📚 Practical Skills 2 for Cambridge AS & A Level Chemistry | 剑桥AS与A Level化学实验技能2
Practical skills 2 builds on the core laboratory techniques from Practical skills 1 and focuses on quantitative analysis, accurate measurement, titration, thermochemistry, reaction kinetics, qualitative inorganic tests, organic purification, chromatography, graphical analysis and error evaluation. A sound grasp of these skills is essential for Cambridge AS & A Level practical examinations and for the Paper 5 planning, analysis and evaluation paper.
实验技能2建立在实验技能1的核心操作基础上，重点训练定量分析、准确测量、滴定、热化学、反应动力学、无机定性检验、有机提纯、色谱、图解分析与误差评价。扎实掌握这些技能对剑桥AS与A Level实验考试以及Paper 5的规划、分析与评价试卷至关重要。
1. Accurate Measurement and Uncertainty | 准确测量与不确定度
Every measured value carries uncertainty. For a burette, the smallest scale division is normally 0.10 cm³, so the reading uncertainty is ±0.05 cm³. A 25.0 cm³ class B pipette often has a tolerance of ±0.06 cm³, while a two-decimal-place analytical balance may have an uncertainty of ±0.005 g.
每一个测量值都带有不确定度。滴定管的最小刻度通常为0.10 cm³，因此读数不确定度为±0.05 cm³。一支25.0 cm³的B级移液管公差常为±0.06 cm³，而两位小数的分析天平不确定度可能为±0.005 g。
Percentage uncertainty is calculated from absolute uncertainty and the measured value. This helps identify which measurement contributes most to the total error.
百分不确定度由绝对不确定度与测量值计算得出，这有助于判断哪一个测量环节对总误差贡献最大。
percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%
For a burette reading of 24.50 cm³, the percentage uncertainty is (0.05 ÷ 24.50) × 100% = 0.204%. In a titration, two burette readings are taken, so the total absolute uncertainty from the burette is ±0.10 cm³.
若滴定管读数为24.50 cm³，百分不确定度为(0.05 ÷ 24.50) × 100% = 0.204%。在一次滴定中要读取两次滴定管读数，因此滴定管带来的总绝对不确定度为±0.10 cm³。
2. Titration Technique and End-points | 滴定技术与终点判断
A burette must be rinsed with the solution it will contain, and the jet must be filled so that no air bubbles remain. A pipette should be rinsed with the solution to be transferred, not with water, because residual water would dilute the solution and change its concentration.
滴定管必须先用待装溶液润洗，并且尖嘴中要充满溶液，不能残留气泡。移液管应先用待移取的溶液润洗，而不是用水润洗，因为残留的水会稀释溶液并改变其浓度。
Read the bottom of the meniscus at eye level to avoid parallax error. A first rough titration gives an approximate titre, then accurate titrations are repeated until two or three concordant results within 0.10 cm³ are obtained.
读数时应视线与凹液面底部平齐以避免视差。第一次粗滴定可得到大致滴定体积，然后重复准确滴定，直到获得两次或三次相差不超过0.10 cm³的吻合结果。
The end-point is the point at which the indicator changes colour, while the equivalence point is the theoretical point at which the reaction is just complete. The indicator must change colour sharply at the equivalence point of the reaction.
终点是指示剂变色的那一点，而化学计量点是反应恰好完成的理论点。指示剂必须在反应的化学计量点附近发生敏锐的颜色变化。
3. Making Standard Solutions | 配制标准溶液
A standard solution is a solution of accurately known concentration. Primary standards such as anhydrous sodium carbonate or potassium hydrogen phthalate have high purity, high molar mass, stability in air and good solubility in water.
标准溶液是浓度准确已知的溶液。基准物质如无水碳酸钠或邻苯二甲酸氢钾具有纯度高、摩尔质量大、在空气中稳定且易溶于水等特点。
To prepare a standard solution, weigh the solute accurately by difference, dissolve it in a beaker with distilled water, transfer the solution and all rinsings to a volumetric flask, then make up to the graduation mark with distilled water and invert several times to mix thoroughly.
配制标准溶液时，先用差减法准确称取溶质，在烧杯中用蒸馏水溶解，将溶液及所有洗液转移至容量瓶中，再用蒸馏水定容至刻度线，最后反复倒置摇匀。
The concentration is calculated using the amount of solute and the final volume. This equation is central to volumetric analysis.
浓度由溶质的物质的量与最终体积计算，该公式是容量分析的核心。
c = n ÷ V
For example, 2.65 g of Na₂CO₃ (M = 106.0 g mol⁻¹) in 250.0 cm³ gives n = 2.65 ÷ 106.0 = 0.0250 mol, so c = 0.0250 mol ÷ 0.250 dm³ = 0.100 mol dm⁻³.
例如，2.65 g Na₂CO₃（M = 106.0 g mol⁻¹）溶于250.0 cm³，n = 2.65 ÷ 106.0 = 0.0250 mol，因此c = 0.0250 mol ÷ 0.250 dm³ = 0.100 mol dm⁻³。
4. Measuring Enthalpy Changes | 测量焓变
Enthalpy changes are usually measured in a simple polystyrene cup calorimeter. The temperature change of a known mass of solution or water is recorded before and after mixing, and the heat change is calculated using the specific heat capacity of the solution.
焓变通常用简易聚苯乙烯杯量热计测量。记录已知质量的溶液或水在混合前后的温度变化，再利用溶液的比热容计算热量变化。
q = mcΔT
Here q is the heat change in joules, m is the mass of the solution in grams, c is the specific heat capacity in J g⁻¹ K⁻¹, and ΔT is the temperature change. If the reaction is exothermic, ΔT is positive when measured in the calorimeter and q is negative for the system.
