IGCSE Chemistry Practical Assessment: A Complete Guide | IGCSE 化学实践评估完全指南

📚 IGCSE Chemistry Practical Assessment: A Complete Guide | IGCSE 化学实践评估完全指南

The IGCSE Chemistry practical assessment is a core component of the Cambridge IGCSE Chemistry (0620) syllabus, designed to test your hands-on laboratory skills, observational accuracy, and analytical thinking. Whether you sit for Paper 5 (Practical Test) or Paper 6 (Alternative to Practical), mastering the techniques covered in this guide is essential for achieving a top grade.

IGCSE 化学实践评估是剑桥 IGCSE 化学（0620）教学大纲的核心组成部分，旨在考查你的动手实验能力、观察准确性以及分析思维能力。无论你参加 Paper 5（实验操作考试）还是 Paper 6（实验笔试替代卷），掌握本指南中所覆盖的技巧都是取得高分的关键。

1. Understanding the Assessment Format | 了解评估形式

The practical assessment is worth 20% of the total IGCSE Chemistry grade. Paper 5 is a school-based practical test where you perform real experiments, while Paper 6 is a written alternative that presents experiments in scenarios and asks you to plan, analyse, and evaluate. Both papers are timed at approximately 1 hour 15 minutes and carry 40 marks each. You should familiarise yourself with the style of questions from past papers, as they consistently range across observation tasks, data recording, graph plotting, and conclusion drawing.

实践评估占 IGCSE 化学总成绩的 20%。Paper 5 是校内实验操作考试，需要你亲手完成真实实验；Paper 6 是实验笔试替代卷，以情景方式呈现实验过程，要求你进行计划、分析与评估。两卷时长均为 1 小时 15 分钟左右，满分 40 分。建议你通过历年真题熟悉题型风格，因为题目始终围绕观察记录、数据整理、作图以及结论推导这几大板块。

Key marks are distributed as follows:

Observation and recording of results — approximately 12–15 marks

观察与结果记录——约 12–15 分
Graph plotting and data interpretation — approximately 10–12 marks

作图与数据解读——约 10–12 分
Planning, evaluation and error analysis — approximately 8–10 marks

计划、评估与误差分析——约 8–10 分
Safety and apparatus handling — approximately 4–6 marks

安全规范与仪器操作——约 4–6 分

2. Essential Apparatus and Measurement | 基本仪器与精确测量

Accurate measurement is the backbone of any chemistry practical. You must know the correct instrument for each task and its associated uncertainty. For volume, the burette measures to the nearest 0.05 cm³ and is used in titrations; the pipette delivers a fixed volume such as 25.0 cm³; the measuring cylinder is less precise, with an uncertainty of ±0.5 cm³, making it unsuitable for titrations. For mass, an electronic balance typically measures to 0.01 g, and you should always record readings to the correct number of decimal places.

精确测量是任何化学实验的基础。你必须了解每项任务应使用何种仪器及其相应误差。体积测量方面，滴定管精确到 0.05 cm³，用于滴定实验；移液管可准确量取固定体积（如 25.0 cm³）；量筒精确度较低，误差为 ±0.5 cm³，不适用于滴定。质量测量方面，电子天平通常精确到 0.01 g，记录读数时务必保留正确的小数位数。

A quick reference table for common apparatus:

Apparatus 仪器	Use 用途	Uncertainty 误差
Burette 滴定管	Accurate dispensing of liquid 精确滴加液体	±0.05 cm³
Pipette 移液管	Fixed volume transfer 固定体积转移	±0.06 cm³
Measuring cylinder 量筒	Approximate volume 粗略体积测量	±0.5 cm³
Balance 天平	Mass measurement 质量测量	±0.01 g
Thermometer 温度计	Temperature reading 温度读数	±0.5 °C

3. Qualitative Analysis: Testing for Cations | 定性分析：阳离子检测

Qualitative analysis requires you to identify unknown ions by observing colour changes and precipitate formation. The flame test is a classic method: lithium produces a crimson flame, sodium a bright yellow flame, potassium a lilac flame, calcium a brick-red flame, and copper(II) a blue-green flame. You should dip a clean nichrome wire loop into concentrated hydrochloric acid, then into the salt sample, and hold it in a non-luminous Bunsen burner flame.

