Notes for Use in Qualitative Analysis | 定性分析使用笔记

📚 Notes for Use in Qualitative Analysis | 定性分析使用笔记

Qualitative analysis in chemistry is the process of identifying the ions present in a substance by observing its physical and chemical properties, particularly through colour changes, precipitate formation, and gas evolution. This guide provides concise, systematic notes for IGCSE students on how to carry out qualitative analysis tests effectively in the laboratory and in examinations.

化学中的定性分析是通过观察物质的物理和化学性质来鉴定其所含离子的过程，尤其是通过颜色变化、沉淀生成和气体放出现象。本指南为IGCSE学生提供简明、系统的笔记，帮助大家在实验室和考试中有效进行定性分析测试。

1. Introduction to Qualitative Analysis | 定性分析导言

Qualitative analysis is a core practical skill tested across all major IGCSE chemistry syllabuses. It involves using a sequence of tests to establish the identity of cations (positive ions), anions (negative ions), and gases without measuring exact quantities.

定性分析是IGCSE化学所有主要考纲中的核心实验技能。它通过一系列测试来确定阳离子（正离子）、阴离子（负离子）和气体的身份，而无需测量精确的物质的量。

The two most common reagent sets used are aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃), both of which react with metal cations to form characteristic precipitates. Additionally, dilute nitric acid and silver nitrate are used to identify halide ions, while barium chloride or barium nitrate identifies sulfate ions.

最常用的两组试剂是氢氧化钠水溶液和氨水，二者都能与金属阳离子反应生成特征性沉淀。此外，稀硝酸和硝酸银用于鉴定卤离子，而氯化钡或硝酸钡用于鉴定硫酸根离子。

2. Safety and Experimental Technique | 安全与实验技术

Before performing any qualitative analysis test, always wear safety goggles and gloves. Many reagents are corrosive or toxic, and gases evolved during tests may be harmful. Use a test tube rack and clean, labelled test tubes to avoid contamination.

在进行任何定性分析测试之前，务必佩戴护目镜和手套。许多试剂具有腐蚀性或毒性，测试中释放的气体可能有害。使用试管架和干净、贴有标签的试管，以避免污染。

Use small volumes (about 1–2 cm³) of test solution to ensure safety and clarity of observations.

使用约1–2 cm³的小体积待测液，以确保安全和观察清晰。
Always add reagents dropwise with gentle shaking; do not boil unless specifically instructed.

始终逐滴加入试剂并轻轻摇动；除非有特别指示，否则不要加热煮沸。
Record observations immediately, including colours of precipitates before and after adding excess reagent.

立即记录观察结果，包括沉淀在加入过量试剂前后的颜色。

3. Testing Cations with Sodium Hydroxide (NaOH) | 用氢氧化钠测试阳离子

Aqueous sodium hydroxide is added dropwise to a sample of the unknown solution. The colour and solubility of any precipitate formed provide key information about the metal cation present.

将氢氧化钠水溶液逐滴加入未知溶液中。所形成沉淀的颜色和溶解性为判断存在的金属阳离子提供了关键信息。

Cation	Colour of precipitate with NaOH	Soluble in excess NaOH?
Cu²⁺	Blue	No
Fe²⁺	Green	No
Fe³⁺	Red-brown	No
Al³⁺	White	Yes
Zn²⁺	White	Yes
Ca²⁺	White	No
NH₄⁺	No precipitate (warming produces NH₃ gas)	N/A

Cu²⁺ + 2OH⁻ → Cu(OH)₂↓ (blue)

Al³⁺ + 3OH⁻ → Al(OH)₃↓ (white)

Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻ (colourless, soluble)

4. Testing Cations with Aqueous Ammonia (NH₃) | 用氨水测试阳离子

Aqueous ammonia is a weak base and can also form precipitates with metal cations. Importantly, certain precipitates that are insoluble in excess NaOH may dissolve in excess NH₃ due to complex formation, which helps distinguish specific ions.

氨水是一种弱碱，也能与金属阳离子形成沉淀。重要的是，某些在过量NaOH中不溶的沉淀可能因配合物生成而溶解于过量NH₃中，这有助于区分特定离子。

Cu²⁺ gives a blue precipitate that dissolves in excess NH₃ to form a deep blue solution.

Cu²⁺生成蓝色沉淀，溶于过量氨水形成深蓝色溶液。
Fe²⁺ and Fe³⁺ form green and red-brown precipitates respectively, neither of which dissolves in excess NH₃.

Fe²⁺和Fe³⁺分别生成绿色和红棕色沉淀，二者均不溶于过量氨水。
Al³⁺ forms a white precipitate that is insoluble in excess NH₃, whereas Zn²⁺ forms a white precipitate that dissolves in excess NH₃.

