📚 Appendix 3: Qualitative analysis notes | 附录3：定性分析笔记
Qualitative analysis is the branch of chemistry that deals with identifying the chemical species present in a sample. In A-Level practical work, you are expected to use a systematic set of test-tube reactions to detect common cations, anions and gases by observing colour changes, precipitate formation, gas evolution and other visible signs. This appendix summarises the key tests, expected observations and underlying chemistry required for Cambridge International A-Level Chemistry.
定性分析是化学中用于鉴定样品中所含化学物种的分支。在A-Level实验操作中，你需要通过系统的试管反应来检测常见阳离子、阴离子和气体，观察颜色变化、沉淀生成、气体放出及其他可见信号。本文总结了剑桥国际A-Level化学所需的关键检验方法、预期现象及背后的化学原理。
1. Introduction to Qualitative Analysis | 定性分析简介
The goal of qualitative analysis is not to measure how much of a substance is present, but to identify which ions or gases are in an unknown mixture. Success relies on carrying out tests in a logical order, using clean apparatus, and carefully noting initial colours of solutions and any immediate or delayed changes.
定性分析的目标不是测量物质的含量，而是鉴定未知混合物中存在哪些离子或气体。成功的关键在于按照合理的顺序进行检验，使用洁净的仪器，并仔细观察溶液的初始颜色以及任何即时或延迟的变化。
A typical flow begins with physical examination (colour, smell), followed by flame tests for cations, addition of sodium hydroxide and then aqueous ammonia for cation confirmation, and a separate set of anion tests. Gas tests are performed whenever a gas is evolved. It is essential to record observations exactly as they appear, without early interpretation.
典型的流程始于外观检查（颜色、气味），接着进行阳离子的焰色试验，然后加入氢氧化钠和氨水以确认阳离子，之后再单独进行阴离子的检验。每当有气体逸出时都要进行气体检验。重要的是如实地记录观察到的现象，不要过早下结论。
2. Testing for Cations Using Sodium Hydroxide | 使用氢氧化钠检验阳离子
Adding a few drops of dilute sodium hydroxide solution to a solution of a metal ion often produces a characteristic precipitate. The colour and the behaviour of the precipitate when excess NaOH is added are the key diagnostic features.
向金属离子溶液中滴加几滴稀氢氧化钠溶液通常会产生特征沉淀。沉淀的颜色以及在加入过量NaOH时的行为是关键的诊断依据。
Cu²⁺ gives a pale blue precipitate of Cu(OH)₂, which does not dissolve in excess NaOH.
Cu²⁺ 生成淡蓝色 Cu(OH)₂ 沉淀，不溶于过量 NaOH。
Fe²⁺ forms a green precipitate of Fe(OH)₂; this turns brown at the surface on standing in air due to oxidation to Fe(OH)₃.
Fe²⁺ 生成绿色 Fe(OH)₂ 沉淀；在空气中放置时，表面变为棕色，因为被氧化成 Fe(OH)₃。
Fe³⁺ directly produces a reddish‑brown precipitate of Fe(OH)₃, which is insoluble in excess NaOH.
Fe³⁺ 直接生成红棕色 Fe(OH)₃ 沉淀，不溶于过量 NaOH。
Al³⁺, Zn²⁺ and Pb²⁺ all give white precipitates of their hydroxides. Importantly, Al(OH)₃, Zn(OH)₂ and Pb(OH)₂ are amphoteric and dissolve in excess NaOH, forming colourless solutions containing [Al(OH)₄]⁻, [Zn(OH)₄]²⁻ and [Pb(OH)₄]²⁻ respectively.
Al³⁺、Zn²⁺ 和 Pb²⁺ 均生成白色的氢氧化物沉淀。重要的是，Al(OH)₃、Zn(OH)₂ 和 Pb(OH)₂ 具有两性，能溶于过量 NaOH，分别形成含有[Al(OH)₄]⁻、[Zn(OH)₄]²⁻ 和 [Pb(OH)₄]²⁻ 的无色溶液。
Ca²⁺ and Mg²⁺ produce white precipitates of Ca(OH)₂ and Mg(OH)₂. These do not dissolve in excess NaOH; however, the Ca(OH)₂ precipitate may appear slightly less dense because calcium hydroxide is sparingly soluble.
