📚 Using Reactions to Identify Chemicals | 利用化学反应识别化学品
In KS3 chemistry, you learn that substances can be identified not only by their appearance but also by how they behave in chemical reactions. A reaction may produce a gas, a colour change, a precipitate, or a characteristic flame colour. These observations act like fingerprints, helping chemists work out what an unknown chemical is. This article introduces the most common tests used at Key Stage 3, including gas tests, indicator tests, acid-carbonate fizz tests, precipitation reactions, and simple flame tests.
在 KS3 化学中，你会学到识别物质不仅可以看外观，还可以看它们在化学反应中的表现。反应可能产生气体、颜色变化、沉淀或特征的火焰颜色。这些现象就像指纹一样，帮助化学家判断未知化学品是什么。本文介绍 KS3 最常用的测试，包括气体测试、指示剂测试、酸与碳酸盐的气泡测试、沉淀反应和简单的焰色试验。
1. Why Use Chemical Reactions to Identify Substances? | 为什么利用化学反应识别物质
Chemists often need to identify an unknown substance in a mixture or a sample. Physical properties such as colour, state, and solubility can give clues, but many substances look alike. Chemical reactions provide stronger evidence because a pure substance reacts in a predictable way with specific reagents. For example, only a carbonate fizzes with dilute acid, and only carbon dioxide turns limewater cloudy.
化学家经常需要识别混合物或样品中的未知物质。颜色、状态和溶解性等物理性质可以提供线索，但许多物质看起来相似。化学反应能提供更有力的证据，因为纯物质与特定试剂会以可预测的方式反应。例如，只有碳酸盐遇稀酸会产生气泡，只有二氧化碳会使石灰水变浑浊。
A positive test result is an observation that is unique enough to confirm the presence of a particular chemical. In the laboratory, you must compare the unknown result with a known standard if possible.
阳性测试结果是一种足够独特的现象，可以确认某种化学物质的存在。在实验室中，如果可能，应将未知结果与已知标准进行对比。
2. Safety First: Handling Unknown Chemicals | 安全第一：处理未知化学品
Before testing any unknown chemical, always wear safety goggles and follow your teacher’s instructions. Never taste a chemical, and do not inhale fumes directly. Use only a small amount of sample, and hold test tubes pointing away from yourself and others. If a gas is produced, test it in a well-ventilated area or a fume cupboard when needed.
在测试任何未知化学品之前，必须始终佩戴护目镜并听从老师的指导。绝不能品尝化学品，也不要直接吸入气体。只用少量样品，并将试管口远离自己和他人。如果产生气体，应在通风良好的地方测试，必要时在通风橱中进行。
Remember that some gases, such as chlorine, are toxic, and some reactions can be exothermic. Always add acid to water slowly and never pour water into concentrated acid.
请记住，有些气体（如氯气）有毒，有些反应会放热。始终将酸缓慢加入水中，切勿将水倒入浓酸中。
3. Testing for Hydrogen Gas | 检测氢气
One of the most famous gas tests is the hydrogen test. Hydrogen is a colourless, odourless and highly flammable gas. To test for hydrogen, hold a lit splint at the mouth of the test tube. If hydrogen is present, it burns rapidly with a distinctive ‘squeaky pop’ sound.
最著名的气体测试之一是氢气测试。氢气是一种无色、无味且极易燃烧的气体。要检测氢气，将点燃的木条放在试管口。如果存在氢气，它会迅速燃烧，发出独特的“尖锐爆鸣声”。
Zn + 2HCl → ZnCl₂ + H₂↑
This reaction between zinc and hydrochloric acid produces hydrogen gas, which can be collected and tested. The pop happens because hydrogen gas ignites and reacts explosively with oxygen in the air.
锌与盐酸反应生成氢气，可以收集并测试。爆鸣声是因为氢气被点燃并与空气中的氧气迅速反应。
4. Testing for Oxygen Gas | 检测氧气
Oxygen supports combustion, so the test is simple. Light a wooden splint, blow it out so that only a glowing tip remains, and place the glowing splint into the gas sample. If oxygen is present, the splint relights immediately.
氧气支持燃烧，因此测试很简单。点燃一根木条，吹灭火焰留下带火星的尖端，然后将带火星的木条伸入气体样品中。如果存在氧气，木条会立即复燃。
2H₂O₂ → 2H₂O + O₂↑
Hydrogen peroxide decomposes slowly at room temperature, but the reaction is speeded up by a catalyst such as manganese dioxide. The gas produced relights a glowing splint, confirming oxygen.
过氧化氢在室温下分解缓慢，但加入催化剂如二氧化锰会加速反应。生成的气体能使带火星的木条复燃，从而确认是氧气。
5. Testing for Carbon Dioxide | 检测二氧化碳
Carbon dioxide is tested using limewater, which is a dilute solution of calcium hydroxide. Bubble the unknown gas through limewater, or shake limewater with the gas. If the limewater turns cloudy or milky white, carbon dioxide is present.
二氧化碳用石灰水检测，石灰水是氢氧化钙的稀溶液。将未知气体通入石灰水中，或将石灰水与气体一起振荡。如果石灰水变浑浊或呈乳白色，则存在二氧化碳。
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O
The cloudiness is caused by tiny solid particles of calcium carbonate formed in the reaction. This test can also be used to confirm that a gas from an acid-carbonate reaction is carbon dioxide.
