📚 Chemical Tests for Gases | 气体化学检验方法
In IGCSE Edexcel Science (Double Award) and Chemistry, being able to identify common gases through simple laboratory tests is a fundamental practical skill. Topic 3.6.7 specifically requires you to describe, and often carry out, the chemical tests for six key gases: hydrogen, oxygen, carbon dioxide, ammonia, chlorine and sulfur dioxide. Mastering these tests not only helps with practical assessments but also frequently appears in written examinations where you must recall reagents, procedures and expected observations.
在IGCSE爱德思科学（双奖）与化学课程中，能通过简单的实验室测试鉴别常见气体是一项基本的实验技能。专题3.6.7明确要求你描述并经常亲手完成六种关键气体的化学检验：氢气、氧气、二氧化碳、氨气、氯气和二氧化硫。掌握这些检验方法不仅有助于实验评估，也常出现在笔试试卷中，需要你准确回忆试剂、操作步骤和预期现象。
1. Overview of Gas Tests | 气体检验概述
Gas tests rely on distinctive chemical or physical properties, such as combustibility, ability to support combustion, acidity, bleaching action or reaction with specific reagents to form a visible change. Most of these tests produce a dramatic and immediate result – a pop sound, rekindling of a glowing splint, a colour change or a precipitate – making them easy to observe and interpret. In the Edexcel specification, you are expected to know both the practical method and the chemical basis for each test.
气体检验依赖于独特的气体化学或物理性质，例如可燃性、助燃性、酸性、漂白作用或与特定试剂发生反应产生可见变化。大多数这些测试都会产生迅速而明显的现象——爆鸣声、余烬复燃、颜色变化或沉淀生成——使它们易于观察和解释。在爱德思考试规范中，你既需要掌握每种检验的操作方法，也要理解其化学原理。
2. Safety Precautions | 安全注意事项
Before performing any gas test, it is essential to consider safety. Several of these gases are toxic, corrosive or flammable. Always work in a well‑ventilated laboratory or a fume cupboard when testing chlorine, ammonia and sulfur dioxide. Wear safety goggles and, when necessary, gloves. When igniting hydrogen, use only small volumes to avoid a violent explosion. Never inhale gases directly; instead, use wafting techniques to detect ammonia’s characteristic odour cautiously.
在进行任何气体检验之前，必须考虑安全事项。其中好几种气体有毒、有腐蚀性或易燃。检验氯气、氨气和二氧化硫时务必在通风良好的实验室或通风橱中操作。佩戴护目镜，必要时应戴手套。点燃氢气时只能使用少量气体，以防剧烈爆炸。切勿直接吸入气体，而应采用扇闻法小心地辨别氨气的特征气味。
3. Test for Hydrogen | 氢气的检验
• Method: Collect a sample of the gas in an inverted test tube. Hold a burning splint near the open mouth of the tube.
• 方法：将气体收集在一个倒置的试管中，把一根燃烧的木条靠近试管口。
• Positive result: Hydrogen burns rapidly with a characteristic ‘squeaky pop’ sound. This is because hydrogen mixes with oxygen in the air and the mixture ignites explosively in a small space.
• 阳性结果：氢气迅速燃烧，发出特有的‘尖啸爆鸣声’。这是因为氢气与空气中的氧气混合，在狭小空间内被点燃发生爆燃。
2H₂(g) + O₂(g) → 2H₂O(l)
4. Test for Oxygen | 氧气的检验
• Method: Insert a glowing splint (a wooden splint that is still hot but no longer flaming) into a test tube containing the gas.
• 方法：将一根带火星的木条（仍炽热但已无明火的木条）伸入盛有该气体的试管中。
• Positive result: The glowing splint immediately rekindles and bursts into flame. Oxygen supports combustion, so it dramatically accelerates the oxidation of the wood.
• 阳性结果：带火星的木条立即复燃，重新燃烧起来。氧气能助燃，因此它会极大地加速木条的氧化反应。
C(s) + O₂(g) → CO₂(g)
5. Test for Carbon Dioxide | 二氧化碳的检验
• Method: Bubble the gas through a small volume of limewater (a dilute calcium hydroxide solution) or shake a test tube of the gas with limewater.
• 方法：将气体通入少量石灰水（稀氢氧化钙溶液）中，或将盛有气体的试管与石灰水一同振荡。
• Positive result: The limewater turns from colourless and transparent to milky or cloudy white. This is due to the formation of a fine white precipitate of calcium carbonate.
• 阳性结果：石灰水由无色透明变为乳白色或浑浊。这是因为生成了白色碳酸钙沉淀。
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
6. Test for Ammonia | 氨气的检验
• Method 1: Hold a piece of damp red litmus paper near the mouth of the test tube or gas source. • Method 2: Dip a glass rod into concentrated hydrochloric acid and hold it close to the gas; a white smoke of ammonium chloride forms. In the exam, you may describe either test.
• 方法1：将一片湿润的红色石蕊试纸靠近试管口或气源。• 方法2：将一根玻璃棒蘸取浓盐酸后靠近气体，会生成氯化铵的白色烟雾。考试中两种方法皆可描述。
• Positive result: Damp red litmus turns blue because ammonia is an alkaline gas. The white smoke is solid ammonium chloride, which appears as a dense white fume.
• 阳性结果：湿润的红色石蕊试纸变蓝，因为氨气是碱性气体。白色烟雾是氯化铵固体，呈浓密白烟状。
NH₃(g) + HCl(g) → NH₄Cl(s)
7. Test for Chlorine | 氯气的检验
• Method: Hold a piece of damp blue litmus paper (or damp universal indicator paper) in the flow of the gas.
