AS Chemistry Unit 2 (CH02) June 2022 Exam Report: Practical Skills & Common Mistakes | AS化学单元2 2022年6月考情报告：实验操作与常见错误

📚 AS Chemistry Unit 2 (CH02) June 2022 Exam Report: Practical Skills & Common Mistakes | AS化学单元2 2022年6月考情报告：实验操作与常见错误

The June 2022 AS Chemistry Unit 2 (CH02) examination series revealed key patterns in how candidates approach practical-based questions. The examiners’ report highlighted that while many students demonstrated a sound understanding of core chemical principles, marks were frequently lost due to imprecise laboratory technique, incomplete data recording, and a lack of familiarity with standard practical procedures. This article breaks down the most common errors and provides targeted advice to help future candidates avoid the same pitfalls and strengthen their practical evaluation skills.

2022年6月的AS化学单元2（CH02）考试系列揭示了考生在应对实验导向题目时的典型模式。考官报告指出，尽管许多考生展示出对核心化学原理的扎实理解，但常因实验操作细节模糊、数据记录不完整以及对标准实验程序不熟悉而失分。本文逐一分析最常见的错误，并提供针对性建议，帮助未来的考生避开同样的问题，提升实验评价能力。

---

1. Overview of the Practical Component | 实验部分概述

In CH02, practical skills are assessed not through a separate practical exam but via written questions that demand candidates interpret experimental procedures, evaluate results, and identify sources of error. The exam assumes familiarity with techniques such as titration, enthalpy change measurement, gas collection, and preparation of standard solutions. Too many answers were generic, lacking the specificity that comes from hands-on experience.

在CH02中，实验技能并非通过单独的动手考试来评估，而是借助要求考生解释实验操作、评价结果和识别误差来源的笔试题。考试假设考生熟悉滴定、焓变测量、气体收集和标准溶液配制等技术。过多的答案流于泛泛而谈，缺少亲身实践带来的具体性。

---

2. Common Errors in Titration | 滴定实验的常见错误

Titration questions were a significant source of avoidable mistakes. Candidates often failed to describe the correct rinsing procedure for the burette and pipette. Many stated that the burette should be rinsed with water, not with the acid it would contain. The correct sequence is: rinse burette with the solution it will deliver, fill and ensure the jet is free of air bubbles, then take the initial reading with the eye level at the bottom of the meniscus.

滴定题目是许多可以避免的错误的重要来源。考生常常未能正确描述滴定管和移液管的润洗程序。许多人声称滴定管应用水冲洗，而不是用即将盛装的酸液。正确的顺序为：用待装溶液润洗滴定管，装液并确保尖嘴处无气泡，然后以视线与弯月面底部齐平读取初始读数。

• Candidates confused the indicator choice: methyl orange is suitable for strong acid–strong base but not for weak acid–weak base titrations. They also struggled to justify why an indicator is better than a pH meter for a sharp end point.

  考生混淆了指示剂的选择：甲基橙适用于强酸–强碱滴定，但不适用于弱酸–弱碱滴定。他们也很难解释为何在突跃终点时指示剂优于pH计。

• Another recurring weakness was failure to account for the effect of rinsing the conical flask with water. Adding distilled water to the flask does not change the number of moles of the analyte and therefore does not affect the titre, yet many incorrectly claimed it would dilute the acid and alter the result.

  另一个反复出现的问题是未能解释用蒸馏水润洗锥形瓶的影响。向锥形瓶中加入蒸馏水不会改变待测物的物质的量，因此不影响滴定体积，但许多考生错误地声称这样会稀释酸液并改变结果。

---

3. Measuring Mass and Using Balances | 质量测量与天平的使用

When describing the preparation of a standard solution from a solid, examiners expected precise mention of the balance type and technique. Weighing by difference was often omitted. Candidates should state that the mass of the weighing bottle plus solid is recorded, the solid is transferred, and the bottle is reweighed to find the mass transferred. Using a 2‑decimal place balance was frequently cited incorrectly; for accurate work a 3‑ or 4‑decimal place analytical balance is needed.

