Required Practical Activities | 必做实验活动

📚 Required Practical Activities | 必做实验活动

The AQA A-Level Chemistry specification requires students to complete a series of twelve required practical activities. These are not merely exercises in following instructions; they are designed to develop your practical competence, data-handling skills, and deep conceptual understanding. Examiners will regularly set questions that assume you have carried out these experiments and reflected on their design, limitations, and possible refinements.

AQA A-Level 化学教学大纲要求学生完成一系列共十二个必做实验活动。这些实验不仅是照搬步骤的操作练习，更在于培养你的实验技能、数据处理能力和深层的概念理解。考官经常设置题目，假定你已经亲身完成过这些实验，并能对其实验设计、局限性和改进方案进行思考。

This guide breaks down the most commonly examined practicals, highlights the key techniques, points out the typical sources of error, and explains how each practical links to the underlying theory. Master these, and you will be prepared both for the written exam and for any practical-based questions that appear.

本指南将拆解最常考的必做实验，强调关键操作、指出典型误差来源，并解释每个实验与基础理论的联系。掌握这些内容，你将能从容应对笔试和实践类题目。

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1. Overview of the Twelve Required Practicals | 十二个必做实验概览

The AQA course defines twelve required practicals, spread across the physical, inorganic, and organic chemistry modules. Each one addresses a distinct skill: volumetric analysis, calorimetry, kinetics, qualitative analysis, organic synthesis, and chromatography.

AQA 课程定义了十二个必做实验，涵盖物理化学、无机化学和有机化学模块。每个实验对应一项独特的技能：容量分析、量热、动力学、定性分析、有机合成和色谱等。

The full list is as follows:

完整列表如下：

Number | 编号	Practical | 实验名称
1	Make up a volumetric solution and carry out an acid-base titration | 配制容量瓶标准溶液并进行酸碱滴定
2	Measurement of an enthalpy change | 焓变的测定
3	Investigation of how rate changes with temperature | 温度对反应速率影响的探究
4	Test-tube reactions to identify cations and anions | 试管反应鉴定阳离子与阴离子
5	Distillation of a reaction product | 反应产物的蒸馏
6	Tests for organic functional groups | 有机官能团的检验
7	Rate of reaction by an initial-rate method | 初始速率法测定反应速率
8	Rate of reaction by continuous monitoring | 连续监测法测定反应速率
9	pH changes during a weak acid/strong alkali titration | 弱酸与强碱滴定过程中的pH变化
10	Preparation of a pure organic solid and purity test | 有机固体的制备与纯度检验
11	Test-tube reactions of transition metal ions | 过渡金属离子的试管反应
12	Separation by thin-layer chromatography (TLC) | 薄层色谱分离

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2. Practical 1: Volumetric Solution and Titration | 实验1：配制标准溶液与酸碱滴定

This is the most frequently examined practical. To prepare a standard solution, you must weigh a solid accurately, dissolve it, and transfer it to a volumetric flask without loss. Use a weighing bottle, not a beaker, and rinse the weighing bottle, stirring rod, and funnel multiple times with distilled water to capture every particle.

这是考查频率最高的实验。配制标准溶液时，必须准确称量固体、溶解并无损转移至容量瓶中。应使用称量瓶而非烧杯，并多次用蒸馏水冲洗称量瓶、玻璃棒和漏斗，确保所有固体颗粒都被转移。

When titrating, the key steps are:

滴定时的关键步骤如下：

• Rinse the burette with the solution it will contain, never with water | 用待装溶液润洗滴定管，切勿用水润洗
• Read the burette at eye level, from the bottom of the meniscus | 读数时视线与液面持平，读取弯月面底部
• Use a white tile beneath the conical flask to observe colour changes clearly | 在锥形瓶下方垫白色瓷砖，便于观察颜色变化
• Add the titrant dropwise near the endpoint | 接近终点时逐滴加入滴定剂
• Repeat until concordant results within 0.10 cm³ | 重复滴定直到两次读数差不超过0.10 cm³

The concentration calculation follows the familiar mole relationship:

浓度计算遵循熟悉的摩尔关系式：

n = c × V and c₁V₁ = c₂V₂ (for a 1:1 mole ratio)

Remember that the moles of acid and alkali are related by the stoichiometric ratio from the balanced equation. For example, sulfuric acid reacts with sodium hydroxide in a 1:2 ratio: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l). Many students lose marks by forgetting to apply the mole ratio.

