📚 Core Principles of CAIE IGCSE Chemistry 0620 Alternative to Practical | CAIE IGCSE化学0620替代实践考试核心原理
The Alternative to Practical paper (Paper 6) in CAIE IGCSE Chemistry 0620 assesses your understanding of experimental techniques, data handling, and scientific reasoning without requiring you to perform a physical laboratory experiment. You must be familiar with common apparatus, measurement accuracy, safety precautions, and methods of analysis. This article covers the core principles you need to master, including titrations, gas collection, ion tests, separation methods, and how to plan and evaluate an investigation.
CAIE IGCSE化学0620的替代实践考试（卷6）考查你对实验技术、数据处理和科学推理的理解，而不需要你实际操作实验。你必须熟悉常用仪器、测量精度、安全措施和分析方法。本文涵盖你必须掌握的核心原理，包括滴定、气体收集、离子检验、分离方法以及如何设计和评价实验。
1. Understanding the Alternative to Practical Paper | 理解替代实践考试
The paper is worth 40 marks and lasts 1 hour. Questions are based on experimental contexts that you have studied in theory and practical lessons. You may be asked to read instruments, complete tables, identify sources of error, suggest improvements, or design a simple investigation. A clear understanding of standard laboratory procedures and the ability to interpret data are essential.
该卷满分40分，时长1小时。题目基于你在理论和实验课上学过的实验情境。你可能需要读取仪器读数、完成表格、指出误差来源、提出改进建议或设计简单实验。清晰地理解标准实验程序以及解读数据的能力至关重要。
2. Common Laboratory Apparatus and Their Uses | 常用实验仪器及其用途
You need to recognise and know the function of standard glassware and equipment. A beaker is used for holding, mixing, and heating liquids; it is not for accurate volume measurement. A conical flask is suitable for titrations and containing reaction mixtures to avoid splashing. A measuring cylinder provides approximate volumes, typically to ±0.5 cm³, while a volumetric pipette delivers a fixed accurate volume and a burette is used for dispensing variable volumes to the nearest 0.05 cm³.
你需要识别并了解标准玻璃器皿和设备的功能。烧杯用于盛装、混合和加热液体，但不用于精确体积测量。锥形瓶适用于滴定和盛装反应混合物以防溅出。量筒提供大致体积，精度通常为±0.5 cm³，而移液管可精确移取固定体积，滴定管用于以0.05 cm³的精度加入可变体积。
Other important apparatus: a thermometer (alcohol or mercury, usually -10 °C to 110 °C, read to 0.5 °C), a Bunsen burner (adjustable air hole for luminous or non-luminous flame), a tripod and wire gauze for supporting beakers during heating, a filter funnel for filtration, and a desiccator for keeping substances dry.
其他重要仪器：温度计（酒精或水银，通常量程-10 °C至110 °C，精确到0.5 °C）、本生灯（通过调节气孔获得黄焰或蓝焰）、三脚架和铁网用于加热时支撑烧杯、漏斗用于过滤、干燥器用于保持物质干燥。
3. Measuring Mass and Volume | 测量质量和体积
Mass is measured using an electronic balance, usually to two decimal places (0.01 g). The reading should be taken when the display is stable. When measuring volume of a liquid, the bottom of the meniscus must be at eye level to avoid parallax errors. For coloured liquids where the meniscus is not visible, the top of the liquid surface is read.
质量使用电子天平测量，通常精确至小数点后两位（0.01 g）。读数应在显示稳定后读取。测量液体体积时，必须在视线水平处读取弯月面底部以避免视差。对于弯月面不可见的有色液体，则读取液体表面顶部。
In titration work, the burette reading is also taken at the bottom of the meniscus. Always record initial and final burette readings and calculate the difference as the titre volume. Repeat titrations until two concordant results (within 0.1 cm³) are obtained.
在滴定操作中，滴定管的液面也应在弯月面底部读取。始终记录初始和最终滴定管读数，并计算差值作为滴定体积。重复滴定直至获得两个一致的结果（相差不超过0.1 cm³）。
4. Methods of Gas Collection | 气体收集方法
The choice of method depends on the density and solubility of the gas. Upward delivery (downward displacement of air) is used for gases denser than air and soluble in water, e.g. chlorine (Cl₂) and hydrogen chloride (HCl). The gas jar is kept upright with the delivery tube reaching the bottom.
收集方法的选择取决于气体的密度和溶解度。向上排空气法（向下排开空气）用于密度大于空气且可溶于水的气体，例如氯气（Cl₂）和氯化氢（HCl）。集气瓶正立放置，导管伸至瓶底。
Downward delivery (upward displacement of air) is for gases less dense than air and soluble in water, e.g. ammonia (NH₃). The inverted gas jar collects the upward-moving gas. Water displacement (over water) is used for gases that are neither very soluble in water nor react with it, such as hydrogen (H₂), oxygen (O₂), and carbon dioxide (CO₂). The gas is bubbled into an inverted water-filled measuring cylinder or gas jar.
