📚 AS Chemistry Experimental Techniques | AS化学实验操作
Experimental techniques form the backbone of AS Chemistry. Mastering the key practical skills – from measuring volumes and masses to carrying out titrations, distillations and safe heating – not only secures high marks in the practical assessment but also deepens understanding of theoretical concepts. This article covers the essential laboratory operations required for the 4.2 scheme of work, providing clear guidance on correct apparatus, step‑by‑step procedures and common sources of error.
实验操作是AS化学的核心。掌握关键的实验技能——从测量体积、质量，到完成滴定、蒸馏和安全加热——不仅能在实验考核中取得高分，还能加深对理论知识的理解。本文涵盖了4.2教学大纲中要求的必备实验室操作，针对正确仪器、分步流程和常见的误差来源提供清晰的指导。
1. Measuring Volume | 测量体积
or approximate volumes, a measuring cylinder is suitable, but when high accuracy is needed – such as in titrations – ±0.06 cm³, while a burette allows variable volumes to be dispensed and read to ±0.05 cm³.
精确测量体积在定量化学中至关重要。仪器的选择取决于所需的精度。粗略量取体积可用量筒，但当需要高准确度时——例如在滴定中——必须使用移液管或滴定管。移液管用来移取单一固定体积（如25.0 cm³），允差为±0.06 cm³，而滴定管可以释放任意体积，读数精度可达±0.05 cm³。
- Volumetric pipettes must be rinsed with the solution they are to deliver, not with water, to avoid dilution. / 移液管必须先用待取溶液润洗，不能用水润洗，以免稀释。
- When reading a burette or measuring cylinder, the bottom of the meniscus should be at eye level. / 读取滴定管或量筒时，视线应与凹液面最低处保持水平。
- The tip of the pipette should be touched against the side of the receiving vessel to ensure complete delivery. / 移液管的尖端应轻触接收容器的内壁，以保证溶液完全流出。
2. Measuring Mass | 测量质量
An electronic balance reading to 0.01 g or 0.001 g is commonly used to measure mass. Before any weighing, the balance must be tared (zeroed) with a clean container or weighing boat on the pan. Substances should never be placed directly on the pan; use a weighing boat or a piece of weighing paper. For hygroscopic solids, rapid weighing in a stoppered weighing bottle prevents absorption of moisture.
通常使用精确到0.01 g或0.001 g的电子天平测量质量。称量前，先将干净的容器或称量舟放在秤盘上，然后按去皮键归零。药品绝不可直接接触秤盘，要使用称量舟或称量纸。对于易吸水的固体，在带盖的称量瓶中快速称量可防止吸收空气中的水分。
- Record the mass immediately after the reading stabilises. / 读数稳定后立即记录质量。
- When weighing a solid for a standard solution, use the ‘weighing by difference’ technique: weigh the bottle + solid, tip most of the solid into the beaker, re‑weigh the bottle, and find the mass transferred by subtraction. / 配制标准溶液称量固体时，采用‘差量法’：称量瓶+固体质量，将大部分固体倒入烧杯，再次称量瓶，通过差量计算出转移的质量。
3. Preparing Standard Solutions | 配制标准溶液
A standard solution has a known concentration, typically prepared by dissolving an accurately weighed mass of primary standard (e.g. anhydrous sodium carbonate for acid‑base titrations). The solid is first dissolved in a beaker with deionised water, then transferred completely to a volumetric flask using a funnel and repeated rinsings of the beaker and funnel. The flask is filled to the calibration mark with deionised water, the stopper inserted, and the solution inverted several times to homogenise.