Al³⁺生成不溶于过量氨水的白色沉淀，而Zn²⁺生成的白色沉淀溶于过量氨水。

This difference allows you to distinguish Al³⁺ from Zn²⁺: both give white precipitates with NaOH and NH₃, but only Zn²⁺ redissolves in excess NH₃.

这一差异可用于区分Al³⁺和Zn²⁺：二者与NaOH和NH₃都生成白色沉淀，但只有Zn²⁺在过量氨水中重新溶解。

5. Flame Tests for Cations | 阳离子焰色反应

Flame tests are used to identify certain metal cations based on the characteristic colour they impart to a non-luminous Bunsen flame. A clean platinum or nichrome wire is dipped into concentrated hydrochloric acid and then into the sample before being placed in the flame.

焰色反应通过金属阳离子赋予无色本生灯火焰的特征颜色来鉴定它们。将洁净的铂丝或镍铬丝蘸取浓盐酸，再蘸取样品，然后置于火焰中。

Metal cation	Flame colour
Li⁺	Crimson / red
Na⁺	Yellow
K⁺	Lilac (pale purple)
Ca²⁺	Brick-red / orange-red
Cu²⁺	Blue-green

Note that sodium compounds often contaminate samples and produce an intense yellow flame; always use a fresh wire and test with a blank control where necessary.

注意钠化合物常污染样品并产生强烈的黄色火焰；应始终使用新洁净的金属丝，并在必要时进行空白对照测试。

6. Testing Anions – Carbonate (CO₃²⁻) | 阴离子测试 – 碳酸根

Carbonate ions are identified by adding dilute hydrochloric acid or dilute nitric acid to the sample. Effervescence indicates the release of carbon dioxide gas, which can be confirmed by bubbling the gas through limewater (aqueous calcium hydroxide).

碳酸根的鉴定方法是向样品中加入稀盐酸或稀硝酸。产生气泡表明有二氧化碳气体释放，可通过将气体通入石灰水（氢氧化钙水溶液）来确认。

CO₃²⁻ + 2H⁺ → CO₂↑ + H₂O

CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O (limewater turns milky)

If the gas produced turns damp blue litmus paper red, the gas is acidic (likely CO₂). For accurate results, test the gas immediately after acid addition.

如果产生的气体使湿润的蓝色石蕊试纸变红，则证明该气体为酸性（可能是CO₂）。为获得准确结果，应在加入酸后立即测试气体。

7. Testing Anions – Sulfate (SO₄²⁻) | 阴离子测试 – 硫酸根

Sulfate ions are detected by adding dilute hydrochloric acid followed by barium chloride solution. The formation of a white precipitate of barium sulfate confirms the presence of sulfate ions.

硫酸根的检测方法是先加入稀盐酸，再加入氯化钡溶液。若生成白色硫酸钡沉淀，则证明硫酸根存在。

Ba²⁺ + SO₄²⁻ → BaSO₄↓ (white)

The hydrochloric acid is added first to remove carbonate ions, which would otherwise also produce a white precipitate with barium ions. If no precipitate appears when carbonate is absent, the white solid confirms sulfate.

先加入稀盐酸是为了去除碳酸根，因为碳酸根也会与钡离子生成白色沉淀。当碳酸根不存在时，仍出现白色沉淀则可确认为硫酸根。

8. Testing Anions – Halides (Cl⁻, Br⁻, I⁻) | 阴离子测试 – 卤离子

Halide ions are identified using dilute nitric acid followed by silver nitrate solution. The colour of the precipitate formed distinguishes the halide. Aqueous ammonia is then used to confirm the identity further, as silver chloride dissolves in dilute ammonia, silver bromide dissolves in concentrated ammonia, and silver iodide is insoluble in ammonia.

卤离子的鉴定方法是先加入稀硝酸，再加入硝酸银溶液。所形成沉淀的颜色可用于区分卤离子。随后用氨水进一步确认：氯化银溶于稀氨水，溴化银溶于浓氨水，而碘化银不溶于氨水。

Halide ion	Precipitate with AgNO₃	Solubility in NH₃
Cl⁻	White	Dissolves in dilute NH₃
Br⁻	Cream / pale yellow	Dissolves in concentrated NH₃
I⁻	Yellow	Insoluble in NH₃

Ag⁺ + Cl⁻ → AgCl↓ (white)

Ag⁺ + Br⁻ → AgBr↓ (cream)

Ag⁺ + I⁻ → AgI↓ (yellow)

9. Testing Anions – Nitrate (NO₃⁻) | 阴离子测试 – 硝酸根

The nitrate ion is typically identified by the brown ring test in A-level work, but at IGCSE the common method is to add aqueous sodium hydroxide and aluminium powder (or Devarda’s alloy), then warm the mixture. Ammonia gas is released if nitrate is present.