向下排空气法（向上排开空气）用于密度小于空气且可溶于水的气体，例如氨气（NH₃）。倒置的集气瓶收集向上运动的气体。排水集气法用于既不溶于水也不与水反应的气体，例如氢气（H₂）、氧气（O₂）和二氧化碳（CO₂）。气体通入装满水并倒置的量筒或集气瓶中。
5. Titration Techniques | 滴定技术
Acid–base titrations are a core skill. A known volume of the alkali (or acid) is measured with a pipette and transferred to a conical flask. A few drops of a suitable indicator, such as methyl orange or phenolphthalein, are added. The reagent in the burette is added slowly while swirling the flask until the end point (the colour change) is reached exactly.
酸碱滴定是核心技能。用移液管量取已知体积的碱（或酸）并转移到锥形瓶。加入几滴合适的指示剂，例如甲基橙或酚酞。不断摇动锥形瓶，从滴定管中缓慢加入试剂，直到恰好到达终点（颜色改变）。
Common indicator colour changes: methyl orange changes from yellow in alkali to orange in neutral to red in acid; phenolphthalein changes from pink in alkali to colourless in acid. Record the burette reading at the end point. Repeat the titration to get consistent results and average the concordant titre values.
常见指示剂颜色变化：甲基橙在碱中呈黄色，中性为橙色，酸中呈红色；酚酞在碱中粉红色，酸中无色。在终点时记录滴定管读数。重复滴定以获得一致的结果，取一致滴定值的平均值。
Errors such as overshooting the end point, failing to remove the funnel from the burette, or not rinsing the pipette/burette with the solution being used can affect accuracy. Rinse pipette with the solution it will contain and burette with the solution it will deliver.
诸如终点滴过、未从滴定管上取下漏斗、或用实验溶液润洗移液管/滴定管不当等误差均会影响准确度。移液管需用将要移取的溶液润洗，滴定管需用即将加入的溶液润洗。
6. Separation Techniques | 分离技术
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel; the residue stays on the paper and the filtrate passes through. Evaporation is used to obtain a soluble solid from a solution by heating until the solvent evaporates; care must be taken to avoid spitting. Crystallisation yields larger, purer crystals by gently heating to concentrate the solution and then allowing it to cool slowly.
过滤用于分离不溶性固体和液体。将混合物通过漏斗中的滤纸过滤；滤渣留在纸上，滤液通过。蒸发用于从溶液中获取可溶性固体，加热至溶剂蒸发殆尽；需注意避免液体飞溅。结晶则可获得更大更纯的晶体，通过温和加热浓缩溶液，然后缓慢冷却。
Simple distillation separates a solvent from a solution or mixtures of liquids with widely different boiling points, e.g. pure water from salt water. The liquid boils, vapour enters the condenser, and is cooled back to liquid collected as distillate. Fractional distillation separates miscible liquids with closer boiling points using a fractionating column. Chromatography separates substances based on their solubility and adsorption; the Rf value is calculated as distance moved by substance divided by distance moved by solvent front.
简单蒸馏可分离溶剂与溶液或沸点相差较大的液体混合物，例如从盐水中获得纯水。液体沸腾，蒸气进入冷凝器，冷却后成为液体被收集为馏出液。分馏利用分馏柱分离沸点相近的互溶液体。色谱法根据溶解度和吸附作用分离物质；Rf值由物质移动距离除以溶剂前沿移动距离计算得到。
Rf = distance moved by spot ÷ distance moved by solvent front
7. Flame Tests and Ion Identification | 焰色试验和离子鉴定
Flame tests identify certain metal cations based on their characteristic flame colours. A clean nichrome or platinum wire is dipped in concentrated hydrochloric acid, then into the sample, and placed in the non-luminous Bunsen flame. The observed colours are: Li⁺ red, Na⁺ intense yellow, K⁺ lilac (often viewed through a blue cobalt glass to filter out yellow), Ca²⁺ brick red, Ba²⁺ apple green, and Cu²⁺ blue‑green.
焰色试验根据金属阳离子特有的火焰颜色进行鉴定。用干净的镍铬丝或铂丝蘸取浓盐酸，再蘸取样品，置于本生灯无光焰中。观察到的颜色为：Li⁺ 红色，Na⁺ 强烈的黄色，K⁺ 淡紫色（常通过蓝色钴玻璃观看以滤除黄色），Ca²⁺ 砖红色，Ba²⁺ 苹果绿色，Cu²⁺ 蓝绿色。
Other cation tests: adding aqueous sodium hydroxide yields coloured precipitates, e.g. Cu²⁺ gives a blue precipitate, Fe²⁺ green, Fe³⁺ reddish‑brown. Ammonium ions (NH₄⁺) can be detected by warming with NaOH; the ammonia gas produced turns damp red litmus blue.