在描述由固体制备标准溶液时，考官期望考生准确提及天平类型和称量技术。差量称量法经常被遗漏。考生应说明先记录称量瓶与固体的总质量，转移固体后重新称量空瓶，从而求得转移的质量。使用小数点后两位精度的天平常被错误引用；精确工作需要小数点后三位或四位分析天平。

Many candidates did not mention that the solid must be completely dissolved in a beaker with a small volume of distilled water before transferring to the volumetric flask, and that multiple rinsings of the beaker are essential to ensure quantitative transfer. Loss of solid at this stage causes systematic error.

许多考生没有提及固体必须先在烧杯中用少量蒸馏水完全溶解，再转移到容量瓶中，而且多次润洗烧杯对于确保定量转移至关重要。此阶段的固体损失会导致系统误差。

---

4. Handling of Solutions and Glassware | 溶液处理与玻璃仪器操作

Questions probing the preparation of a standard solution often revealed uncertainty about the function of the volumetric flask. The mark scheme required statements such as: ‘fill to the graduation mark with distilled water using a dropping pipette for the last few drops’ and ‘stopper and invert several times to mix thoroughly’. Omitting the mixing step was a frequent error. Candidates also confused the role of a wash bottle with that of a pipette filler.

探究标准溶液配制的题目常常暴露出考生对容量瓶作用的不确定性。评分方案要求写出：“用滴管滴加蒸馏水至刻度线”以及“塞住瓶塞并反复倒转几次以充分混匀”。遗漏混匀步骤是常见的错误。考生还混淆了洗瓶和移液球的作用。

When describing how to use a volumetric pipette, many neglected to mention using a pipette filler for safety, or they omitted the step of touching the tip to the inside of the flask to ensure the calibrated volume is delivered without blowing out the last drop. Leaving liquid in the tip is correct for a TD (to deliver) pipette.

在描述如何使用移液管时，许多人忽略了出于安全使用洗耳球，或者遗漏了将管尖接触瓶壁以确保流出校准体积且不吹出最后一滴的步骤。对于移出式移液管，管尖残留液体是正确的。

---

5. Temperature Measurement and Control | 温度测量与控制

Calorimetry and enthalpy change experiments were frequently scrutinised. Candidates needed to explain why a polystyrene cup is used: it is a good thermal insulator, minimising heat loss to the surroundings. When plotting temperature against time to determine the maximum temperature rise, many could not describe the extrapolation method for compensating heat loss. The examiners expected a description of drawing lines of best fit before and after the addition of the reagent, and reading the temperature at the time of addition from the intersection.

量热和焓变实验经常被细致考查。考生需要解释为何使用聚苯乙烯杯：它是一种良好的隔热体，能最大限度减少环境热损失。在绘制温度-时间图以确定最大温升时，很多人无法描述补偿热损失的作图外推法。考官期望的描述是：在加入试剂前和反应完成后分别画最适线，通过交点读取加入瞬间的温度。

Failure to mention stirring during temperature measurement was a common omission. The thermometer bulb must be fully immersed in the solution and the solution stirred continuously to ensure even temperature distribution. Recording the temperature at regular intervals (e.g., every 30 s) for a few minutes before mixing allows establishment of a stable baseline.

遗漏温度测量过程中的搅拌是常见的失误。温度计球泡需完全浸入溶液中，溶液需持续搅拌以确保温度分布均匀。在混合前以固定时间间隔（如每30秒）记录温度几分钟，可以建立稳定的基线。

---

6. Gas Collection and Measurement | 气体收集与测量

Experiments involving gas volume measurement, such as following the rate of a reaction that produces CO₂, were handled poorly by many. A gas syringe or an inverted measuring cylinder over water are acceptable methods, but candidates need to describe the set‑up correctly. For the water displacement method, it must be stated that the measuring cylinder is filled with water and inverted in a trough of water, with the delivery tube placed under the mouth of the cylinder. The volume is read at eye level, ensuring the cylinder is vertical.