定性分析要求你通过观察颜色变化和沉淀生成来鉴定未知离子。焰色反应是经典方法：锂产生深红色火焰，钠产生亮黄色火焰，钾产生淡紫色火焰，钙产生砖红色火焰，铜(II)产生蓝绿色火焰。操作时，将干净的镍铬丝环蘸取浓盐酸，再蘸取盐样品，放入不发光的本生灯火焰中灼烧。

For the sodium hydroxide precipitation test, add aqueous NaOH to the unknown solution:

Cu²⁺ gives a blue precipitate of Cu(OH)₂, insoluble in excess

Cu²⁺ 产生蓝色 Cu(OH)₂ 沉淀，过量 NaOH 不溶解
Fe²⁺ gives a dirty green precipitate of Fe(OH)₂

Fe²⁺ 产生灰绿色 Fe(OH)₂ 沉淀
Fe³⁺ gives a red-brown precipitate of Fe(OH)₃

Fe³⁺ 产生红棕色 Fe(OH)₃ 沉淀
Zn²⁺ gives a white precipitate, which dissolves in excess NaOH to form a colourless solution

Zn²⁺ 产生白色沉淀，过量 NaOH 中溶解形成无色溶液
Al³⁺ gives a white precipitate, also soluble in excess NaOH

Al³⁺ 产生白色沉淀，同样溶于过量 NaOH
Ca²⁺ gives a white precipitate, insoluble in excess NaOH

Ca²⁺ 产生白色沉淀，过量 NaOH 中不溶解

4. Qualitative Analysis: Testing for Anions and Gases | 定性分析：阴离子与气体检测

Testing for anions requires specific reagents that produce characteristic precipitates or colour changes. For carbonate (CO₃²⁻), add dilute hydrochloric acid — effervescence occurs and the gas produced turns limewater milky. For sulfate (SO₄²⁻), add barium chloride solution followed by dilute hydrochloric acid; a white precipitate of BaSO₄ confirms its presence. For halides, add silver nitrate solution acidified with dilute nitric acid: chloride gives a white precipitate, bromide gives a cream precipitate, and iodide gives a pale yellow precipitate.

阴离子检测需要使用特定试剂，产生特征性沉淀或颜色变化。碳酸根（CO₃²⁻）：加入稀盐酸，产生气泡，生成的气体使石灰水变浑浊。硫酸根（SO₄²⁻）：加入氯化钡溶液和稀盐酸，产生白色 BaSO₄ 沉淀即可确认。卤离子：加入用稀硝酸酸化的硝酸银溶液——氯离子产生白色沉淀，溴离子产生淡奶油色沉淀，碘离子产生淡黄色沉淀。

Gas testing is equally vital. Carbon dioxide turns limewater milky; hydrogen burns with a squeaky pop when a lighted splint is inserted; oxygen relights a glowing splint; ammonia turns damp red litmus paper blue; chlorine bleaches damp litmus paper; and sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless. Always state both the observation and the conclusion clearly in your answer.

气体检测同样至关重要。二氧化碳使石灰水变浑浊；氢气用燃着的火柴伸入时发出”扑”的爆鸣声；氧气使带火星的木条复燃；氨气使湿润的红色石蕊试纸变蓝；氯气使湿润的石蕊试纸褪色；二氧化硫使酸化的高锰酸钾溶液由紫色褪为无色。作答时务必同时清楚说明观察现象和对应结论。

5. Quantitative Analysis: Titration Technique | 定量分析：滴定操作

Titration is perhaps the most important quantitative skill in IGCSE chemistry. The procedure is as follows: rinse the burette with the acid solution and the pipette with the alkali solution, then use a pipette filler to transfer exactly 25.0 cm³ of the alkali into a conical flask. Add two or three drops of an appropriate indicator, such as methyl orange or phenolphthalein. Run the acid from the burette into the flask while swirling continuously, until the indicator just changes colour. Record the initial and final burette readings to the nearest 0.05 cm³.

