📚 Chemical Tests for Cations, Anions and Gases | 阳离子、阴离子及气体的化学测试
Qualitative analysis helps us identify the ions present in a compound. In Edexcel IGCSE Science, you need to recall the tests, observations and conclusions for common cations, anions and gases. Understanding these chemical tests is essential for practical exams and application questions.
定性分析帮助我们鉴定化合物中存在的离子。在爱德思 IGCSE 科学课程中，你需要记住常见阳离子、阴离子和气体的测试方法、观察现象以及结论。掌握这些化学测试对实验考试和简答题至关重要。
1. Introduction to Qualitative Analysis | 定性分析简介
Qualitative analysis determines the identity of ions, not their amounts. It relies on distinctive observations such as colour changes, precipitate formation, effervescence or a specific gas produced. These tests often involve adding a reagent and observing the result under controlled conditions.
定性分析确定离子的种类，而非其数量。它基于独特的观察现象，例如颜色变化、沉淀生成、冒泡或生成特定气体。这些测试通常涉及加入一种试剂，并在受控条件下观察结果。
2. Flame Tests | 火焰测试
Flame tests are used to identify certain metal cations by the characteristic colour they impart to a flame. Clean a nichrome wire loop with concentrated hydrochloric acid, dip it into the sample, and hold it at the edge of a blue Bunsen flame.
火焰测试利用某些金属离子使火焰呈现特征颜色的性质来鉴定它们。用浓盐酸清洗镍铬丝环，蘸取样品后置于蓝色本生火焰外焰边缘。
| Metal ion | Formula | Flame colour |
|---|---|---|
| Lithium | Li⁺ | Red |
| Sodium | Na⁺ | Yellow |
| Potassium | K⁺ | Lilac |
| Calcium | Ca²⁺ | Orange-red |
| Copper | Cu²⁺ | Blue-green |
Potassium's lilac flame is often masked by sodium's intense yellow, so viewing through a blue cobalt glass helps to filter out the yellow light.
钾的淡紫色火焰常被钠的强黄色掩盖，因此透过蓝色钴玻璃观察有助于滤除黄光。
3. Testing Cations with Sodium Hydroxide Solution | 用氢氧化钠溶液测试阳离子
Many metal cations form characteristic precipitates with sodium hydroxide. Add a few drops of NaOH solution to the sample, then add excess to see if the precipitate redissolves. The colour and solubility give the identification.
许多金属阳离子与氢氧化钠形成特征沉淀。向样品中加入几滴氢氧化钠溶液，然后加入过量试剂观察沉淀是否重新溶解。沉淀的颜色和可溶性可作为鉴定依据。
| Cation | With small amount of NaOH | With excess NaOH |
|---|---|---|
| Cu²⁺ | Blue precipitate | Insoluble |
| Fe²⁺ | Green precipitate | Insoluble |
| Fe³⁺ | Brown precipitate | Insoluble |
| Al³⁺ | White precipitate | Soluble, colourless solution |
| Zn²⁺ | White precipitate | Soluble, colourless solution |
| Ca²⁺ | White precipitate | Insoluble |
Zinc and aluminium ions both give a white precipitate that dissolves in excess NaOH, but they can be further distinguished by adding aqueous ammonia: aluminium hydroxide does not dissolve in excess ammonia, whereas zinc hydroxide does.
锌离子和铝离子均生成白色沉淀，且溶于过量的氢氧化钠，但可通过加氨水进一步区分：氢氧化铝不溶于过量氨水，而氢氧化锌可溶。
4. Testing for Ammonium Ions | 铵离子测试
Ammonium ions (NH₄⁺) do not give a precipitate with NaOH; instead, they release ammonia gas upon warming. Add sodium hydroxide solution and gently heat. Test the gas with damp red litmus paper; it turns blue.
铵离子（NH₄⁺）与氢氧化钠不产生沉淀，而是在加热时放出氨气。加入氢氧化钠溶液并微热，用湿润的红色石蕊试纸检验气体；试纸变蓝。
This test is often combined with the cation precipitation scheme because ammonia gas is only detected when the sample contains ammonium salts.
该测试常与阳离子沉淀方案结合使用，因为只有当样品中含有铵盐时才会检测到氨气。
5. Testing for Carbonate Ions | 碳酸根离子测试
Add dilute hydrochloric acid to the solid or solution. If carbonate ions (CO₃²⁻) are present, effervescence occurs. The gas produced is carbon dioxide, which turns limewater milky (a white precipitate of calcium carbonate forms).
向固体或溶液中加入稀盐酸。若存在碳酸根离子（CO₃²⁻），会冒泡。产生的气体是二氧化碳，能使石灰水变浑浊（生成碳酸钙白色沉淀）。
Some sulfites may also produce a gas with acid, so limewater is used to confirm the identity. A positive result is the milky appearance.
某些亚硫酸盐与酸也可能产生气体，因此用石灰水确认气体种类。阳性结果是石灰水变浑浊。
6. Testing for Sulfate Ions | 硫酸根离子测试
Add a few drops of dilute hydrochloric acid, then add barium chloride solution. A white precipitate of barium sulfate (BaSO₄) indicates the presence of sulfate ions (SO₄²⁻).
