📚 Identify the Unknown Compound | 识别未知化合物
In AQA A-level Chemistry, "identify" questions ask you to use experimental evidence — colour changes, precipitates, gas evolution, spectra — to name an unknown ion, functional group, or compound. This guide breaks down the identification toolkit, from classic wet tests to modern spectroscopy, so you can answer Paper 2 and Paper 3 structured questions with confidence.
在 AQA A-level 化学中，“identify”类题目要求你根据实验证据——颜色变化、沉淀生成、气体放出、谱图数据——判断未知离子、官能团或化合物。本指南将系统整理识别工具包，从经典湿法检验到现代波谱方法，帮助你在 Paper 2 和 Paper 3 的结构题中稳稳拿分。
1. What Does "Identify" Mean in AQA Chemistry? | AQA 化学中“Identify”的含义
The command word "identify" means you should use given data to name a species or structure. It is less demanding than "explain" or "justify": you do not need a full mechanism or extended reasoning, but you must select the correct chemical from the evidence. For example, an IR spectrum with a sharp peak at about 1700 cm⁻¹ allows you to identify a carbonyl group, and a silver mirror in Tollens' test identifies an aldehyde.
指令词“identify”表示你要根据给出的数据确定某一种粒子或结构的名称。它比“explain”或“justify”要求更直接：你不需要写出完整机理或长篇论证，但必须从证据中选出正确的化学物质。例如，红外光谱在约 1700 cm⁻¹ 有尖锐吸收峰，可识别羰基；Tollens 试验出现银镜，可识别醛。
Common evidence types in AQA questions include precipitation colours, flame colours, gas tests, colour changes with oxidising agents, infrared absorption frequencies, and mass-to-charge ratios. Examiners expect you to link the evidence to a named particle or functional group, not to describe the procedure in excessive detail.
AQA 题目中常见的证据类型包括沉淀颜色、火焰颜色、气体检验、与氧化剂作用时的颜色变化、红外吸收频率以及质荷比。考官希望你把这些证据与具体的粒子或官能团联系起来，而不是过度描述操作步骤。
2. Step 1: Use Physical Data and Appearance | 第一步：利用物理数据与外观
Before destructive tests, read the question for state, colour, solubility, and any simple test results. Solid ionic compounds are often white; transition metal compounds may be coloured. Organic unknowns may be described as colourless liquids with a characteristic smell. These observations narrow the list of candidates quickly.
在进行破坏性检验之前，先阅读题干中给出的状态、颜色、溶解性以及简单的测试结果。离子化合物固体往往为白色；过渡金属化合物可能有颜色。有机未知物常被描述为无色液体并具有特征气味。这些观察能迅速缩小待选范围。
For example, an aqueous solution containing Cu²⁺ is blue, Fe²⁺ is pale green, Fe³⁺ is yellow-brown, and MnO₄⁻ is purple. A colourless gas that turns limewater milky is likely CO₂. A white solid that dissolves in water to give a neutral solution is unlikely to be a transition metal compound.
例如，含 Cu²⁺ 的水溶液呈蓝色，Fe²⁺ 呈浅绿色，Fe³⁺ 呈黄棕色，MnO₄⁻ 呈紫色。能使石灰水变浑浊的无色气体很可能是 CO₂。一种白色固体溶于水后形成中性溶液，就不太可能是过渡金属化合物。
3. Step 2: Flame Tests for Metal Cations | 第二步：金属阳离子的焰色试验
Flame tests are quick and decisive for Group 1 and Group 2 metals. Clean a nichrome wire in concentrated HCl, dip it into the sample, and place it in a roaring Bunsen flame. The characteristic colours are: lithium gives a crimson red flame; sodium gives a strong yellow flame; potassium gives a
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