滴定可以说是 IGCSE 化学中最重要的定量技能。操作流程如下：用酸溶液润洗滴定管，用碱溶液润洗移液管；用洗耳球吸取碱液，精确转移 25.0 cm³ 至锥形瓶中；滴加 2–3 滴合适的指示剂，如甲基橙或酚酞。从滴定管中逐滴滴加酸，同时不断旋摇锥形瓶，直至指示剂恰好变色。记录滴定管的初读数和末读数，精确到 0.05 cm³。

Repeat the titration until you obtain two consistent results within 0.10 cm³ of each other. Calculate the mean titre from the concordant results only, ignoring any anomalous readings. The core calculation uses the relationship:

n = c × V ÷ 1000

where n is the number of moles, c is the concentration in mol/dm³, and V is the volume in cm³. Remember to use the mole ratio from the balanced equation to find the unknown concentration.

重复滴定，直至获得两次差值在 0.10 cm³ 以内的平行结果。仅用平行结果计算平均值，忽略异常读数。核心计算使用如下关系式：

n = c × V ÷ 1000

其中 n 为物质的量（摩尔），c 为浓度（mol/dm³），V 为体积（cm³）。务必利用化学方程式的摩尔比来求解未知浓度。

6. Rates of Reaction: Measurement and Calculation | 反应速率：测量与计算

Rates of reaction experiments are frequently examined. Common methods include measuring the volume of gas evolved with time, measuring the loss of mass over time, or measuring the time taken for a precipitate to obscure a cross drawn on paper. The rate can be expressed as the change in concentration, mass, or volume per unit time. For the gas collection method, use a gas syringe or an inverted measuring cylinder over water.

反应速率实验是高频考点。常用方法包括：随时间测量产生的气体体积、测量质量随时间减少的量、或者测量沉淀物遮住纸下十字标志所需的时间。速率可以表示为浓度、质量或体积随单位时间的变化量。收集气体时使用气针筒（气体注射器），或倒扣在水槽中的量筒来收集。

Factors affecting rate include temperature, concentration, surface area, pressure (for gases), and the presence of a catalyst. For example, increasing the temperature by 10 °C roughly doubles the rate of many reactions. When interpreting data, you may be asked to calculate the average rate over an interval:

Average rate = (final volume − initial volume) ÷ (final time − initial time)

You may also be asked to draw a tangent to a curved graph to find the instantaneous rate at a specific time.

影响反应速率的因素包括温度、浓度、表面积、压强（针对气体）以及催化剂的存在。例如，温度每升高 10 °C，许多反应的速率大约翻倍。解读数据时，你可能会被要求计算某一时间区间内的平均速率：

平均速率 =（末体积 − 初体积）÷（末时间 − 初时间）

你还可能需要在曲线图上作切线，以求出某一特定时刻的瞬时速率。

7. Energy Changes: Enthalpy Experiments | 能量变化：焓变实验

Measuring enthalpy changes involves recording the temperature change of a known mass of solution when a reaction occurs. For an exothermic reaction, the temperature rises; for an endothermic reaction, it falls. The key calculation uses the equation:

ΔH = −m × c × ΔT ÷ n

where m is the mass of the solution (in grams, usually approximated as 1 g per cm³ of water), c is the specific heat capacity (4.18 J/g·°C), ΔT is the temperature change, and n is the number of moles of the limiting reactant.

测量焓变需要记录反应过程中已知质量溶液的温度变化。放热反应温度升高，吸热反应温度降低。关键计算使用如下方程：

ΔH = −m × c × ΔT ÷ n

其中 m 为溶液质量（单位克，通常将 1 cm³ 水近似为 1 g），c 为比热容（4.18 J/g·°C），ΔT 为温度变化，n 为限制性反应物的物质的量。

A common experimental setup is the “polystyrene cup” calorimeter, which minimises heat loss due to its insulating properties. When drawing a temperature-time graph, plot the points, draw a best-fit line, and extrapolate backwards to estimate the temperature change at the instant of mixing. This corrects for heat loss to the surroundings.