Ca²⁺ 和 Mg²⁺ 生成白色的 Ca(OH)₂ 和 Mg(OH)₂ 沉淀。它们不溶于过量 NaOH；但由于氢氧化钙微溶，Ca(OH)₂ 沉淀可能显得稍稀薄。
Cu²⁺ and Fe³⁺ can often be distinguished at a glance by the colours of their original solutions (blue and yellow‑brown respectively) before any reagent is added.
在加入任何试剂之前，通常就能通过原溶液的颜色（分别为蓝色和黄棕色）迅速区分 Cu²⁺ 和 Fe³⁺。
3. Testing for Cations Using Aqueous Ammonia | 使用氨水检验阳离子
Aqueous ammonia provides a second, complementary test for metal cations. The precipitate colours are similar to those with NaOH, but the solubility in excess ammonia often gives crucial distinguishing information, especially for copper and zinc.
氨水提供了对金属阳离子的第二种补充检验。沉淀颜色与使用 NaOH 时相似，但在过量氨水中的溶解性常常提供关键的区分信息，特别是对铜和锌。
Cu²⁺ forms a pale blue precipitate of Cu(OH)₂, which readily dissolves in excess ammonia to produce a deep blue solution containing the complex ion [Cu(NH₃)₄]²⁺. This colour change is highly characteristic.
Cu²⁺ 生成淡蓝色 Cu(OH)₂ 沉淀，该沉淀易溶于过量氨水，生成含有[Cu(NH₃)₄]²⁺ 络离子的深蓝色溶液。这一颜色变化极具特征性。
Zn²⁺ gives a white precipitate of Zn(OH)₂, which also dissolves in excess ammonia, forming a colourless solution containing [Zn(NH₃)₄]²⁺.
Zn²⁺ 生成白色 Zn(OH)₂ 沉淀，该沉淀也溶于过量氨水，形成含有[Zn(NH₃)₄]²⁺ 的无色溶液。
Al³⁺ produces a white precipitate of Al(OH)₃, but unlike the reaction with NaOH, this precipitate does not dissolve in excess ammonia. This difference allows you to distinguish Al³⁺ from Zn²⁺.
Al³⁺ 生成白色的 Al(OH)₃ 沉淀，但与 NaOH 的反应不同，该沉淀不溶于过量氨水。这一差异使我们可以区分 Al³⁺ 和 Zn²⁺。
Fe²⁺ and Fe³⁺ give green and reddish‑brown precipitates respectively, both insoluble in excess ammonia. Like NaOH, the Fe²⁺ precipitate darkens on standing.
Fe²⁺ 和 Fe³⁺ 分别生成绿色和红棕色沉淀，两者均不溶于过量氨水。与 NaOH 一样，Fe²⁺ 沉淀在放置时会变暗。
Ca²⁺ and Mg²⁺: with dilute ammonia, no precipitate or only a faint white turbidity may appear because the concentration of OH⁻ is too low to exceed the solubility product. This contrasts with the immediate precipitate seen with NaOH and can be used as an additional clue.
Ca²⁺ 和 Mg²⁺：使用稀氨水时，可能无沉淀或仅出现微弱的白色浑浊，因为 OH⁻ 浓度太低，不足以超过溶度积。这与加入 NaOH 时立即产生沉淀的现象形成对比，可作为额外线索。
4. Flame Tests for Metal Ions | 金属离子的焰色试验
Flame tests offer a rapid means of identifying certain metal ions by the characteristic colour they impart to a Bunsen flame. A clean nichrome or platinum wire is dipped in concentrated hydrochloric acid and then into the sample, and the colour is observed.
焰色试验提供了一种快速鉴定某些金属离子的方法，通过它们使本生灯火焰呈现的特征颜色。将洁净的镍铬丝或铂丝蘸取浓盐酸，再蘸取样品，然后观察火焰颜色。
Lithium, Li⁺: crimson red.
锂，Li⁺：深红色。
Sodium, Na⁺: intense, persistent yellow.
钠，Na⁺：强烈持久的黄色。
Potassium, K⁺: lilac, often viewed through a blue cobalt glass to mask sodium contamination.
钾，K⁺：淡紫色，通常透过蓝色钴玻璃观察以屏蔽钠的干扰。
Calcium, Ca²⁺: brick‑red.
钙，Ca²⁺：砖红色。
Barium, Ba²⁺: pale green.
钡，Ba²⁺：淡绿色。
Copper, Cu²⁺: blue‑green (green with a blue centre).
铜，Cu²⁺：蓝绿色（外焰绿色，中心蓝色）。
Because the yellow emission from sodium is so bright, even traces can mask other colours. The cobalt glass absorbs the yellow light and allows the lilac of potassium to be seen clearly.
