AS Chemistry: Practical Skills Guide | AS 化学：实验操作指南

📚 AS Chemistry: Practical Skills Guide | AS 化学：实验操作指南

Practical work is at the heart of AS Chemistry, not only to consolidate theory but also to develop essential laboratory skills that are assessed in written papers and, where applicable, in practical endorsements. This guide covers key techniques, from making accurate measurements to handling organic preparations, with an emphasis on why each step matters and how to minimise common errors.

实验操作是 AS 化学的核心，不仅巩固理论知识，更是培养关键实验技能的途径——这些技能在笔试卷和（若适用）实验认证中都会考察。本指南涵盖从精确测量到有机合成的关键技术，重点解释每一步的意义以及如何减少常见错误。

1. Safety and Laboratory Recording | 实验室安全与记录

Always wear eye protection, a lab coat, and gloves when handling chemicals. Before starting an experiment, carry out a risk assessment — identify the hazards of each reagent (corrosive, toxic, flammable) and the control measures needed.

处理化学品时务必佩戴护目镜、实验服和手套。开始实验前要进行风险评估：识别每种试剂（腐蚀性、毒性、易燃）的危害及所需的控制措施。

Record all observations directly into a lab notebook in ink, not in pencil. A common mistake is writing results on scrap paper and copying them later — this introduces copying errors and loses the original record. Include dates, names of reagents, concentrations, and any unexpected colour changes or temperature shifts.

实验记录应直接使用墨水记录在实验室笔记本上，不要用铅笔。一个常见的错误是先用废纸记录结果再誊写——这样会引入誊写错误并丢失原始记录。记录应包含日期、试剂名称、浓度以及任何意外的颜色变化或温度变化。

Label all glassware and containers with the substance and hazard symbol. Never return unused reagents to stock bottles to avoid contamination.

所有玻璃器皿和容器需标明物质名称及危险符号。切勿将未用完的试剂倒回原瓶，以免污染。

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2. Measuring Mass and Volume | 质量与体积的测量

a weighing boat or paper on the pan, tare (zero) the balance, then add the solid. Recording mass “by difference” — weighing the container with the solid and then subtracting the mass of the empty container — improves accuracy, especially for hygroscopic powders.

电子天平的读数精度通常为 ±0.01 g 或 ±0.001 g。称量时先将称量舟或纸放在托盘上，去皮（调零）后再加入固体。使用“差减法”记录质量——称量容器加固体的质量，再减去空容器的质量——能提高准确性，尤其适用于易吸潮的粉末。

For volumes, a volumetric flask is used for making solutions of exact concentrations; a graduated cylinder is suitable for approximate volumes. When reading the level of a liquid, the bottom of the concave meniscus should align with the graduation mark at eye level to avoid parallax error.

测量体积时，容量瓶用于配制精确浓度的溶液；量筒适用于粗略体积。读取液面时，应确保视线与凹月面底部和刻度线水平，以避免视差。

Pipettes (usually 10.0 cm³ or 25.0 cm³) and burettes (50.0 cm³) deliver volumes with high precision. Rinse them with the solution they will contain to prevent dilution or contamination. A pipette filler should always be used; never pipette by mouth.

移液管（常见 10.0 cm³ 或 25.0 cm³）和滴定管（50.0 cm³）可提供高精度体积。使用前需用待盛溶液润洗，以防止稀释或污染。必须使用洗耳球，严禁用嘴吸液。

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3. Preparing a Standard Solution | 配制标准溶液

A standard solution is one whose concentration is accurately known. To prepare it, weigh a required mass of a primary standard — a solid of high purity, stability in air, and known formula, such as anhydrous sodium carbonate (Na₂CO₃) or potassium hydrogenphthalate (KHC₈H₄O₄).

标准溶液是浓度精确已知的溶液。配制时称取所需质量的一级标准物质——一种高纯度、在空气中稳定、化学式确定的固体，如无水碳酸钠 (Na₂CO₃) 或邻苯二甲酸氢钾 (KHC₈H₄O₄)。

Transfer the solid to a beaker, dissolve it in deionised water, and then pour the solution into a 250.0 cm³ volumetric flask through a funnel. Rinse the beaker and funnel several times with deionised water, transferring all washings into the flask. Finally, add water until the bottom of the meniscus sits exactly on the graduation mark. Stopper and invert the flask ten times to mix thoroughly.

将固体转移到烧杯中，用去离子水溶解，然后将溶液通过漏斗倒入 250.0 cm³ 容量瓶中。用去离子水冲洗烧杯和漏斗数次，所有洗涤液都转入容量瓶。最后加水直至弯月面底部恰好与刻度线齐平。盖好瓶塞，翻转容量瓶十次，混合均匀。

The concentration c (mol dm⁻³) is calculated using:

c = n / V, where n = m / M

(m = mass in g, M = molar mass in g mol⁻¹, V = volume in dm³).