常见的实验装置是”聚苯乙烯杯”量热计，因其保温性能好而能减少热量散失。绘制温度-时间图时，先描点，再画最佳拟合直线，并向后外推以估算混合瞬间的温度变化。这样可以修正向环境散失的热量。

8. Separation and Purification Techniques | 分离与提纯技术

Separation techniques are examined both theoretically and practically. Filtration separates an insoluble solid from a liquid using filter paper; the solid residue is the precipitate and the liquid that passes through is the filtrate. Crystallisation involves heating a solution to evaporate some solvent, then allowing it to cool so that pure crystals form. Distillation separates a liquid from a solution, or two liquids with different boiling points.

分离技术在理论和实践两方面都会考查。过滤使用滤纸将不溶性固体与液体分离，留在滤纸上的固体称为残渣，滤过的液体称为滤液。结晶法通过加热溶液蒸发部分溶剂，然后冷却使纯净晶体析出。蒸馏用于分离溶液中的液体，或分离沸点不同的两种液体。

Paper chromatography is another key technique. Spot the mixture onto the pencil baseline, place the paper in a solvent, and allow the solvent front to rise. Calculate the Rf value using:

Rf = distance moved by solute ÷ distance moved by solvent

Different substances have different Rf values under the same solvent system, enabling identification. In your answers, always describe the procedure in full sentences with correct terminology, mentioning the pencil baseline, the solvent front, and the precautions such as not allowing the baseline to be submerged in the solvent.

纸色谱法是另一项重要技术。将混合物点在铅笔底线上，将滤纸放入溶剂中，让溶剂前沿上升。用以下公式计算 Rf 值：

Rf = 溶质移动距离 ÷ 溶剂移动距离

在相同溶剂体系中，不同物质的 Rf 值不同，从而可以实现鉴定。作答时务必用完整句子和规范术语描述操作流程，提到铅笔基线、溶剂前沿，以及不要让基线浸入溶剂等注意事项。

9. Errors, Uncertainties and Data Processing | 误差、不确定性与数据处理

Every measurement carries uncertainty, and examiners expect you to identify and quantify it. The percentage error of an apparatus is calculated as:

Percentage error = (absolute uncertainty ÷ measured value) × 100%

For example, if a burette reading has an uncertainty of ±0.05 cm³ and you measure 25.0 cm³, the percentage error is 0.2%. Systematic errors affect accuracy (how close your result is to the true value), while random errors affect precision (how reproducible your results are).

每项测量都存在不确定性，考官期望你能够识别并量化它。仪器误差百分比如下计算：

误差百分比 =（绝对不确定度 ÷ 测量值）× 100%

例如，滴定管读数不确定度为 ±0.05 cm³，测量体积为 25.0 cm³，则误差百分比为 0.2%。系统误差影响准确度（结果与真实值的接近程度），随机误差影响精密度（结果的可重复性）。

When plotting graphs, use a sharp pencil, choose a suitable scale (usually at least half of the graph grid in each direction), label both axes with quantity and unit, plot points as small crosses, and draw a best-fit straight line or smooth curve. Calculate the gradient by choosing two widely separated points on the line, using the formula:

Gradient = (y₂ − y₁) ÷ (x₂ − x₁)

绘图中，使用削尖的铅笔，选择适当比例（通常各方向至少占用坐标纸一半以上），标注两轴的量与单位，用小叉描点，并画出最佳拟合直线或平滑曲线。计算斜率时，在线上选择两个相隔较远的点，使用公式：

斜率 =（y₂ − y₁）÷（x₂ − x₁）

10. Safety in the Chemistry Laboratory | 化学实验室安全规范

Safety is a fundamental component of the practical assessment. You must wear safety goggles at all times, especially when heating or handling corrosive substances. Dilute acids and alkalis are corrosive; if spilled, rinse with plenty of cold water and inform the teacher. Flammable substances, such as ethanol, must be kept away from naked flames. When heating liquids in a test tube, point the mouth away from yourself and others, and use a test-tube holder. Never add water to concentrated acid — always add acid to water slowly while stirring.