• 方法：将一片湿润的蓝色石蕊试纸（或湿润的通用指示试纸）放在气流中。
• Positive result: The paper initially turns red due to the acidic nature of chlorine in water, and then it is bleached to white. Chlorine reacts with water to form hydrochloric acid and hypochlorous acid, the latter being a powerful bleaching agent. This sequential colour change – red then white – is a unique indicator for chlorine.
• 阳性结果：试纸首先因氯气溶于水呈酸性而变红，随后被漂白为白色。氯气与水反应生成盐酸和次氯酸，后者是一种强漂白剂。这种先变红后褪色的顺序变化是氯气的独特鉴别特征。
Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq)
8. Test for Sulfur Dioxide | 二氧化硫的检验
• Method: Pass the gas over or bubble it through a dilute acidified solution of potassium manganate(VII) (potassium permanganate). The solution must be acidified with dilute sulfuric acid.
• 方法：将气体通过或通入稀的酸化高锰酸钾(KMnO₄)溶液中。该溶液必须用稀硫酸酸化。
• Positive result: The purple solution turns colourless. Sulfur dioxide acts as a reducing agent, reducing the purple manganate(VII) ions (MnO₄⁻) to very pale pink or virtually colourless manganese(II) ions (Mn²⁺). An alternative test using acidified potassium dichromate(VI) turns the solution from orange to green, but manganate(VII) is the standard Edexcel test.
• 阳性结果：紫色溶液变为无色。二氧化硫作为还原剂，将紫色的高锰酸根离子(MnO₄⁻)还原为颜色极浅或几乎无色的锰(II)离子(Mn²⁺)。另一种使用酸化重铬酸钾(VI)的检验会使溶液从橙色变为绿色，但高锰酸钾法是爱德思的标准检验。
5SO₂(g) + 2MnO₄⁻(aq) + 2H₂O(l) → 2Mn²⁺(aq) + 5SO₄²⁻(aq) + 4H⁺(aq)
9. Summary Table of Tests | 检验方法总结表
| Gas / 气体 | Reagent / 试剂 | Positive Result / 阳性结果 |
|---|---|---|
| Hydrogen / 氢气 | Burning splint / 燃烧木条 | Squeaky pop / 爆鸣声 |
| Oxygen / 氧气 | Glowing splint / 带火星木条 | Rekindles / 复燃 |
| Carbon dioxide / 二氧化碳 | Limewater / 石灰水 | Turns milky / 变乳白色 |
| Ammonia / 氨气 | Damp red litmus paper / 湿润红色石蕊试纸 | Turns blue / 变蓝 |
| Chlorine / 氯气 | Damp blue litmus paper / 湿润蓝色石蕊试纸 | Bleached (red then white) / 漂白(先红后白) |
| Sulfur dioxide / 二氧化硫 | Acidified potassium manganate(VII) / 酸化高锰酸钾 | Purple to colourless / 紫色变无色 |
Use this table as a quick reference when revising. Pay attention to the exact wording required in mark schemes, especially for chlorine’s two‑step colour change and the nature of the reagent used for sulfur dioxide.
在复习时请用此表快速参考。注意评分方案中要求的确切表述，特别是氯气的两步颜色变化以及二氧化硫检验所用的试剂性质。
10. Common Examination Mistakes | 常见考试错误
• Confusing litmus colours: Many students remember that chlorine bleaches but forget that damp blue litmus must first turn red before bleaching – this sequence is crucial for the mark. Always state ‘turns red then is bleached’ or ‘red then white’.
• 混淆石蕊试纸颜色：许多学生记得氯气有漂白性，但忘记了湿润的蓝色石蕊试纸必须先变红再被漂白——这一顺序是得分关键。务必写明‘先变红后漂白’或‘先红后白’。
• Omitting ‘damp’ or ‘moist’: For ammonia and chlorine tests, the litmus paper must be damp. A dry paper will not show a rapid change, as the gases need to dissolve and ionise. Always specify ‘damp red litmus’ or ‘moist blue litmus’.
• 遗漏‘湿润’二字：在氨气和氯气的检验中，石蕊试纸必须是湿润的。干燥试纸不会迅速变色，因为气体需要溶解并电离。务必写明‘湿润的红色石蕊试纸’或‘湿润的蓝色石蕊试纸’。
• Using the wrong acid for manganate(VII): Potassium manganate(VII) solution must be acidified with dilute sulfuric acid, not hydrochloric acid, because chloride ions would also react with the oxidising agent. In the exam, saying ‘acidified potassium manganate(VII)’ is sufficient, but be ready to explain why.
• 高锰酸钾用错酸：高锰酸钾溶液必须用稀硫酸酸化，而不能用盐酸，因为氯离子也会与氧化剂反应。考试中写出‘酸化高锰酸钾’即可，但要能解释原因。
• Forgetting to mention limewater’s appearance: The phrase ‘limewater turns milky’ or ‘cloudy’ is expected; just saying ‘a precipitate forms’ without referencing the observed change may lose marks.
• 忘记描述石灰水的外观变化：要求写出‘石灰水变乳白色’或‘变浑浊’；只写‘生成沉淀’而不提观察到什么变化可能会丢分。
11. Conclusion | 总结
Chemical tests for gases are among the most straightforward yet highly assessed topics in Edexcel IGCSE Science. By learning each test’s reagent, method and, most importantly, the specific observable change, you can secure valuable marks both in multiple‑choice and structured questions. Practise linking the observation to the chemical equation, and you will be well prepared for any question on gas identification.
气体化学检验是爱德思IGCSE科学中最直接却又高频率考查的主题之一。牢记每种检验的试剂、方法，尤其是特定的可观察变化，你就能在选择题和结构化问题中稳稳拿到分数。练习将现象与化学方程式联系起来，你就能应对任何有关气体鉴别的题目做好充分准备。
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