浑浊是由反应中生成的微小碳酸钙固体颗粒引起的。该测试也可用于确认酸与碳酸盐反应产生的气体是二氧化碳。
6. Testing for Chlorine Gas | 检测氯气
Chlorine is a toxic greenish-yellow gas with a strong smell. You should only test chlorine if your teacher has shown you the safe method. Damp blue litmus paper is held near the gas. Chlorine first turns the litmus paper red because it forms an acid, then quickly bleaches it white.
氯气是一种有毒的黄绿色气体，有强烈气味。只有在老师示范安全方法后才能测试氯气。将湿润的蓝色石蕊试纸靠近气体。氯气先使石蕊试纸变红，因为它生成酸，然后迅速将其漂白为白色。
The bleaching effect is a key difference from other acidic gases, which turn litmus red but do not whiten it. Chlorine is used in small amounts to kill bacteria in water.
漂白作用是与其他酸性气体的关键区别，其他酸性气体使石蕊变红但不会使其变白。氯气少量用于杀灭水中的细菌。
7. Using Litmus and Universal Indicator | 使用石蕊和通用指示剂
Indicators are chemicals that change colour in acids and alkalis. Blue litmus paper turns red in acids, and red litmus paper turns blue in alkalis. Universal indicator solution or paper gives a range of colours from red for strong acids to purple for strong alkalis, with green for neutral solutions.
指示剂是遇到酸和碱会变色的化学品。蓝色石蕊试纸在酸中变红，红色石蕊试纸在碱中变蓝。通用指示剂溶液或试纸显示一系列颜色，从强酸的红色到强碱的紫色，中性溶液为绿色。
| pH range | pH范围 | Colour | 颜色 | Type of solution | 溶液类型 |
|---|---|---|
| 0-2 | Red | 红色 | Strong acid | 强酸 |
| 3-6 | Orange / yellow | 橙色 / 黄色 | Weak acid | 弱酸 |
| 7 | Green | 绿色 | Neutral | 中性 |
| 8-11 | Blue / dark green | 蓝色 / 深绿色 | Weak alkali | 弱碱 |
| 12-14 | Purple | 紫色 | Strong alkali | 强碱 |
You can use universal indicator to estimate the pH of an unknown solution and decide whether it is acidic, neutral, or alkaline.
你可以用通用指示剂估计未知溶液的 pH，判断它是酸性、中性还是碱性。
8. Acid + Carbonate Reaction: Fizz Test | 酸与碳酸盐反应：气泡测试
Many rocks and minerals contain carbonate ions. To test a solid for carbonate, add a few drops of dilute hydrochloric acid. If the solid fizzes and releases a gas, collect the gas and bubble it through limewater. The limewater turning cloudy confirms the gas is carbon dioxide, so the solid is a carbonate.
许多岩石和矿物含有碳酸根离子。要测试固体是否含碳酸盐，滴加几滴稀盐酸。如果固体冒泡并释放气体，收集该气体并通入石灰水。石灰水变浑浊可确认该气体是二氧化碳，因此该固体是碳酸盐。
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
This fizz test is a good example of using a reaction to identify an unknown anion. It works for many carbonates, including calcium carbonate, sodium carbonate, and copper carbonate.
这个气泡测试是利用反应识别未知阴离子的一个很好例子。它适用于许多碳酸盐，包括碳酸钙、碳酸钠和碳酸铜。
9. Precipitation Reactions: Forming Solids | 沉淀反应：生成固体
A precipitate is an insoluble solid that forms when two solutions are mixed. The colour of the precipitate can help identify the ions present. For example, when silver nitrate solution is added to a solution containing chloride ions, a white precipitate of silver chloride forms.
沉淀是两种溶液混合时生成的不溶性固体。沉淀的颜色可以帮助识别存在的离子。例如，当硝酸银溶液加入含氯离子的溶液中时，会生成白色氯化银沉淀。
AgNO₃(aq) + NaCl(aq) → AgCl↓ + NaNO₃(aq)
Other useful precipitates include blue copper hydroxide formed when sodium hydroxide is added to copper sulfate solution, and white calcium carbonate formed in the limewater test. Precipitation is a quick visual clue, but some ions give similar colours, so confirm with further tests.
其他有用的沉淀包括向硫酸铜溶液中加入氢氧化钠时生成的蓝色氢氧化铜，以及石灰水测试中生成的白色碳酸钙。沉淀是一种快速的视觉线索，但有些离子颜色相似，因此需要进一步测试确认。
10. Flame Tests for Metal Ions (Intro) | 金属离子的焰色试验（入门）
Flame tests can identify certain metal ions. A clean nichrome wire loop is dipped into the sample, then placed in the hot part of a Bunsen flame. Different metal ions give different flame colours because the heat excites their electrons, which release light at characteristic wavelengths.
焰色试验可以识别某些金属离子。将洁净的镍铬丝环浸入样品，然后放在本生灯火焰的高温部分。不同的金属离子会呈现不同的火焰颜色，因为热量激发电子，电子释放特征波长的光。
Lithium gives a red flame, sodium gives a yellow flame, potassium gives a lilac flame, calcium gives a brick-red flame, and copper gives a
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