安全是实践评估的基本组成部分。你应始终佩戴护目镜，尤其在加热或接触腐蚀性物质时。稀酸和稀碱具有腐蚀性，如溅到皮肤上应立即用大量冷水冲洗并告知老师。乙醇等易燃物质必须远离明火。在试管中加热液体时，试管口不可朝向自己或他人，并应使用试管夹。切勿向浓酸中加水——必须将酸缓慢加入水中并搅拌。

You should also be able to identify common hazard symbols: the corrosive symbol indicates skin burns, the flammable symbol indicates fire risk, the toxic symbol indicates danger upon inhalation or ingestion, and the irritant/harmful symbol indicates lower-level health risks. In planning questions, always state the hazards and the corresponding safety precautions you would take.

你还应能够识别常见危险品标志：腐蚀性标志表示灼伤皮肤，易燃标志表示火灾风险，有毒标志表示吸入或食入有危险，刺激性/有害标志表示较低级别的健康风险。在有计划性题目中，务必同时说明危害以及你将要采取的相应安全预防措施。

11. Planning Experiments: Design and Evaluation | 实验计划：设计与评价

Planning questions require you to design a complete experimental method. A good plan includes: the independent variable (what you change), the dependent variable (what you measure), and the control variables (what you keep constant). You must specify the apparatus, the quantities of reagents, the step-by-step procedure, and the safety precautions. For example, to investigate the effect of concentration on reaction rate, you would change the concentration of one reactant, keep the temperature and total volume constant, and measure the time for a fixed volume of gas to evolve.

计划题要求你设计完整的实验方案。一份好的计划包括：自变量（你要改变的）、因变量（你要测量的）以及控制变量（你要保持不变的）。你必须说明仪器、试剂用量、分步操作流程和安全注意事项。例如，研究浓度对反应速率的影响时，应改变一种反应物的浓度，保持温度和总体积不变，并测量产生固定体积气体所需的时间。

Evaluation involves identifying limitations of your method and suggesting improvements. Common issues include heat loss in enthalpy experiments, difficulty in seeing the end point of a titration, and gas leakage in rate experiments. For each limitation, state a specific improvement: use a lid on the polystyrene cup, use a more sensitive indicator or a pH meter, and check all connections for airtightness.

评价部分需要识别方法的局限性并提出改进建议。常见问题包括焓变实验中的热量散失、滴定终点难以判断，以及速率实验中气体泄漏等。针对每个局限性，要给出具体改进方案：为聚苯乙烯杯加杯盖、使用更灵敏的指示剂或 pH 计、检查所有连接处的气密性。

12. Exam Technique: Maximising Your Marks | 考试技巧：最大化得分

Specific exam techniques can significantly boost your marks. Read every question carefully and underline key words such as “state”, “describe”, “explain”, or “suggest”. “State” requires a brief factual answer, “describe” requires a detailed account of what you see or do, “explain” requires a reason using chemical concepts, and “suggest” requires you to propose a scientifically plausible idea. Use the marks allocated as a guide to how much detail is expected — a 2-mark question needs at least two distinct points.

特定的考试技巧可以显著提高你的得分。细心阅读每题并在关键词下划线，如”指明”、”描述”、”解释”或”建议”。”指明”需要简明事实性回答，”描述”需要详细说明所见或所做的事，”解释”需要用化学概念说明原因，”建议”则需提出科学上合理的设想。以题目分值作为详细程度的参考——2 分题至少需要两个独立的要点。

Always quote units in your final answers, record readings to the appropriate precision, and show your working in calculations. If you are asked to draw a graph, do not use a dot-to-dot line unless specifically instructed; a best-fit straight line or smooth curve is expected. Finally, review your answers to ensure every observation is paired with a correct conclusion and that your evaluation points are specific rather than generic.

最终答案中务必标注单位，读数记录保持适当精确度，计算题展示完整步骤。如需作图，除非特别说明，不要连点成线；应画出最佳拟合直线或平滑曲线。最后，检查所有答案，确保每项观察现象都与正确结论配对，且评价要点具体明确而非泛泛而谈。

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IGCSE Chemistry Practical Assessment: A Complete Guide | IGCSE 化学实践评估完全指南