper. Hold the moistened red litmus paper near the mouth of the test tube (never inside the solution if testing a heated reaction mixture). The paper will turn blue because ammonia dissolves in the water on the paper to form ammonium hydroxide, which is alkaline: NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
氨气是一种无色但有强烈刺激性气味的气体，但在 IGCSE 中，单凭气味不可作为合格的鉴定检验。正确的化学检验是用一片湿润的红色石蕊试纸。把湿润的红色石蕊试纸靠近试管口（如果检验的是加热反应混合物，绝不能把试纸伸入溶液内）。试纸会变成蓝色，因为氨气溶于试纸上的水中形成氢氧化铵，呈碱性： NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)。
H₄Cl(s). This test is also acceptable, but the damp red litmus method is more commonly quoted as the standard IGCSE test.
另一种检验方法是用蘸过浓盐酸的玻璃棒。当把它靠近氨气时，会出现浓厚的白烟，这是氯化铵颗粒： NH₃(g) + HCl(g) → NH₄Cl(s)。该检验同样被认可，但湿润的红色石蕊试纸法更常被引为 IGCSE 标准检验。
Always note that ammonia gas must be tested at the mouth of the tube to avoid drawing a false conclusion from hot alkali spray that may also turn litmus blue.
务必注意，氨气的检验必须在试管口进行，以免热的碱液飞溅沾到试纸导致变色，从而得出错误结论。
6. Test for Chlorine (Cl₂) | 氯气的检验
Chlorine is a greenish-yellow toxic gas with a characteristic choking smell. Its chemical test relies on its bleaching action. Place a piece of damp blue litmus paper in the flow of gas. Chlorine initially turns the paper red due to the formation of acidic HCl and HOCl, but then rapidly bleaches the colour to white. The key observation is bleaching of damp litmus, which distinguishes chlorine from other acid gases that only turn litmus red.
氯气是一种黄绿色、有特征性窒息气味的有毒气体。它的化学检验利用其漂白作用。将一片湿润的蓝色石蕊试纸放在气流中。氯气首先会使试纸变红，因为形成了酸性的 HCl 和次氯酸 HOCl，但随后会迅速将颜色漂白为白色。关键的观察点是湿润石蕊试纸的漂白现象，这可以把氯气与其他只能让石蕊试纸变红的酸性气体区别开来。
Alternatively, you may use starch-iodide paper: chlorine oxidises iodide ions to iodine, which forms a blue-black complex with starch. However, the IGCSE Edexcel specification favours the damp blue litmus/bleaching test. Note that both ammonia and chlorine tests use damp litmus, so you must describe the sequence of colour changes carefully.
或者，你也可以使用淀粉碘化钾试纸：氯气把碘离子氧化成碘单质，碘与淀粉形成蓝黑色复合物。不过，IGCSE Edexcel 考纲更倾向于使用湿润的蓝色石蕊试纸并观察漂白结果。注意氨气和氯气的检验都用到了湿润的石蕊试纸，因此必须仔细描述颜色变化的顺序。
Safety is paramount: chlorine is poisonous. The test should be carried out in a fume cupboard or well-ventilated space, and you should only use very small amounts.
安全至关重要：氯气有毒。检验必须在通风橱或通风良好的地方进行，而且只能使用极少量的氯气。
7. Summary Table of Gas Tests | 气体检验汇总表
The table below brings together all five standard gas tests. Memorising this table will help you answer practical-based questions quickly and accurately.
下表汇总了全部五种标准气体检验。牢记这个表格能帮助你快速准确地回答实验类问题。
| Gas 气体 | Test 检验方法 | Positive Result 阳性结果 |
|---|---|---|
| H₂ | Hold a lit splint at the mouth of the tube | Squeaky pop |
| O₂ | Insert a glowing splint | Splint relights |
| CO₂ | Bubble through limewater | Limewater turns milky/cloudy |
| NH₃ | Damp red litmus paper | Paper turns blue |
| Cl₂ | Damp blue litmus paper | Turns red then bleaches white |
Notice that for ammonium ions in a salt, you often add NaOH(aq) and warm, then test the evolved gas with damp red litmus – this tests for NH₃ derived from the ammonium salt, linking the test to cation analysis.
请注意，对于盐中的铵根离子，通常做法是加入氢氧化钠溶液并微热，然后用湿润的红色石蕊试纸检验释放出的气体——这就把气体检验和阳离子分析联系起来了。
8. Common Mistakes in Gas Tests | 气体检验中的常见错误
One frequent error is describing the test for hydrogen as 'burns with a pop'. While hydrogen does burn, the official description focuses on the sound: a burning splint gives a 'squeaky pop'. Another mistake is stating that oxygen turns limewater milky or that carbon dioxide relights a glowing splint – these are fundamentally wrong and show confusion between gases. Similarly, some learners claim that ammonia bleaches litmus; this is incorrect – it only turns red litmus blue with no bleaching.