加入几滴稀盐酸，然后加入氯化钡溶液。若生成硫酸钡（BaSO₄）白色沉淀，表明存在硫酸根离子（SO₄²⁻）。
The initial acidification removes any carbonate or sulfite ions that could also form a white precipitate with barium ions, ensuring the test is specific to sulfate.
先酸化可除去任何可能同样与钡离子形成白色沉淀的碳酸根或亚硫酸根离子，从而确保该测试对硫酸根具有专一性。
7. Testing for Halide Ions (Chloride, Bromide, Iodide) | 卤化物离子测试
To test for halide ions, add nitric acid, then silver nitrate solution. The colour of the precipitate identifies the halide. Acidification removes interfering carbonate ions.
要测试卤素离子，先加硝酸，再加硝酸银溶液。沉淀的颜色可确定是哪种卤离子。酸化的目的是消除碳酸根离子的干扰。
- Chloride (Cl⁻): white precipitate of AgCl, soluble in dilute ammonia.
氯化物 (Cl⁻)：白色 AgCl 沉淀，溶于稀氨水。 - Bromide (Br⁻): cream precipitate of AgBr, soluble only in concentrated ammonia.
溴化物 (Br⁻)：淡黄色 AgBr 沉淀，仅溶于浓氨水。 - Iodide (I⁻): yellow precipitate of AgI, insoluble in both dilute and concentrated ammonia.
碘化物 (I⁻)：黄色 AgI 沉淀，不溶于稀氨水和浓氨水。
This solubility in ammonia provides a clear way to distinguish between the three halide precipitates.
这种在氨水中的溶解性差异为区分三种卤化银沉淀提供了明确的方法。
8. Testing for Nitrate Ions | 硝酸根离子测试
The nitrate test involves reducing nitrate ions (NO₃⁻) to ammonia using aluminium powder and sodium hydroxide solution. Warm the mixture gently; ammonia gas is released, turning damp red litmus paper blue.
硝酸根测试需用铝粉和氢氧化钠溶液将硝酸根离子（NO₃⁻）还原成氨。微热混合物，放出氨气，使湿润的红色石蕊试纸变蓝。
This test is also known as the "aluminium and NaOH test" or the "Devarda's test". It confirms nitrate ions through the production of ammonia, which does not occur with other common anions like sulfate or chloride.
该测试也称为“铝和氢氧化钠测试”或“德瓦达测试”。通过产生氨气来确证硝酸根离子，而其他常见阴离子如硫酸根或氯离子则不会产生氨气。
9. Testing for Common Gases | 常见气体的测试
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen, H₂ | Lit splint | Squeaky pop |
| Oxygen, O₂ | Glowing splint | Relights |
| Carbon dioxide, CO₂ | Bubble through limewater | Limewater turns milky |
| Chlorine, Cl₂ | Damp blue litmus paper | Bleaches (turns white) |
| Ammonia, NH₃ | Damp red litmus paper | Turns blue |
| Sulfur dioxide, SO₂ | Acidified potassium dichromate paper | Turns from orange to green |
For chlorine, note that the litmus paper first turns red due to the acidic nature, then is bleached white by the hypochlorite ions formed in solution.
对于氯气，注意石蕊试纸先因酸性而变红，随后被溶液中生成的次氯酸根离子漂白变白。
10. Summary Table of Tests and Key Observations | 测试与关键现象总结表
Below is a compact summary linking ions to their characteristic tests. Use this as a quick revision checklist for your Edexcel IGCSE Science papers.
下面是一个紧凑的总结表，将离子与其特征测试联系起来。可作为 Edexcel IGCSE 科学考试前的快速复习清单。
| Ion / Gas | Reagent(s) | Observation |
|---|---|---|
| Cu²⁺ | NaOH solution | Blue precipitate |
| Fe²⁺ | NaOH solution | Green precipitate |
| Fe³⁺ | NaOH solution | Brown precipitate |
| NH₄⁺ | NaOH + heat | Damp red litmus → blue |
| CO₃²⁻ | Dilute HCl, limewater | Effervescence, limewater milky |
| SO₄²⁻ | HCl, BaCl₂ solution | White precipitate |
| Cl⁻ | HNO₃, AgNO₃, dilute NH₃ | White ppt, soluble in dilute NH₃ |
| Br⁻ | HNO₃, AgNO₃, conc. NH₃ | Cream ppt, soluble in conc. NH₃ |
| I⁻ | HNO₃, AgNO₃ | Yellow ppt, insoluble in NH₃ |
| NO₃⁻ | Al + NaOH, heat | Damp red litmus → blue |
| H₂ | Lit splint | Squeaky pop |
| O₂ | Glowing splint | Relights |
| CO₂ | Limewater | Turns milky |
| Cl₂ | Damp blue litmus | Bleaches |
| NH₃ | Damp red litmus | Turns blue |
| SO₂ | Acidified K₂Cr₂O₇ paper | Orange to green |
Memorising this table alone is not enough – practise writing balanced ionic equations for the precipitation reactions and gas tests. Understanding why each reagent is chosen will help you answer unfamiliar application questions confidently.
仅背诵此表是不够的——还要练习书写沉淀反应和气体检验的离子方程式。理解每种试剂的选择原因，将有助于你自信地回答陌生的应用问题。
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