浓度 c (mol dm⁻³) 用下式计算：

c = n / V, 其中 n = m / M

（m = 质量/g，M = 摩尔质量/g mol⁻¹，V = 体积/dm³）。

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4. Titration Technique | 滴定技术

Rinse the burette with the titrant (the solution of known concentration), then fill it, ensuring the tip is free of air bubbles. Record the initial burette reading to the nearest 0.05 cm³. Use a pipette to transfer a fixed volume of the analyte into a clean conical flask and add two or three drops of a suitable indicator.

用滴定液（已知浓度的溶液）润洗滴定管，注满并确保尖嘴无气泡。记录初始读数，精确到 0.05 cm³。用移液管定量移取待测液至洁净锥形瓶中，加入 2–3 滴适宜的指示剂。

Place the conical flask on a white tile to make colour changes easier to see. Swirl the flask gently while adding titrant from the burette. Near the end point, add the titrant dropwise, washing down the sides of the flask with deionised water if necessary. The end point is reached when a single drop causes a permanent colour change.

将锥形瓶置于白色瓷板上，便于观察颜色变化。一边旋摇锥形瓶，一边从滴定管加入滴定液。接近终点时，逐滴滴加，必要时用去离子水冲洗瓶壁。当一滴液体导致颜色持久变化时，即为终点。

Common indicators: phenolphthalein (colourless in acid, pink in alkali, pH range 8.2–10.0) and methyl orange (red in acid, yellow in alkali, pH range 3.1–4.4). Choose an indicator whose pH range aligns with the steep part of the titration curve.

常用指示剂：酚酞（酸中无色，碱中粉红，pH 范围 8.2–10.0）和甲基橙（酸中红，碱中黄，pH 范围 3.1–4.4）。应选择 pH 变化范围与滴定曲线陡峭部分匹配的指示剂。

Repeat the titration until two concordant results (within 0.10 cm³ of each other) are obtained. Calculate the mean titre using only concordant values.

重复滴定，直到获得两个一致的测量值（彼此相差不超过 0.10 cm³）。仅用一致的数据计算平均滴定体积。

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5. Measuring Temperature Changes | 测量温度变化

In thermometric experiments, such as calorimetry, a 0– ecord the temperature at regular intervals (e.g., every 30 seconds) to follow the change and to allow extrapolation for correcting heat loss.

在量热等测温实验中，通常使用刻度为 0.5 °C 或 0.2 °C 的 0–100 °C 温度计。若需更高精度，数字温度探头可读取至 ±0.1 °C。每隔固定时间（如 30 秒）记录温度，以便跟踪变化趋势并外推校正热量损失。

When mixing solutions in a polystyrene cup, stir continuously and record the maximum or minimum temperature reached. To compensate for cooling or warming from the surroundings, plot temperature against time and extrapolate the linear sections back to the time of mixing.

在聚苯乙烯杯中混合溶液时持续搅拌，记录达到的最高或最低温度。为补偿环境引起的冷却或升温，可绘制温度–时间图，将线性部分回推至混合时刻。

The heat energy transferred, q, is calculated by

q = m c ΔT

where m is the mass of the solution (g), c is the specific heat capacity (usually 4.18 J g⁻¹ °C⁻¹ for aqueous solutions), and ΔT is the temperature change. For enthalpy changes per mole, divide q by the number of moles of the limiting reactant.

传递的热量 q 由

q = m c ΔT

计算，其中 m 为溶液质量 (g)，c 为比热容（水溶液通常取 4.18 J g⁻¹ °C⁻¹），ΔT 为温度变化。求算摩尔焓变时，将 q 除以限量反应物的物质的量。

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6. Determining Melting and Boiling Points | 测定熔点与沸点

To determine the melting point of a solid, a small amount of dry sample is placed in a capillary tube sealed at one end, then attached to a thermometer and heated in a suitable liquid bath (e.g., oil or Thiele tube). Record the temperature range from the first appearance of liquid to complete melting. A pure substance melts sharply over 1–2 °C; impurities broaden the range and lower the melting point.

测定固体熔点时，将少量干燥样品装入一端封闭的毛细管中，捆在温度计上，放入合适的液浴（如油浴或提勒管）中加热。记录从初熔至全熔的温度范围。纯物质的熔点范围较窄（1–2 °C）；杂质会使范围变宽并降低熔点。

For boiling point, use a distillation apparatus or a boiling point tube with a thermometer bulb positioned just below the side arm of the distillation head. The liquid is heated until boiling gently and the steady temperature is recorded. Also measure the atmospheric pressure, as boiling point changes with pressure.