标准溶液具有准确已知的浓度，通常由精确称量的基准物质（如酸碱滴定中的无水碳酸钠）溶解配制。固体先在小烧杯中用去离子水溶解，然后通过漏斗全部转移到容量瓶中，烧杯和漏斗要反复冲洗，洗液一并转入。向容量瓶中加去离子水至刻度线，塞好瓶塞，上下颠倒多次使溶液混匀。
- The volumetric flask should not be heated, as the calibration is accurate only at the specified temperature (normally 20 °C). / 容量瓶不能加热，因为其刻度仅在特定温度（通常20 °C）下精确。
- The bottom of the meniscus must be exactly on the graduation line when viewed at eye level. / 视线与凹液面最低处相切，必须恰好对准刻度线。
4. Titration Technique | 滴定技术
Titration is used to determine the concentration of an unknown solution by reacting it with a standard solution. A typical acid‑base titration uses a burette filled with the standard solution and a conical flask containing a known volume of the unknown solution plus a few drops of indicator. The titrant is added with swirling until the endpoint – a permanent colour change – is reached.
滴定法利用标准溶液与未知浓度溶液反应，从而测定后者的浓度。典型的酸碱滴定中，滴定管盛放标准溶液，锥形瓶内装有已知体积的待测溶液和几滴指示剂。不断旋摇锥形瓶，逐滴加入滴定剂，直至到达终点——即出现持久的颜色变化。
- A trial (rough) titration is performed first to find the approximate endpoint. / 先进行一次粗滴定以确定大致终点。
- Accurate titres should agree within ±0.10 cm³. / 精确滴定的读数差值应在±0.10 cm³以内。
- Read the burette to the nearest 0.05 cm³, recording both the initial and final readings. / 滴定管读数估读至0.05 cm³，记录初读数和末读数。
- Common indicators: phenolphthalein (colourless in acid, pink in alkali) or methyl orange (red in acid, yellow in alkali). / 常用指示剂：酚酞（酸中无色，碱中粉红）或甲基橙（酸中红色，碱中黄色）。
c₁V₁ = c₂V₂
or at the equivalence point: n₁ = n₂ (for 1:1 reactions)
5. Heating Methods | 加热方法
Different heating requirements call for different apparatus. For gentle heating of non‑flammable liquids, a Bunsen burner with a tripod and gauze can be used. For flammable solvents, a water bath or electric heating mantle is essential to avoid naked flames. For strong heating of solids, a crucible on a pipeclay triangle over a Bunsen burner is suitable. A spirit burner or micro‑burner provides a cooler flame for small‑scale work.
不同的加热需求应选择不同的装置。对不易燃液体温和加热，可以用本生灯搭配三脚架和石棉网。对于易燃溶剂，必须使用水浴或电热套以避开明火。固体强热时，将坩埚放在泥三角上，用本生灯加热为宜。酒精灯或微型灯可提供温度较低的火焰，适合小型实验。
- Never heat a closed system – always provide an air vent to prevent pressure build‑up. / 绝不能加热密闭体系——必须留有排气口，防止压力积聚。
- When heating a liquid in a test tube, point the mouth away from people and use a gentle, circular motion to prevent bumping. / 试管加热液体时，管口朝向无人处，并轻轻圆周晃动以防止暴沸。
- Quickfit apparatus can withstand moderate heat but avoid direct strong flames on ground glass joints. / 快速装配仪器能承受中温，但不要用强火直接加热磨口玻璃接头。
6. Collecting Gases | 气体收集
The method of collecting a gas depends on its density and solubility in water. Gases that are almost insoluble in water – such as hydrogen, oxygen and carbon dioxide – can be collected over water in an inverted measuring cylinder or gas jar. Gases denser than air (e.g. chlorine, sulfur dioxide) are collected by downward delivery (upward displacement of air), while less dense gases (e.g. ammonia, hydrogen) are collected by upward delivery.