注意：酸和碱的摩尔数必须通过化学计量比相联系。例如硫酸与氢氧化钠按1:2反应：H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)。许多学生因忘记应用化学计量比而失分。

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3. Practical 2: Measurement of an Enthalpy Change | 实验2：焓变的测定

In this practical you typically measure the enthalpy of neutralisation or the enthalpy of solution. A polystyrene cup acts as a simple calorimeter: it reduces heat loss, and the reaction mixture is stirred with a thermometer or a temperature probe.

该实验通常测定中和焓或溶解焓。聚苯乙烯泡沫杯用作简易量热计：它可以减少热量散失，反应混合物用温度计或温度传感器搅拌。

The central calculation uses the heat equation:

核心计算使用热量方程：

q = mcΔT

where q is the heat absorbed, m is the mass of the solution (assume 1.00 g cm⁻³ and full volume of the combined solutions), c is the specific heat capacity of the solution (use 4.18 J g⁻¹ K⁻¹), and ΔT is the temperature change.

其中 q 为热量，m 为溶液质量（假设密度为1.00 g cm⁻³，质量取混合溶液的总体积对应值），c 为溶液比热容（取4.18 J g⁻¹ K⁻¹），ΔT 为温度变化。

A graph of temperature against time should be plotted for reactions that are not instantaneous. Extrapolate the cooling curve back to the time of mixing to obtain the maximum temperature rise, which corrects for heat loss to the surroundings. The enthalpy change per mole is then:

对于非瞬时完成的反应，应绘制温度-时间曲线图。将冷却曲线外推回混合时刻，得到校正了环境热损失的最大温升。单位摩尔的焓变则为：

ΔH = −q / n

The sign is important: an exothermic reaction gives a negative ΔH. In exam questions, you may be asked to suggest why the experimental value is less exothermic than the theoretical value: this is due to heat loss to the surroundings, incomplete reaction, or the assumption that the calorimeter itself absorbs no heat.

符号很重要：放热反应 ΔH 为负值。考题可能要求解释为什么实验值比理论值放热少：原因是热量散失到环境中、反应不完全，或假设量热计本身不吸收热量。

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4. Practical 3: How Temperature Affects Reaction Rate | 实验3：温度对反应速率的影响

This practical usually involves the reaction between sodium thiosulfate and hydrochloric acid, where a precipitate of sulfur is formed:

该实验通常涉及硫代硫酸钠与盐酸的反应，生成硫沉淀：

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(aq) + S(s) + H₂O(l)

You place a flask containing thiosulfate over a cross drawn on paper, add acid, and time how long it takes for the cross to disappear. The rate is taken as 1/t. Experiments are repeated at different initial temperatures of the thiosulfate solution.

将盛有硫代硫酸钠的锥形瓶放在画有十字标记的白纸上，加入酸后，记录十字被硫沉淀遮蔽所需的时间。速率取 1/t。在不同初始温度下重复实验。

The results show that as temperature increases, the time decreases and the rate increases. The Maxwell-Boltzmann distribution explains this: at higher temperature, a greater proportion of collisions exceed the activation energy Eₐ. A common exam question asks you to sketch two Maxwell-Boltzmann curves on the same axes and shade the area representing molecules with energy greater than Eₐ.

结果表明：温度升高时，时间缩短、速率增大。麦克斯韦-玻尔兹曼分布解释了这一点：温度更高时，超过活化能 Eₐ 的碰撞比例更大。常见考题要求在同一坐标轴上画出两条麦克斯韦-玻尔兹曼分布曲线，并标出能量大于 Eₐ 的分子区域。

In more quantitative A-Level questions, you may be expected to use the Arrhenius equation in graphical form:

在更定量的 A-Level 题目中，可能要求应用阿伦尼乌斯方程的图形形式：

ln k = ln A − Eₐ/(R × T)

A plot of ln rate against 1/T in kelvin gives a straight line of gradient −Eₐ/R. Remember to convert temperatures to kelvin before performing this calculation.