沸点测定可使用蒸馏装置或沸点管，温度计水银球应放在蒸馏头支管下方。加热至液体微沸，记录恒定温度。同时测定大气压，因为沸点随压力变化。

Safety note: never heat a closed system; ensure the apparatus is vented. When using an oil bath, be aware of the flash points of the oil and avoid overheating.

安全提示：切勿加热封闭体系，确保装置有通气孔。使用油浴时应注意油的闪点，避免过热。

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7. Chromatography | 色谱法

Paper chromatography or thin‑layer chromatography (TLC) separates components of a mixture based on their partitioning between a stationary phase and a mobile phase. Draw a pencil baseline (never ink) on the paper or TLC plate, spot tiny drops of the sample and known standards, then stand the plate in a developing chamber containing a shallow layer of solvent. The solvent level must be below the baseline.

纸色谱或薄层色谱 (TLC) 根据各组分在固定相和流动相之间的分配不同来分离混合物。在纸或薄层板上画一条铅笔基线（绝不能用墨水），点上样品和已知标准品的微小液滴，然后将板放入含浅层展开剂的层析缸中。展开剂液面必须低于基线。

Allow the solvent to travel up the plate; when the solvent front is about 1 cm from the top, remove the plate and mark the solvent front immediately. Visualise spots using UV light, iodine vapour, or ninhydrin as appropriate. Calculate the Rf value:

Rf = distance moved by spot / distance moved by solvent front

. Compare Rf with known values for identification.

让溶剂沿板上升，待溶剂前沿距顶端约 1 cm 时取出，并立即标记溶剂前沿。根据需要使用紫外光、碘蒸气或茚三酮显色。计算 Rf 值：

Rf = 斑点移动距离 / 溶剂前沿移动距离

。通过与已知值比较进行鉴定。

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8. Filtration and Recrystallisation | 过滤与重结晶

Vacuum filtration (Büchner flask, Büchner funnel and filter paper, water pump) is used to separate a solid product from a liquid quickly. Wet the filter paper with the solvent to ensure a good seal, pour the suspension onto the paper, and rinse the solid with a small amount of cold solvent. Dry the crystals by pulling air through them.

真空抽滤（布氏烧瓶、布氏漏斗和滤纸、水泵）用于快速分离固体产物与液体。用溶剂润湿滤纸以确保密封，将悬浮液倒入滤纸，并用少量冷溶剂洗涤固体。通过抽吸空气使晶体干燥。

Recrystallisation purifies an impure solid. Dissolve the crude solid in the minimum volume of hot solvent, filter while hot to remove insoluble impurities, then allow the solution to cool slowly. Pure crystals form while soluble impurities remain in the solution (the mother liquor). Collect the crystals by vacuum filtration and wash with a little cold solvent.

重结晶用于提纯不纯的固体。将粗产品溶于最少量的热溶剂中，趁热过滤除去不溶性杂质，然后让溶液缓慢冷却。纯晶体析出，可溶性杂质留在溶液中（母液）。通过真空抽滤收集晶体，并用少量冷溶剂洗涤。

To test purity, check the melting point or run a TLC against a pure reference. The melting point should be sharp and match the literature value.

检验纯度可测熔点或与纯参照物对照进行 TLC。熔点应尖锐且与文献值一致。

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9. Gas Collection | 气体收集

Gases can be collected by upward or downward delivery, or over water, depending on their density relative to air and their solubility. A gas denser than air, e.g., carbon dioxide (CO₂), is collected by upward delivery (gas jar upright, delivery tube to the bottom). A gas less dense than air, e.g., hydrogen (H₂), is collected by downward delivery (inverted gas jar).

气体可根据其相对于空气的密度和溶解度，选用向上排空气、向下排空气或排水集气法收集。密度大于空气的气体，如二氧化碳 (CO₂)，用向上排空气法收集（集气瓶正放，导管伸到底部）。密度小于空气的气体，如氢气 (H₂)，用向下排空气法收集（集气瓶倒置）。

If a gas is only slightly soluble in water, it can be collected over water — for example, oxygen (O₂) or hydrogen (H₂). The gas displaces water in an inverted measuring cylinder or gas jar. This method gives a visual indication of the volume collected but the gas becomes saturated with water vapour.

若气体微溶于水，可采用排水集气法——例如氧气 (O₂) 或氢气 (H₂)。气体将倒置的量筒或集气瓶中的水排出。此法可直观获得收集到的体积，但气体会被水蒸气饱和。

In reactions producing a gas, use a gas syringe for accurate volume measurements. Ensure the syringe barrel moves freely and that the system is sealed. Record the volume change alongside any temperature or pressure fluctuations.