涉及气体体积测量的实验，例如跟踪产生CO₂的反应速率，许多考生处理不佳。气体注射器或在水上用倒置量筒都是可接受的方法，但考生需要正确描述装置。对于排水集气法，必须说明量筒装满水后在水中倒置，导气管口放在筒口下方。读数时视线水平，并保证量筒竖直。

Leak checking was rarely mentioned. Candidates should indicate that all connections must be airtight and that a preliminary leak test should be performed. When a gas syringe is used, the plunger must move freely. The effect of not starting the stop‑watch at the exact moment of mixing was another area where marks were dropped.

检漏很少被提及。考生应指出所有连接处必须气密且应进行初步漏气测试。使用气体注射器时，活塞应能自由滑动。未在混合瞬间立即启动秒表的影响是另一个失分点。

---

7. Safety and Risk Assessment | 实验安全与风险评估

Examiners continue to expect context‑specific safety precautions. Generic statements such as ‘wear safety goggles’ are insufficient; candidates must link the precaution to the particular hazard. For example, when heating flammable solvents, a water bath should be used instead of a Bunsen burner to avoid ignition. When handling concentrated acids or alkalis, mention the use of gloves and the risk of corrosive burns.

考官持续期望针对具体情境的安全预防措施。泛泛而谈“佩戴护目镜”是不够的；考生必须将预防措施与特定危险联系起来。例如，加热易燃溶剂时应使用水浴代替本生灯以防止点燃。处理浓酸或浓碱时，应提及使用手套并说明腐蚀灼伤风险。

When barium chloride solution is used in precipitation reactions, the toxicity of barium ions must be recognised: ‘avoid ingestion – barium salts are toxic’ and ‘wash hands after use’. The absence of such specific comments was penalised.

在沉淀反应中使用氯化钡溶液时，必须认识到钡离子的毒性：“避免入口——钡盐有毒”以及“使用后洗手”。缺少此类具体说明会被扣分。

---

8. Recording and Presenting Data | 数据记录与呈现

Many candidates lost marks by recording raw data without units or by using inappropriate precision. Titration burette readings should always be recorded to two decimal places (e.g., 23.45 cm³). If two decimal places are not possible, the zero must still be shown (23.40 cm³). Temperature readings are expected to be to the nearest 0.5 °C or 0.1 °C depending on the thermometer graduation.

许多考生因记录原始数据时无单位或精度不当而失分。滴定管读数应始终记录至小数点后两位（如23.45 cm³）。若无法出现两位小数，仍须写出零（23.40 cm³）。温度读数根据温度计刻度应读到0.5 °C或0.1 °C。

When constructing a results table, all columns must have a heading that includes the quantity and the unit, separated by a solidus or presented in brackets, e.g., ‘Time / s’ or ‘Time (s)’. Repeating identical errors in a table heading was noted across many scripts.

制作结果表格时，所有列必须有标题，包含量和单位，以斜线分隔或用括号表示，如“Time / s”或“Time (s)”。许多答卷在表格标题上反复出现相同的错误。

---

9. Use of significant figures and units | 有效数字与单位

Significant figure (sf) errors were ubiquitous. When calculating a mean titre from concordant results, candidates often gave the mean to an unjustifiable number of sf. The rule is that the mean should be stated to the same number of decimal places as the original readings. Quoting an enthalpy change as –56.879 kJ mol⁻¹ when the data allow only three sf is unacceptable. Answers must reflect the least precise measurement used in the calculation.