由于钠的黄色发射非常明亮，即使微量也能掩盖其他颜色。钴玻璃能吸收黄光，从而清晰地看到钾的淡紫色。
5. Testing for the Ammonium Ion (NH₄⁺) | 铵根离子（NH₄⁺）的检验
The ammonium ion is detected by heating the sample with sodium hydroxide solution and testing the gas evolved. The reaction is: NH₄⁺ + OH⁻ → NH₃ + H₂O.
检验铵根离子的方法是将样品与氢氧化钠溶液共热，并检验生成的气体。反应为：NH₄⁺ + OH⁻ → NH₃ + H₂O。
Place a small volume of the test solution in a test tube, add an equal volume of dilute NaOH, and warm gently. Hold a piece of damp red litmus paper near the mouth of the tube. The paper turns blue, confirming the presence of ammonia gas and thus the ammonium ion.
向试管中加入少量待测溶液和等体积的稀 NaOH，温和加热。将一片湿润的红色石蕊试纸置于管口附近。试纸变蓝，即可确认氨气的存在，进而证明铵根离子的存在。
This test is extremely specific and should be performed whenever an unknown sample possibly contains NH₄⁺, as its presence can interfere with cation tests that use NaOH, producing ammonia rather than metal hydroxide precipitates in some cases.
这一检验极为专一，当未知样品可能含有 NH₄⁺ 时应进行该检验，因为它的存在会干扰使用 NaOH 的阳离子检验，在某些情况下会产生氨气而非金属氢氧化物沉淀。
6. Testing for Common Anions: General Approach | 常见阴离子检验：一般方法
Anion tests are typically performed on a fresh portion of the sample solution, often after removing any interfering cations. The principal anions tested in Cambridge A‑Level Chemistry are carbonate (CO₃²⁻), sulfate (SO₄²⁻), and the halides (Cl⁻, Br⁻, I⁻). Nitrate (NO₃⁻) and sulfite (SO₃²⁻) may also be encountered.
阴离子检验通常使用一份新的样品溶液，往往在除去干扰阳离子后进行。剑桥A‑Level化学中主要检验的阴离子包括碳酸根（CO₃²⁻）、硫酸根（SO₄²⁻）和卤离子（Cl⁻、Br⁻、I⁻）。也可能遇到硝酸根（NO₃⁻）和亚硫酸根（SO₃²⁻）。
Always prepare a fresh solution in deionised water for anion tests. Carbonates and sulfites react with acids, so these tests must be performed first, ideally on the solid sample or directly on the original solution before acidification.
阴离子检验永远要用去离子水配制新鲜溶液。碳酸根和亚硫酸根会与酸反应，因此这些检验必须最先进行，最好使用固体样品或在酸化前直接使用原溶液。
7. Testing for Carbonate and Sulfate Ions | 碳酸根与硫酸根离子的检验
To test for carbonate, add dilute nitric acid or hydrochloric acid to the solid or solution. Effervescence indicates the release of carbon dioxide. Bubble the gas through limewater; if the limewater turns milky, CO₃²⁻ is present.
检验碳酸根：向固体或溶液中加入稀硝酸或稀盐酸。起泡表示释放出二氧化碳。将气体通入石灰水中；若石灰水变浑浊，则存在 CO₃²⁻。
Sulfate ions are detected by adding dilute nitric acid (or hydrochloric acid) followed by barium chloride or barium nitrate solution. A white precipitate of barium sulfate, BaSO₄, forms, which is insoluble in dilute acids.
硫酸根离子的检验是先加入稀硝酸（或盐酸），然后加入氯化钡或硝酸钡溶液。生成不溶于稀酸的白色硫酸钡（BaSO₄）沉淀。
Acidification is essential to decompose any carbonate or sulfite ions that would otherwise produce a white precipitate of BaCO₃ or BaSO₃, leading to a false positive result for sulfate.
酸化是必不可少的，其目的是分解任何可能存在的碳酸根或亚硫酸根离子，否则它们会生成白色的 BaCO₃ 或 BaSO₃ 沉淀，导致硫酸根检验出现假阳性。
8. Testing for Halide Ions with Silver Nitrate | 用硝酸银检验卤离子
The classic test for halide ions uses silver nitrate solution acidified with dilute nitric acid. The colour of the silver halide precipitate and its solubility in ammonia solutions provide the definitive identification.