以 ln(速率) 对 1/T（开尔文温度）作图，得到斜率为 −Eₐ/R 的直线。计算前务必把温度换算为开尔文。

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5. Practical 4: Identifying Cations and Anions | 实验4：阳离子与阴离子的鉴定

This practical develops your qualitative analysis skills. For cations, flame tests are used for Group 2 and the s-block metals: lithium gives a crimson flame, sodium a yellow flame, potassium a lilac flame, calcium a brick-red flame, and copper(II) a blue-green flame. By contrast, transition metal ions are identified by the colour of the precipitate formed with sodium hydroxide solution.

该实验培养定性分析技能。对于阳离子，焰色测试用于第2族和s区金属：锂呈深红色火焰、钠呈黄色火焰、钾呈淡紫色火焰、钙呈砖红色火焰、铜(II)呈蓝绿色火焰。相比之下，过渡金属离子则通过加入氢氧化钠溶液后生成沉淀的颜色来鉴定。

Ion | 离子	Observation with NaOH | 加入NaOH的现象
Cu²⁺	Blue precipitate, insoluble in excess | 蓝色沉淀，过量NaOH中不溶解
Fe²⁺	Green precipitate, turns brown on standing | 绿色沉淀，久置变棕色
Fe³⁺	Orange-brown precipitate | 橙棕色沉淀
Al³⁺	White precipitate, dissolves in excess NaOH | 白色沉淀，过量NaOH中溶解
Zn²⁺	White precipitate, dissolves in excess NaOH | 白色沉淀，过量NaOH中溶解
NH₄⁺	Ammonia gas evolved, turns damp red litmus blue | 放出氨气，使湿润红色石蕊变蓝

For anions, the key tests are: carbonate ions release CO₂ with dilute acid, which turns limewater cloudy; sulfate ions give a white precipitate of BaSO₄ with barium chloride; and halide ions give precipitates with silver nitrate: AgCl white, AgBr cream, AgI pale yellow. Always acidify the test solution with dilute nitric acid first to remove interfering ions such as carbonate or sulfite.

对于阴离子，关键检验包括：碳酸根离子遇稀酸放出CO₂，使石灰水变浑浊；硫酸根离子与氯化钡反应生成BaSO₄白色沉淀；卤离子与硝酸银生成沉淀：AgCl白色、AgBr乳黄色、AgI淡黄色。务必先用稀硝酸酸化待测液，以消除碳酸根、亚硫酸根等干扰离子。

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6. Practical 5: Distillation of an Organic Product | 实验5：有机产物的蒸馏

Distillation is used to separate a liquid product from a reaction mixture based on differences in boiling point. The key apparatus includes a round-bottom flask, an anti-bumping granule, a still head with thermometer, a condenser (water enters at the bottom and leaves at the top), and a collecting flask.

蒸馏利用沸点差异将液体产物从反应混合物中分离。关键仪器包括圆底烧瓶、防暴沸石、带温度计的蒸馏头、冷凝管（水从下进上出）和接收瓶。

During the distillation, the thermometer bulb must be positioned at the junction where the still head meets the condenser, so that it records the temperature of the vapour being condensed. The anti-bumping granule prevents vigorous, spurting boiling. The receiving flask should be cooled in an ice bath when the product is volatile, and you must never heat the flask to dryness.

蒸馏时，温度计的水银球必须放在蒸馏头连接冷凝管的位置，以记录被冷凝蒸汽的温度。防暴沸石防止剧烈喷溅沸腾。如果产物易挥发，接收瓶应置于冰水浴中冷却，且绝不能将烧瓶加热至干。

Exam questions often ask why the product collected over a range of temperatures is impure: it will contain other components with close boiling points. You can suggest using a fractionating column to improve separation, or redistilling over a narrower temperature range.

考题常问：为什么在某个温度范围内收集的产物不纯？因为其中含有沸点相近的其他组分。可建议使用分馏柱增强分离效果，或在更窄的温度范围内重新蒸馏。

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7. Practical 7: The Iodine Clock — Initial Rate Method | 实验7：碘钟实验——初始速率法

This experiment measures the initial rate of reaction between peroxodisulfate ions and iodide ions:

该实验测定过二硫酸根离子与碘离子反应的初始速率：

S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

The iodine produced immediately reacts with thiosulfate ions, which are also present:

生成的碘立即与同时存在的硫代硫酸根离子反应：

I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)

Once the thiosulfate is consumed, the free iodine reacts with a starch indicator to give an intense blue-black colour. The time taken for the colour change to appear is inversely proportional to the initial rate: initial rate ∝ 1/time.