在有气体生成的反应中，使用气体注射器进行精确的体积测量。确保注射器的活塞可自由移动且系统密闭。记录体积变化及任何温度或压力波动。

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10. Reflux and Distillation | 回流与蒸馏

Many organic reactions, such as the oxidation of alcohols, require heating under reflux to prevent volatile reactants or products from escaping. Assemble the round‑bottom flask, condenser, and water tubing (water in at the bottom, out at the top). Add anti‑bumping granules to ensure smooth boiling. Heat using a heating mantle or water bath, never with an open flame near flammable solvent.

许多有机反应（如醇的氧化）需要回流加热，以防挥发性反应物或产物逸出。组装圆底烧瓶、冷凝管和水管（下进上出）。加入沸石防暴沸。使用加热套或水浴加热，切勿在有易燃溶剂的场合用明火。

Distillation separates a liquid mixture based on differences in boiling points. For simple distillation, the thermometer bulb is placed at the junction of the condenser and the side arm. Collect the distillate that comes over at a steady temperature corresponding to the boiling point of the desired component. Discard the first few drops (forerun) if necessary. For fractional distillation, a fractionating column provides better separation of liquids with close boiling points.

蒸馏利用沸点差异分离液体混合物。进行简单蒸馏时，温度计水银球应位于冷凝管与支管连接处。收集在对应目标组分沸点的稳定温度下蒸出的馏分。必要时弃去前几滴（前馏分）。分馏时，使用分馏柱能更好地分离沸点接近的液体。

Never distil to dryness; leave a small amount of liquid in the flask to prevent overheating and decomposition of residues.

切勿蒸干，应在烧瓶中留少量液体，以防过热和残留物分解。

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11. Colorimetry | 比色法 / 吸光光度法

Colorimetry measures the absorbance of light by a coloured solution to determine its concentration. Select a filter or wavelength of light that is complementary to the colour of the solution (e.g., for a blue solution, use a red/orange filter, around 600–650 nm).

比色法通过测定有色溶液的吸光度来确定其浓度。选择与溶液颜色互补的滤光片或波长（例如蓝色溶液用红/橙光区，约 600–650 nm）。

filled with distilled water (the blank). Measure the absorbance of standard solutions of known concentration and plot a calibration curve of absorbance (y‑axis) against concentration (x‑axis). The relationship should be linear (Beer– measure the absorbance of the unknown solution and read its concentration from the graph.

用蒸馏水（空白）装满比色皿，调零分光光度计。测量一系列已知浓度标准溶液的吸光度，绘制吸光度 (y 轴) 对浓度 (x 轴) 的校正曲线。两者应呈线性关系（比尔‑朗伯定律：A = εcl）。然后测量未知溶液的吸光度，从图中读出浓度。

Always handle cuvettes by the frosted sides to avoid fingerprints on the clear faces. Rinse the cuvette with the solution to be measured before filling.

拿取比色皿时持磨砂面，避免在透明面上留下指纹。装液前用待测溶液润洗比色皿。

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12. Data Handling and Errors | 数据处理与误差

.10 cm³. Calculate percentage uncertainty as (absolute uncertainty / measured value) × 100%.

所有测量都带有不确定度。数字仪表单次读数的不确定度为 ± 最小分度值。模拟刻度（如滴定管）需合并两次读数的不确定度（各 ±0.05 cm³），总不确定度为 ±0.10 cm³。百分不确定度 = (绝对不确定度 / 测量值) × 100%。

Identify random errors (e.g., inconsistent readings) from the spread of repeated measurements; these can be reduced by taking more replicates and averaging. Systematic errors (e.g., an incorrectly calibrated balance or a contaminated reagent) affect all results in the same direction and cannot be eliminated by averaging. Identifying and correcting the source is essential.

随机误差（如读数不一致）可从重复测量的散布程度识别；可通过增加重复次数取平均来减少。系统误差（如天平未校准或试剂受污染）会使所有结果朝同一方向偏移，无法通过取平均消除。识别并纠正误差源至关重要。

When plotting a graph, draw a line of best fit (or a curve where appropriate) that passes through or near the points. Outliers should be circled and not included in the line. Use the gradient or intercept as required; for enthalpy calculations, extrapolation corrects for heat loss.

作图时绘制一条通过或接近各点的最佳拟合线（或曲线）。异常点应圈出，不纳入拟合。根据需要使用斜率或截距；对于焓变计算，外推可校正热量损失。

Always relate the calculation back to the chemical equation. Identify the limiting reactant, calculate the theoretical yield, and compare with the actual yield to determine percentage yield:

% yield = (actual yield / theoretical yield) × 100%

. For atom economy, use:

% atom economy = (Mr of desired product / sum of Mr of all reactants) × 100%

.

计算始终要关联化学方程式。确定限量反应物，算出理论产量，与实际产量比较得出产率：

产率 = (实际产量 / 理论产量) × 100%

。原子经济性：

原子经济性 = (目标产物 Mr / 所有反应物 Mr 之和) × 100%

。

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