收集气体的方法取决于其密度和在水中的溶解性。几乎不溶于水的气体——如氢气、氧气和二氧化碳——可用排水集气法，在倒置的量筒或集气瓶中收集。密度大于空气的气体（如氯气、二氧化硫）用向下排空气法收集，而密度小于空气的气体（如氨气、氢气）用向上排空气法收集。
- To collect over water, fill a trough with water, place a gas jar full of water upside‑down, and bubble the gas into it. / 用排水法收集时，在水槽中注满水，将充满水的集气瓶倒置，通入气体至瓶内水排尽。
- For downward delivery, the delivery tube must reach near the bottom of the gas jar. / 向下排空气法时，导气管需接近集气瓶底部。
- Testing for a full jar: for chlorine, the green colour is visible; for oxygen, a glowing splint re‑lights. / 判断集满：氯气呈黄绿色；氧气可使带火星的木条复燃。
7. Drying Gases | 干燥气体
Moisture in an evolved gas can be removed by passing the gas through a drying agent. Concentrated sulfuric acid is effective for most non‑basic gases (e.g. hydrogen, oxygen, chlorine, carbon dioxide) but cannot be used for ammonia or basic gases due to an acid‑base reaction. Anhydrous calcium chloride is a versatile desiccant, suitable for most gases except ammonia, with which it forms a complex. Quicklime (calcium oxide) is used specifically for ammonia.
生成的气体常常含有水汽，可通过干燥剂除去。浓硫酸可干燥多数非碱性气体（如氢气、氧气、氯气、二氧化碳），但由于酸碱反应，不能用于氨气等碱性气体。无水氯化钙是一种通用干燥剂，适用于大部分气体，但会与氨气形成络合物，不可用。生石灰（氧化钙）专用于干燥氨气。
- The drying agent is placed in a U‑tube or a drying tower, and the gas is bubbled or passed through. / 将干燥剂装入U形管或干燥塔中，气体以鼓泡或通过的方式流过。
- Always check the compatibility of the gas with the drying agent before use. / 使用前务必确认气体与干燥剂是否相容。
8. Reflux and Distillation | 回流与蒸馏
Reflux is used to carry out a reaction at the boiling point of the solvent without losing volatile components. The reaction flask is fitted with a vertical condenser; vapours condense and return to the flask. For separating liquids with different boiling points, simple distillation is employed, where the vapour is directed into a Liebig condenser and the distillate collected in a receiving flask. Fractional distillation with a fractionating column can separate liquids with close boiling points.
回流常用于在溶剂沸点温度下进行反应，同时避免挥发性组分损失。反应瓶上安装竖直冷凝管，蒸气冷凝后流回瓶内。对于沸点相差较大的液体混合物，使用简单蒸馏，蒸气导入直形冷凝管，馏出液收集于接收瓶中。分馏柱用于沸点接近的液体分馏。
- Anti‑bumping granules must be added to the distilling flask to ensure smooth boiling. / 蒸馏瓶中必须加入沸石（防暴沸粒），使沸腾平稳。
- In simple distillation, the thermometer bulb should be placed exactly at the side arm of the distillation head to measure the vapour temperature. / 简单蒸馏时，温度计水银球应恰好位于蒸馏头支管口处，以测量蒸气温度。
- Water flows into the condenser jacket at the bottom and out at the top to maintain efficient cooling. / 冷凝管外套中的冷却水应从下方进口流入，上方出口流出，以保证冷却效率。
9. Filtration and Drying Solids | 过滤与干燥固体
Solid precipitates are separated from a liquid by filtration. Gravity filtration using fluted filter paper in a funnel is suitable for small quantities. For faster filtration or when the solid is to be retained, vacuum filtration (Büchner flask and Büchner funnel) is used. The solid cake is washed with a small amount of cold solvent to remove soluble impurities, then dried – either in an oven at a moderate temperature or in a desiccator over silica gel.
固体沉淀物通过过滤与液体分离。重力过滤使用折叠滤纸和漏斗，适合少量物质。若需要快速过滤或保留固体，可采用抽滤（布氏烧瓶和布氏漏斗）。滤饼用少量冷溶剂洗涤以除去可溶性杂质，然后干燥——可在烘箱中适度温度烘干，或在干燥器中用硅胶干燥。
- Wet filter paper with the solvent before transferring the precipitate to improve adhesion. / 移入沉淀前先用溶剂润湿滤纸，增加贴合度。
- In vacuum filtration, the filter paper should cover all holes in the Büchner funnel. / 抽滤时，滤纸应完全覆盖布氏漏斗底部所有孔洞。
- Desiccator lid should be gently rotated into place, not slid, to preserve the vacuum and grease seal. / 干燥器盖子应旋转盖紧，不可滑动，以保持真空和密封脂的密封性。
10. Use of Quickfit Apparatus | 快速装配仪器的使用
Quickfit glassware, with standard ground‑glass joints, allows rapid assembly of apparatus for reflux, distillation and organic synthesis. The joints are secured with plastic clips and require no grease unless vacuum is applied. After use, all Quickfit components must be dismantled and cleaned immediately, preferably with an appropriate solvent, to prevent joints from seizing.