其他阳离子检验：加入氢氧化钠溶液会产生有色沉淀，例如Cu²⁺ 生成蓝色沉淀，Fe²⁺ 绿色，Fe³⁺ 红棕色。铵离子(NH₄⁺)可通过与NaOH加热鉴别；产生的氨气使湿润的红色石蕊试纸变蓝。
8. Testing for Gases and Anions | 气体和阴离子的检验
You must know standard gas tests. Oxygen relights a glowing splint. Hydrogen gives a ‘pop’ with a lighted splint. Carbon dioxide turns limewater cloudy (milky). Chlorine bleaches damp litmus or universal indicator paper (first red then white). Ammonia turns damp red litmus blue. Sulfur dioxide turns acidified potassium dichromate(VI) paper from orange to green.
你必须掌握标准气体检验方法。氧气使带火星的木条复燃。氢气遇点燃的木条发出‘噗’的爆鸣声。二氧化碳使石灰水变浑浊。氯气使湿润的石蕊试纸或通用指示剂试纸褪色（先红后白）。氨气使湿润的红色石蕊试纸变蓝。二氧化硫使酸化重铬酸钾(VI)试纸由橙色变为绿色。
Anion tests: Chloride ions (Cl⁻) are tested by adding dilute nitric acid then silver nitrate solution; a white precipitate forms. Sulfate ions (SO₄²⁻) are identified by adding dilute hydrochloric acid then barium chloride solution; a white precipitate appears. Carbonate ions (CO₃²⁻) react with dilute acid to produce carbon dioxide gas, which can be bubbled through limewater. Nitrate ions can be tested using the brown ring test (with iron(II) sulfate and concentrated sulfuric acid) but often a simpler approach with aluminium foil and sodium hydroxide releasing ammonia is used.
阴离子检验：氯离子(Cl⁻)通过先加入稀硝酸再加入硝酸银溶液鉴定，产生白色沉淀。硫酸根离子(SO₄²⁻)通过加入稀盐酸然后氯化钡溶液鉴定，产生白色沉淀。碳酸根离子(CO₃²⁻)与稀酸反应生成二氧化碳，可将气体通入石灰水检验。硝酸根离子可用棕色环实验（硫酸亚铁和浓硫酸）测试，但常采用铝箔和氢氧化钠加热释放氨气的简便方法。
9. Planning and Evaluating Experiments | 实验设计与评价
In the planning section, you must outline a clear, logical procedure, list apparatus, control variables, state how to record results, and mention safety precautions. A fair test is designed by changing only the independent variable and keeping all other factors constant. Reproducibility is improved by repeating measurements and averaging.
在实验设计题中，你必须写出清晰、逻辑的步骤，列出仪器，确定控制变量，说明如何记录结果，并提及安全措施。通过只改变自变量并保持其他因素不变来设计公平测试。重复测量并取平均值可提高重复性。
When evaluating, identify sources of error, such as heat loss in calorimetry, incomplete reaction, or loss of precipitate during filtration. Suggest practical improvements like using a lid, insulating the apparatus, or washing the precipitate with distilled water. Recognise anomalous results and explain their possible causes.
评价时，要指出误差来源，例如量热中的热量损失、反应不完全或过滤时沉淀损失。提出切实可行的改进措施，如加盖盖子、给装置保温或用蒸馏水洗涤沉淀。识别异常结果并解释其可能的原因。
10. Data Recording and Graph Plotting | 数据记录与作图
Draw neat tables with headings that include units, e.g. ‘Volume of acid added / cm³’ and ‘Temperature / °C’. The independent variable is usually placed in the left column. Record values to the appropriate number of decimal places consistent with the instrument’s precision.
绘制整洁的表格，表头需包含单位，如‘加入酸的体积 / cm³’和‘温度 / °C’。自变量通常放在左栏。记录数值的小数位数应与仪器精度相匹配。
When plotting a graph, label the axes with the variable and unit, use sensible linear scales (avoid awkward divisions like 3, 7, etc.), plot points with small crosses or dots with circles, and draw a best‑fit straight line or a smooth curve. A line of best fit should have roughly equal numbers of points above and below the line. The slope can be calculated using a large triangle on the straight‑line graph.
作图时，需标出变量名称和单位，使用合理的线性比例（避免如3、7等不便的刻度），用叉号或加点的小圆圈标出数据点，并画出最佳拟合直线或平滑曲线。最佳拟合线应使位于线上方和下方的点数大致相等。可通过在直线图上作一大三角形来计算斜率。
slope = change in y ÷ change in x
If the graph is a curve through zero, it may indicate direct proportionality. If the curve does not pass through the origin, there may be a systematic error. Data interpolation (reading between points) and extrapolation (extending the line beyond points) can be required.
如果图像是过原点的曲线，可能表明直接比例关系。如果曲线不过原点，可能存在系统误差。有时会要求进行数据内插（在数据点间读取）和外推（将线延伸至数据点之外）。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导