当硫代硫酸根耗尽后，剩余的游离碘与淀粉指示剂反应，出现强烈的蓝黑色。颜色变化所需的时间与初始速率成反比：初始速率 ∝ 1/时间。

By varying the initial concentration of iodide ions while keeping other concentrations constant, you can determine the order of reaction with respect to iodide. If doubling [I⁻] halves the time, the order is 1; if the time is unchanged, the order is 0; if the time is quartered, the order is 2. This is a classic AQA data-analysis question.

通过固定其他浓度、改变碘离子的初始浓度，可以确定反应对碘离子的反应级数。若 [I⁻] 加倍使时间减半，则级数为1；若时间不变，级数为0；若时间变为原来的四分之一，则级数为2。这是 AQA 经典的数据分析题型。

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8. Practical 8: Continuous Monitoring — Gas Volume | 实验8：连续监测法——气体体积

This practical monitors a reaction that produces a gas, such as magnesium reacting with dilute hydrochloric acid, or calcium carbonate with acid. You measure the volume of gas evolved at regular time intervals, using a gas syringe or an inverted measuring cylinder filled with water.

该实验监测产生气体的反应，例如镁与稀盐酸反应，或碳酸钙与酸反应。使用气体注射器（gas syringe）或倒扣在水槽中装满水的量筒，每隔一定时间记录释放气体的体积。

The data produce a graph of gas volume against time, which increases steeply at first and then levels off as the limiting reactant is used up. From this graph you can determine:

实验数据绘制气体体积-时间曲线：开始时上升陡峭，随后趋于平缓，直至限量反应物耗尽。从曲线可以确定：

• The initial rate, as the gradient of the tangent at t = 0 | 初始速率：t = 0 处切线的斜率
• The total volume of gas, which corresponds to the limiting reactant | 气体总体积，对应限量反应物的量
• How concentration changes over time (volume is proportional to concentration) | 浓度随时间的变化（体积与浓度成正比）

Temperature should be kept constant for a fair comparison, and the reaction needs to be started quickly once the reagents are mixed. A slower reaction such as magnesium with ethanoic acid can be followed more easily than a very fast reaction where data may be missed at the start.

为了公平比较，温度必须保持恒定，且试剂混合后要迅速开始计时。较慢的反应（如镁与乙酸）比极快的反应更容易准确监测，因为快速反应在开始时容易丢失数据点。

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9. Practical 9: pH Titration Curves | 实验9：pH滴定曲线

This practical uses a pH meter or a data-logger with a pH electrode to record pH continuously as a weak acid is titrated with a strong alkali. Each addition of alkali from the burette is followed by a pH reading, or the pH is recorded automatically while the alkali is added steadily.

该实验使用 pH 计或连接 pH 电极的数据采集器，在弱酸滴定强碱过程中连续记录 pH 值。每加入一定体积的碱后记录读数，或匀速加入碱液并自动记录 pH。

The resulting titration curve has a characteristic feature: the pH rises slowly, then rapidly through a steep section around the equivalence point, before levelling off. For a weak acid–strong base titration, the equivalence point is above pH 7 (typically pH 8–9), because the salt formed undergoes hydrolysis to produce a basic solution.

所得滴定曲线特征明显：pH 缓慢上升，然后经过等当点附近的一段陡峭变化，最后趋于平缓。对于弱酸-强碱滴定，等当点高于 pH 7（通常为8–9），因为生成的盐发生水解产生碱性溶液。

Two further points are commonly examined. First, the pH at the half-equivalence point equals the pKₐ of the weak acid. This allows Kₐ to be calculated using the Henderson–Hasselbalch relationship in its simple form:

另外两个考点常被考查。第一，半等当点处的 pH 等于弱酸的 pKₐ。据此可以利用简单的Henderson–Hasselbalch关系计算 Kₐ：

pH = pKₐ + log₁₀([A⁻]/[HA])

At the half-equivalence point, [A⁻] = [HA], so pH = pKₐ. Second, a suitable indicator for a weak acid–strong alkali titration is phenolphthalein, because its colour-change range (pH 8.3–10.0) lies within the steep part of the curve. Methyl orange would be unsuitable here.