硝酸根在A-level中通常通过棕色环实验鉴定，但IGCSE常用方法是加入氢氧化钠水溶液和铝粉（或Devarda合金），然后微热混合物。若存在硝酸根，则会释放出氨气。

NO₃⁻ + 4Al + 7OH⁻ + H₂O → NH₃↑ + 4AlO₂⁻

The ammonia gas is confirmed by its pungent smell and by turning damp red litmus paper blue.

通过刺激性气味和使湿润的红色石蕊试纸变蓝来确认氨气。

10. Testing Gases | 气体测试

Several gases commonly appear in qualitative analysis questions. A summary of their tests is essential for quick revision.

几种常见气体经常出现在定性分析题目中。掌握其测试方法汇总对快速复习至关重要。

Gas	Test and observation
CO₂	Turns limewater milky
NH₃	Turns damp red litmus blue; pungent smell
H₂	Squeaky pop with a lighted splint
O₂	Relights a glowing splint
Cl₂	Bleaches damp litmus paper
SO₂	Turns aqueous potassium dichromate(VI) from orange to green

In the exam, always state both the reagent and the expected observation for full marks.

在考试中，务必同时说明试剂和预期现象，才能得满分。

11. Systematic Approach to Unknown Solutions | 未知溶液的系统分析方法

When analysing an unknown compound, perform tests in a logical sequence to minimise ambiguity. First test for gases by adding dilute acid, then carry out anion tests, followed by cation tests using NaOH and NH₃, and finally flame tests if required.

分析未知化合物时，应按逻辑顺序进行测试以尽量减少歧义。首先加入稀酸检测气体，然后进行阴离子测试，随后用NaOH和NH₃进行阳离子测试，最后根据需要做焰色反应。

Step 1: Observe physical appearance – colour, crystal shape, odour.

第一步：观察物理外观 – 颜色、晶体形状、气味。
Step 2: Add dilute HCl – note effervescence (CO₃²⁻, SO₃²⁻) and colour of gas.

第二步：加入稀盐酸 – 注意气泡（CO₃²⁻、SO₃²⁻）和气体颜色。
Step 3: Perform barium chloride test for sulfate, silver nitrate test for halides.

第三步：用氯化钡测试硫酸根，用硝酸银测试卤离子。
Step 4: Add NaOH and NH₃ to separate samples to identify cations via precipitates.

第四步：分别向不同样品中加入NaOH和NH₃，通过沉淀鉴定阳离子。

This logical order helps you build conclusive evidence while reducing interference between tests.

这一逻辑顺序有助于你构建确凿的证据，同时减少各测试之间的干扰。

12. Common Pitfalls and Exam Tips | 常见易错点与考试技巧

Students often lose marks by drawing conclusions too quickly or by recording incomplete observations. Always note the colour of the precipitate before and after adding excess reagent, and state whether it dissolves.

学生常因过早下结论或记录不完整的观察而失分。务必记录沉淀在加入过量试剂前后的颜色，并说明其是否溶解。

Use dilute nitric acid, not sulfuric acid, when testing halides – sulfate ions would interfere.

测试卤离子时使用稀硝酸而非稀硫酸 – 硫酸根会干扰结果。
Do not use hydrochloric acid when testing for sulfates – it is fine, but use only a small amount; alternatively, use dilute nitric acid for acidification.

测试硫酸根时可以用稀盐酸酸化，但只需少量；也可以用稀硝酸酸化。
Al³⁺ and Zn²⁺ both give white precipitates with NaOH; excess NaOH dissolves both. Use NH₃ in excess to differentiate: Zn²⁺ dissolves, Al³⁺ does not.

Al³⁺和Zn²⁺与NaOH都生成白色沉淀；过量NaOH使二者都溶解。用过量的NH₃可区分：Zn²⁺溶解，Al³⁺不溶解。
Fe²⁺ solutions are pale green; Fe³⁺ solutions are yellow-brown. Always mention solution colour where relevant.

Fe²⁺溶液呈浅绿色；Fe³⁺溶液呈黄棕色。在相关处务必提及溶液颜色。

Conclusion | 结语

Mastering qualitative analysis requires memorisation of key observations and the logic to apply them to unknown samples. Use plain-language flashcards and practise with past-paper questions to build speed and accuracy. The table summaries above are designed to be your quick-reference companion during revision.

掌握定性分析需要熟记关键现象以及将其应用于未知样品的逻辑。使用简明语言制作闪卡并练习过往真题，以提升速度和准确性。以上表格汇总可作为你复习阶段的快速参考手册。

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Notes for Use in Qualitative Analysis | 定性分析使用笔记