一个常见的错误是把氢气检验描述为“燃烧并发出爆鸣声”。虽然氢气确实燃烧，但官方表述集中在声音上：点燃的木条会听到“爆鸣声”。另一个错误是说氧气能使石灰水变浑浊，或者说二氧化碳能使带火星木条复燃——这些都是根本性错误，表现出对不同气体检验的混淆。同样，有些学生声称氨气能漂白石蕊试纸；这是错误的——氨气只会让红色石蕊试纸变蓝，而不发生漂白。
Using dry litmus paper is also a common pitfall. The litmus paper must be damp, otherwise the gases will not dissolve and react. For chlorine, the observation of bleaching happens quite quickly, but if the paper is dry, you may miss the red intermediate stage and only see gradual white bleaching, losing the key detail.
使用干的石蕊试纸也是一个常见陷阱。石蕊试纸必须湿润，否则气体无法溶解和反应。对于氯气，漂白的观察发生得很快，但如果试纸是干的，你可能会错过红色中间阶段，只看到逐渐变白，从而丢失关键细节。
In written answers, avoid vague language like 'it will change colour'. Always specify the starting colour of the indicator and the final colour observed.
在书面作答中，避免使用“它会变色”这样模糊的语言。一定要指出指示剂的起始颜色和所观察到的最终颜色。
9. Practical Tips for Collecting and Testing Gases | 气体收集与检验的实用技巧
Success in a gas test often begins with the correct collection method. Hydrogen, being less dense than air, is collected by downward delivery; carbon dioxide and chlorine, being denser, are collected by upward delivery. Oxygen is almost exactly the same density as air, so it is usually collected over water. Ammonia is highly soluble in water, so it must be collected by upward delivery of dry apparatus.
气体检验的成功往往开始于正确的收集方法。氢气的密度小于空气，使用向下排空气法收集；二氧化碳和氯气密度较大，使用向上排空气法。氧气的密度几乎与空气完全相同，所以通常用排水法收集。氨气极易溶于水，因此必须使用干燥仪器并通过向上排空气法收集。
When bubbling gas into limewater, use a delivery tube that goes beneath the surface of the limewater to ensure adequate contact. For the pop test, make sure you have a secure grip on the test tube and that the splint is not so large that it extinguishes before reaching the gas.
往石灰水中通入气体时，要将导管口伸入液面以下，确保充分接触。进行爆鸣检验时，要确保握住试管稳固，并且木条不要太大，以至于在凑近气体之前就熄灭了。
Always wear chemical splash goggles and follow your school's safety rules when handling ammonia, chlorine or strong alkalis.
在处理氨气、氯气或强碱时，务必佩戴化学护目镜，并遵守学校的安全规定。
10. How Gas Tests Are Assessed in Edexcel IGCSE | Edexcel IGCSE 如何考查气体检验
Exam questions frequently give a list of observations and ask you to identify the gas or gases present. For example, 'A colourless gas turns damp red litmus paper blue' immediately points to ammonia. Another common scenario involves a table showing the results of several gases on limewater and litmus; you are asked to name gas X, Y and Z. This assesses not only your factual recall but also your ability to compare and contrast.
考试题目经常给出一系列观察结果，要求你识别出存在哪种（些）气体。例如，“一种无色气体使湿润的红色石蕊试纸变蓝”立刻指向氨气。另一种常见情境是给出一张表格，列示了若干气体在石灰水和石蕊试纸上的结果，然后要求你说出气体 X、Y 和 Z 的名称。这不仅要考察你对事实的回忆，还要考察你比较和对比的能力。
Extended writing tasks may ask you to describe a full procedure for testing the gases produced in a reaction, for instance when ammonium chloride is heated. You would need to describe the collection, the use of safety equipment, and the expected results. Linking the test to the chemical equation also demonstrates deeper understanding and can secure top-band marks.
扩展写作题可能会要求你完整描述检验某个反应所产生气体的步骤，例如加热氯化铵。你需要描述气体收集、安全设备的使用以及预期的结果。将检验与化学方程式联系起来，还能展示更深层次的理解，帮你获得最高档分数。
of apparatus → test → observation → conclusion. This is the format expected by examiners.
因此，练习时不仅要能背出检验方法，还要把它们写成一套逻辑严谨的操作流程。使用正确的顺序：收集→选择仪器→检验→观察→结论。这是考官期待的格式。
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