有效数字错误普遍存在。从吻合结果计算平均滴定量时，考生常给出不合理的有效数字位数。规则是平均值的小数位数应与原始读数一致。当数据只允许三位有效数字时，将焓变写为–56.879 kJ mol⁻¹是不可接受的。答案必须反映计算中所用最不精确的测量。

Unit conversions also caused problems. Changing cm³ to dm³ requires division by 1000; many divided by 100 instead. When using the ideal gas equation, conversion of units for volume, pressure, and temperature must be checked carefully. Misplacing the decimal point in the molar mass determination was a frequent arithmetic slip.

单位换算也造成问题。将cm³转换为dm³需除以1000；许多人错误地除以100。使用理想气体方程时，必须仔细检查体积、压力和温度的单位换算。摩尔质量计算中小数点错位是常见的计算疏忽。

---

10. Planning and Evaluating Experiments | 实验规划与评价

Extended response questions asking candidates to plan an investigation were often answered vaguely. A clear logical sequence is essential: list the apparatus, state how variables are controlled, describe the step‑by‑step procedure, and indicate what measurements will be taken and how results will be processed. Leaving out the range and interval of the independent variable was a common mistake.

要求考生规划探究的扩展回答题往往答案模糊。清晰的逻辑顺序至关重要：列出仪器，说明如何控制变量，描述分步操作，并指出将测量什么以及如何处理结果。遗漏自变量的范围和间隔是常见错误。

In evaluation, candidates must distinguish between random and systematic errors. For example, heat loss in a calorimetry experiment is a systematic error leading to a lower temperature rise; it can be minimised by insulating the apparatus but not eliminated. Ignoring this distinction often led to incorrect suggestions for improvement.

在评价中，考生必须区分随机误差和系统误差。例如，量热实验中的热损失是导致温升偏低的系统误差；可通过隔热装置减小但无法消除。忽视这种区别常常导致改进建议不正确。

---

11. Drawing Conclusions from Practical Data | 从实验数据得出结论

When asked to comment on the reliability of results, candidates should refer to the consistency of repeated readings or the scatter of points on a graph. Small scatter implies good precision, but without knowledge of the ‘true’ value accuracy cannot be assessed. Too many answers incorrectly stated that accuracy can be deduced from the spread of repeated results alone.

当被问及结果的可靠性时，考生应提及重复读数的吻合程度或图形点的离散程度。散点小意味着精密度好，但若不知道“真值”，则无法评价准确度。太多答案错误地声称仅凭重复结果的离散程度就能判断准确度。

Linking the conclusion back to the hypothesis or the aim of the experiment is vital. If measuring the activation energy, candidates need to state how the gradient of an Arrhenius plot relates to Ea. Generic conclusions such as ‘the experiment worked’ receive no credit.

将结论与假设或实验目的连接起来至关重要。若测量活化能，考生需要说明Arrhenius图的斜率如何与Ea相关。诸如“实验成功”的笼统结论不得分。

---

12. Tips for Improvement | 改进建议

To excel in practical-based questions, students should engage in hands‑on laboratory work wherever possible, even if only through simulations or detailed video analyses. Rehearse the language of practical write‑ups: use phrases like ‘rinse the burette with the acid solution’, ‘add dropwise near the end point with constant swirling’, and ‘record the mass of the empty weighing bottle after transfer’. Practise identifying the resolution and uncertainty of common instruments.

要应对好实验类问题，学生应尽可能参与到动手实验中，哪怕只是通过模拟或详细的视频分析。反复练习实验报告的语言：使用“用酸液润洗滴定管”、“近终点时逐滴加入并持续摇荡”、“转移后称量空称量瓶质量”等句式。练习识别常用仪器的分辨率和不确定度。

Regularly review examiners’ reports and mark schemes to internalise the expected level of detail. Create a checklist for each common practical scenario, covering apparatus, measurement technique, safety, data presentation, and error analysis. This will turn vague knowledge into precise, exam-ready answers.

定期复习考官报告和评分方案，内化所期望的详细程度。为每种常见实验情景制作一份包括仪器、测量技术、安全、数据呈现和误差分析的清单。这将把模糊的知识转化为精准、适应考试要求的答案。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）