📚 Appendix 3: Qualitative Analysis Notes | 定性分析笔记
Qualitative analysis in A-level chemistry uses simple test-tube reactions and flame tests to identify the ions present in an unknown compound or mixture. Precise observations of colour changes, precipitate formation, gas evolution, and solubility in excess reagents are the evidence on which reliable conclusions depend.
在 A-level 化学中，定性分析利用简单的试管反应和焰色试验来鉴定未知化合物或混合物中的离子。准确记录颜色变化、沉淀生成、气体放出以及沉淀在过量试剂中的溶解性，是得出可靠结论的依据。
1. Core Principles of Qualitative Analysis | 定性分析核心原则
Always carry out the test on both the unknown sample and a known reference sample. Record observations before adding excess reagent, because a precipitate may dissolve or change colour on further addition.
检验未知样品时，应同时用已知样品作对照。加入过量试剂之前必须记录现象，因为沉淀可能在继续加入试剂后溶解或改变颜色。
Clean all apparatus and use small volumes. The most common source of false positives is contamination from a previous sample, especially when flame tests or silver nitrate tests are repeated.
所有仪器必须清洁，溶液用量要少。假阳性最常见的来源是前一样品的残留污染，尤其是在重复进行焰色试验或硝酸银试验时。
Do not rely on a single test alone. Whenever possible, confirm an ion with at least two independent observations, such as a precipitation colour and a flame colour or gas test.
不能只依赖单一实验。只要可能，应至少用两个独立观察来确认一种离子，例如沉淀颜色加上焰色或气体检验。
2. Flame Tests for Metal Cations | 金属阳离子的焰色试验
Clean a nichrome or platinum wire by dipping it in concentrated hydrochloric acid, then place it in the blue Bunsen flame until no colour is seen. Dip the hot wire into the solid sample and observe the flame colour against a dark background.
将镍铬丝或铂丝蘸取浓盐酸，置于蓝色本生灯火焰中灼烧至无颜色。再将热丝蘸取固体样品，在暗色背景下观察火焰颜色。
| Ion | Flame colour | 焰色 | Comment | 备注 |
|---|---|---|
| Li⁺ | crimson red | 洋红色 | often less intense than Sr²⁺ | 通常不如 Sr²⁺ 强烈 |
| Na⁺ | intense yellow | 亮黄色 | persistent, masks many colours | 持久，易掩盖其他颜色 |
| K⁺ | lilac | 淡紫色 | use cobalt glass to filter Na⁺ | 用钴玻璃滤去 Na⁺ |
| Ca²⁺ | brick red | 砖红色 | distinct from Sr²⁺ crimson | 与 Sr²⁺ 的深红色不同 |
| Sr²⁺ | crimson | 深红色 | deeper red than Li⁺ | 比 Li⁺ 更深的红色 |
| Ba²⁺ | apple green | 苹果绿色 | characteristic for barium | 钡的特征焰色 |
| Cu²⁺ | blue-green | 蓝绿色 | often seen with copper halides | 常见于铜的卤化物 |
The flame test is a rapid preliminary method, but it cannot distinguish ions with similar colours, such as Na⁺ and some organic compounds that burn yellow.
焰色试验是一种快速初步方法，但无法区分颜色相似的离子，例如 Na⁺ 和某些燃烧呈黄色的有机化合物。
3. Sodium Hydroxide Tests for Cations | 阳离子的氢氧化钠检验
Add aqueous sodium hydroxide dropwise to the unknown solution, then add an excess and shake. Precipitate colour and solubility in excess NaOH are diagnostic for several metal cations.
向未知溶液中逐滴加入氢氧化钠溶液，再加入过量并振荡。沉淀的颜色及其在过量 NaOH 中的溶解性可用于鉴定多种金属阳离子。
Cu²⁺ gives a pale blue precipitate, Cu(OH)₂, which is insoluble in excess NaOH.
Cu²⁺ 生成淡蓝色沉淀 Cu(OH)₂，过量 NaOH 中不溶解。
Cu²⁺ + 2OH⁻ → Cu(OH)₂(s)
Fe²⁺ gives a green precipitate, Fe(OH)₂, which is insoluble in excess NaOH and slowly turns brown at the surface on standing due to oxidation to Fe(OH)₃.
Fe²⁺ 生成绿色沉淀 Fe(OH)₂，过量 NaOH 中不溶，放置后表面逐渐被氧化为 Fe(OH)₃ 而变成棕色。
Fe³⁺ gives a reddish-brown precipitate, Fe(OH)₃, which is insoluble in excess NaOH.
Fe³⁺ 生成红棕色沉淀 Fe(OH)₃，过量 NaOH 中不溶。
Mg²⁺ gives a white precipitate, Mg(OH)₂, which is insoluble in excess NaOH.
Mg²⁺ 生成白色沉淀 Mg(OH)₂，过量 NaOH 中不溶。
Ca²⁺ may give a white precipitate of Ca(OH)₂ because calcium hydroxide is sparingly soluble. The precipitate appears more clearly with moderately concentrated NaOH.
Ca²⁺ 可能生成白色 Ca(OH)₂ 沉淀，因为氢氧化钙微溶。使用中等浓度的 NaOH 时沉淀更明显。
Al³⁺ gives a white precipitate, Al(OH)₃, which dissolves in excess NaOH to form a colourless solution of [Al(OH)₄]⁻.
Al³⁺ 生成白色沉淀 Al(OH)₃，过量 NaOH 中溶解，形成无色 [Al(OH)₄]⁻ 溶液。
Zn²⁺ gives a white precipitate, Zn(OH)₂, which dissolves in excess NaOH to form a colourless solution of [Zn(OH)₄]²⁻.
Zn²⁺ 生成白色沉淀 Zn(OH)₂，过量 NaOH 中溶解，形成无色 [Zn(OH)₄]²⁻ 溶液。
Pb²⁺ gives a white precipitate, Pb(OH)₂, which dissolves in excess NaOH to give a colourless solution.
Pb²⁺ 生成白色沉淀 Pb(OH)₂，过量 NaOH 中溶解，得到无色溶液。
Cr³⁺ gives a grey-green precipitate, Cr(OH)₃, which dissolves in excess NaOH to give a dark green solution.
Cr³⁺ 生成灰绿色沉淀 Cr(OH)₃，过量 NaOH 中溶解，得到深绿色溶液。
4. Ammonia Solution Tests for Cations | 阳离子的氨水检验
Aqueous ammonia is a weak base, so it first precipitates insoluble metal hydroxides. Excess ammonia can act as a ligand and dissolve certain precipitates by forming ammine complexes.
氨水是弱碱，因此先使不溶性金属氢氧化物沉淀。过量氨可作为配体，与某些沉淀形成氨配合物而使其溶解。
Cu²⁺ gives a blue precipitate that dissolves in excess NH₃ to form a deep blue solution containing [Cu(NH₃)₄]²
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