在半等当点处，[A⁻] = [HA]，因此 pH = pKₐ。第二，弱酸-强碱滴定适合使用酚酞指示剂，因为其变色范围（pH 8.3–10.0）位于曲线陡峭部分内；甲基橙则不适用。

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10. Practical 10: Synthesis and Purification of Aspirin | 实验10：阿司匹林的制备与纯化

The preparation of aspirin combines organic synthesis with purification by recrystallisation. Salicylic acid is reacted with ethanoic anhydride in the presence of concentrated phosphoric acid as a catalyst:

阿司匹林的制备结合了有机合成与重结晶纯化。水杨酸与乙酸酐在浓磷酸催化下反应：

C₆H₄(OH)COOH + (CH₃CO)₂O → C₆H₄(OCOCH₃)COOH + CH₃COOH

The mixture is warmed gently in a water bath, then poured into cold water to precipitate the crude solid. The crude product is filtered under reduced pressure, washed with cold water, and recrystallised from a hot solvent such as ethanol (with water added to adjust solubility) to remove unreacted starting materials and by-products.

混合物在水浴中微微加热，然后倒入冷水中使粗产物沉淀。减压过滤后，用冷水洗涤，再在热溶剂（如乙醇，可加水调节溶解度）中进行重结晶，以除去未反应物和副产物。

The purity of the recrystallised product is checked by measuring its melting point. A pure compound melts sharply at its literature value; aspirin melts at approximately 135 °C. An impure product melts over a wider range and at a lower temperature. This is known as melting-point depression. The melting point apparatus is heated slowly, and the thermometer reading is taken when the sample begins to melt and again when melting is complete.

产物纯度通过熔点测定来检验。纯化合物在其文献值处尖锐熔化；阿司匹林的熔点约为135 °C。杂质导致熔点降低且熔程变宽，这称为熔点下降。用熔点仪缓慢加热，分别记录样品开始熔化与完全熔化的温度。

A common calculation involves the percentage yield. Always identify the limiting reagent, convert masses to moles, use the 1:1 stoichiometry of the balanced equation to find the theoretical mass, and compare it with the actual mass recovered.

常见的计算是百分产率。务必先确定限量反应物，将质量换算成物质的量，利用平衡方程1:1的化学计量关系求出理论产量，再与实际产量比较。

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11. Practical 12: Thin-Layer Chromatography | 实验11：薄层色谱法

Thin-layer chromatography is used to separate components of a mixture and to assess purity. A small spot of the sample is applied to a silica-coated plate using a capillary tube, the plate is placed in a developing tank containing a small volume of solvent, and the solvent travels up the plate by capillary action, carrying the components at different rates.

薄层色谱法用于分离混合物组分并评估纯度。用毛细管将少量样品点在硅胶薄层板上，将板放入盛有少量溶剂的层析缸中，溶剂通过毛细作用沿板上升，各组分以不同速率移动。

The crucial measurement is the retention factor:

关键测量值是比移值：

Rf = distance moved by the substance / distance moved by the solvent front

Each pure compound has a characteristic Rf value under specified conditions. A pure sample gives a single spot; an impure sample gives multiple spots. To improve separation, you might change the solvent polarity, adjust the stationary phase, or use a longer plate so the components travel further.

每种纯化合物在指定条件下具有特征的 Rf 值。纯样品只有一个斑点；不纯样品则出现多个斑点。为改善分离效果，可以更换溶剂极性、调整固定相或使用更长的薄层板使组分移动更远。

If the compounds are colourless, the plate is visualised by shining UV light (many compounds fluoresce) or by placing it in a jar containing iodine crystals, which stain the spots brown. Exam questions may ask why the spot must be applied above the solvent level: if the spot is submerged, it will dissolve into the solvent instead of moving up the plate.

若化合物无色，可用紫外灯照射显色（许多化合物有荧光），或将薄层板放入碘蒸气缸中使斑点显棕色。考题可能问

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