卤离子的经典检验方法是使用经稀硝酸酸化的硝酸银溶液。卤化银沉淀的颜色及其在氨水中的溶解性提供了确切的鉴定依据。
Add a few drops of dilute nitric acid to the test solution, followed by silver nitrate solution. Observe the precipitate immediately.
向待测溶液中加入几滴稀硝酸，再加入硝酸银溶液，立即观察沉淀。
Chloride, Cl⁻, gives a white precipitate of AgCl. This precipitate dissolves in dilute ammonia solution, forming the colourless complex [Ag(NH₃)₂]⁺.
氯离子，Cl⁻，生成白色的 AgCl 沉淀。该沉淀溶于稀氨水，生成无色络离子 [Ag(NH₃)₂]⁺。
Bromide, Br⁻, gives a cream or pale yellow precipitate of AgBr. It is insoluble in dilute ammonia but dissolves in concentrated ammonia solution.
溴离子，Br⁻，生成奶油色或淡黄色的 AgBr 沉淀。它不溶于稀氨水，但溶于浓氨水。
Iodide, I⁻, gives a bright yellow precipitate of AgI. This precipitate does not dissolve even in concentrated ammonia, providing a clear distinction from bromide and chloride.
碘离子，I⁻，生成亮黄色的 AgI 沉淀。该沉淀即使在浓氨水中也不溶解，从而能够与溴离子和氯离子明确区分。
It is important to add nitric acid before silver nitrate to remove carbonate or hydroxide ions that would otherwise produce spurious precipitates.
在加入硝酸银之前必须先加入硝酸，以除去碳酸根或氢氧根离子，否则会产生干扰性沉淀。
9. Testing for Gases | 气体的检验
Whenever a gas is evolved in a test, its identity should be confirmed using the standard gas tests summarised below. Lighted splints, glowing splints, indicator papers and specific reagents are used.
每当检验中有气体逸出时，都应使用以下总结的标准气体检验方法来确认其身份，包括点燃的木条、带火星的木条、试纸和特定试剂。
Oxygen, O₂: relights a glowing splint.
氧气，O₂：能使带火星的木条复燃。
Hydrogen, H₂: produces a ‘squeaky pop’ with a lighted splint.
氢气，H₂：用点燃的木条检验时产生“爆鸣声”。
Carbon dioxide, CO₂: turns limewater milky. Prolonged bubbling may cause the precipitate to dissolve as calcium hydrogencarbonate forms.
二氧化碳，CO₂：使石灰水变浑浊。长时间鼓泡可能导致沉淀溶解，因为生成了碳酸氢钙。
Chlorine, Cl₂: bleaches damp litmus paper; the paper may first turn red because chlorine dissolves to form acidic species.
氯气，Cl₂：能漂白湿润的石蕊试纸；试纸可能先变红，因为氯气溶解生成酸性物种。
Ammonia, NH₃: turns damp red litmus paper blue and forms dense white fumes with concentrated hydrochloric acid (NH₄Cl).
氨气，NH₃：使湿润的红色石蕊试纸变蓝，并与浓盐酸接触产生浓厚的白烟（NH₄Cl）。
Sulfur dioxide, SO₂: turns acidified potassium dichromate(VI) solution from orange to green, as Cr₂O₇²⁻ is reduced to Cr³⁺.
二氧化硫，SO₂：使酸性重铬酸钾(VI)溶液由橙色变为绿色，因为 Cr₂O₇²⁻ 被还原为 Cr³⁺。
Nitrogen dioxide, NO₂: a brown gas that turns moist blue litmus paper red and may leave a brown stain.
二氧化氮，NO₂：一种棕色气体，能使湿润的蓝色石蕊试纸变红，并可能留下棕色痕迹。
10. Key Summary Table and Practical Tips | 关键总结表与实用技巧
Use the following table as a quick reference for cation precipitate colours and solubility in excess NaOH and NH₃.
使用下表作为阳离子沉淀颜色及其在过量 NaOH 和 NH₃ 中溶解性的快速参考。
| Ion | NaOH (few drops) | In excess NaOH | NH₃ (few drops) | In excess NH₃ |
|---|---|---|---|---|
| Cu²⁺ | Pale blue ppt | Insoluble | Pale blue ppt | Dissolves, deep blue solution |
| Fe²⁺ | Green ppt → brown | Insoluble | Green ppt → brown | Insoluble |
| Fe³⁺ | Red‑brown ppt | Insoluble | Red‑brown ppt | Insoluble |
| Al³⁺ | White ppt | 更多咨询请联系16621398022（同微信）
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