快速装配玻璃仪器带有标准磨口接头，可快速搭建回流、蒸馏和有机合成装置。接头用塑料夹子固定，除非抽真空，一般不需涂油脂。使用后必须立即拆开并清洗所有组件，最好用合适的溶剂，以防接头粘死。
- Never apply excessive force when assembling or dismantling; a gentle twisting motion helps separate stuck joints. / 安装或拆卸时绝不可用力过猛；轻微旋转动作有助于分开卡住的接头。
- All glassware should be checked for cracks or star fractures before heating. / 加热前所有玻璃仪器都应检查是否出现裂纹或星形裂痕。
11. Handling Hazardous Chemicals | 危险化学品操作
AS students must follow strict safety rules when handling corrosive, toxic or flammable substances. Concentrated acids and alkalis require the use of safety goggles, gloves and a fume cupboard if vapours are produced. Flammable solvents such as ethanol, propanone and hydrocarbons must be kept away from open flames; heating is performed on a water bath or electric mantle. Spills should be dealt with immediately using neutralising or absorbent materials.
AS学生处理腐蚀性、有毒或易燃化学品时，必须严格遵守安全规则。浓酸和浓碱须佩戴护目镜、手套，如果产生蒸气，应在通风橱中进行操作。乙醇、丙酮和烃类等易燃溶剂必须远离明火；加热需用水浴或电热套。溢出物应立即使用中和剂或吸收材料处理。
- Always read the hazard symbols (GHS pictograms) on reagent bottles before starting an experiment. / 开始实验前，务必阅读试剂瓶上的危险标志（GHS象形图）。
- Never pipette by mouth; use a safety pipette filler. / 绝不用嘴吸移液管，必须使用洗耳球。
- Fume cupboards are mandatory for experiments releasing toxic gases like SO₂, NO₂ or chlorine. / 释放有毒气体（如SO₂, NO₂或氯气）的实验，必须在通风橱内进行。
12. Recording and Analysing Data | 实验数据记录与分析
All observations and measurements must be recorded directly into a laboratory notebook using ink. A neat table with clear headings and units should be designed before starting the practical. For titration, a table showing initial burette readings, final readings and titre volumes is typical. Graphs should be drawn on graph paper or plotted using software, with both axes labelled, a suitable scale, and a line of best fit where appropriate. Percentage errors and uncertainties should be discussed to evaluate the reliability of results.
所有的观察和测量结果必须用墨水笔直接记录在实验记录本上。实验开始前，应先设计好格式清晰的表格，列明表头与单位。对滴定实验，通常要记录初读数、末读数和滴定体积。图表应绘于坐标纸或使用软件绘制，两坐标轴均需标记，选用合适的比例，并尽可能绘制最佳拟合线。应讨论百分误差和不确定度，以评价结果的可靠性。
- Calculate mean titres only from concordant readings (within 0.10 cm³). / 仅从接近一致的读数（差值在0.10 cm³以内）中计算平均滴定体积。
- Percentage error = (│experimental value – theoretical value│ / theoretical value) × 100%. / 百分误差 = (│实验值 – 理论值│ / 理论值) × 100%。
- ty of a burette is ±0.05 cm³ per reading, so total uncertainty for a titre is ±0.10 cm³. / 滴定管单次读数的不确定度为±0.05 cm³，因此一次滴定的总不确定度为±0.10 cm³。
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