其中q为热量变化，单位焦耳；m为溶液质量，单位克；c为比热容，单位J g⁻¹ K⁻¹；ΔT为温度变化。若反应放热，量热计中测得的ΔT为正值，而体系的热量变化q为负值。
The enthalpy change per mole, ΔH, is then found by dividing the heat change by the moles of the limiting reagent. Inaccurate results often come from heat loss to the surroundings, incomplete reaction, or assuming the solution has the same density and specific heat capacity as pure water.
摩尔焓变ΔH由热量变化除以限量试剂的物质的量得到。结果不准确的原因常包括热量散失到环境中、反应不完全，或假设溶液密度与比热容与纯水相同。
5. Investigating Reaction Rates | 研究反应速率
Reaction rate is the change in concentration of a reactant or product per unit time. It can be followed by measuring gas volume, loss in mass, colour intensity, turbidity, pH change or electrical conductivity.
反应速率是单位时间内反应物或产物浓度的变化。可通过测量气体体积、质量减少、颜色强度、浑浊度、pH变化或电导率来跟踪反应。
rate = Δ[A] ÷ Δt
The initial rate method measures the rate at the very start of the reaction, where the concentrations are known. A clock reaction, such as the iodine clock, gives a convenient fixed visual end-point and allows relative rates to be compared.
初始速率法测量反应刚开始时的速率，此时各物质浓度已知。时钟反应如碘钟反应能提供固定的视觉终点，便于比较相对速率。
For the reaction aA + bB → products, the rate equation may be written as rate = k[A]ᵐ[B]ⁿ, where m and n are the orders with respect to A and B. These are determined experimentally, not from the stoichiometric equation.
对于反应aA + bB → 产物，速率方程可写作rate = k[A]ᵐ[B]ⁿ，其中m和n分别为对A和B的反应级数。反应级数由实验测定，而不是从化学计量方程式直接得出。
6. Gas Collection and Molar Volume | 气体收集与摩尔体积
Gases produced in a reaction can be collected in a gas syringe, over water in an inverted measuring cylinder, or by displacement of air. The method chosen must suit the solubility and density of the gas.
反应产生的气体可用气体注射器收集、通过排水集气法在倒置量筒中收集，或用排空气法收集。选择的方法必须适合气体的溶解性和密度。
For an ideal gas, pressure, volume, temperature and amount are related by the ideal gas equation. This can be used to find the molar volume or the molar mass of a gas.
对于理想气体，压强、体积、温度与物质的量之间由理想气体状态方程关联。该方程可用于求气体的摩尔体积或摩尔质量。
pV = nRT
At room temperature and pressure (RTP), usually taken as 20 °C and 1 atm, the molar volume of an ideal gas is approximately 24.0 dm³ mol⁻¹. At standard temperature and pressure (STP, 0 °C and 1 atm) it is 22.4 dm³ mol⁻¹.
在常温常压（RTP）下，通常取20 °C和1 atm，理想气体的摩尔体积约为24.0 dm³ mol⁻¹。在标准状况（STP，0 °C和1 atm）下为22.4 dm³ mol⁻¹。
7. Qualitative Analysis: Tests for Ions | 定性分析：离子检验
Qualitative analysis identifies ions or functional groups by observing colour changes, precipitates and gas evolution. Cation tests include flame tests and precipitation with sodium hydroxide or aqueous ammonia, while anion tests often use dilute acid, silver nitrate or barium chloride.
定性分析通过观察颜色变化、沉淀和气体产生来鉴定离子或官能团。阳离子检验包括焰色反应以及用氢氧化钠或氨水沉淀，阴离子检验常用稀酸、硝酸银或氯化钡。
| Ion | Test reagent | Positive observation |
|---|---|---|
| NH₄⁺ | Warm with NaOH(aq) | Pungent gas turns damp red litmus blue |
| CO₃²⁻ | Add dilute HCl | Effervescence; gas turns limewater milky |
| SO₄²⁻ | Dilute HNO₃ then BaCl₂(aq) | White precipitate of BaSO₄ |
| Cl⁻ | Dilute HNO₃ then AgNO₃(aq) | White precipitate soluble in dilute NH₃ |
| Br⁻ | Dilute HNO₃ then AgNO₃(aq) | Cream precipitate partially soluble in conc NH₃ |
| I⁻ | Dilute HNO₃ then AgNO₃(aq) | Yellow precipitate insoluble in conc NH₃ |
Common gas tests include hydrogen, which gives a squeaky pop with a lighted splint; oxygen, which relights a glowing splint; carbon dioxide, which turns limewater milky; and ammonia, which turns damp red litmus blue.
常见气体检验包括：氢气遇点燃木条有爆鸣声；氧气使带火星的木条复燃；二氧化碳使石灰水变浑浊；氨气使湿润的红色石蕊试纸变蓝。
8. Separation and Purification Techniques | 分离与提纯技术
Filtration separates an insoluble solid from a liquid. Simple distillation separates a solvent from a solution or a liquid from a mixture of liquids with sufficiently different boiling points. Fractional distillation improves separation of miscible liquids with closer boiling points.
过滤用于分离不溶性固体与液体。简单蒸馏可将溶剂从溶液中蒸出，或分离沸点差异足够大的液体混合物。分馏可改善沸点接近的互溶液体的分离效果。
A separating funnel separates immiscible liquids, such as an aqueous layer and an organic layer. Drying agents, such as anhydrous magnesium sulfate or calcium chloride, remove traces of water from organic liquids.
分液漏斗用于分离互不相溶的液体，例如水层与有机层。干燥剂如无水硫酸镁或氯化钙可除去有机液体中残留的水分。
Reflux allows a reaction mixture to be heated at the boiling point for an extended time without losing volatile components. The condenser returns vaporised liquid to the flask, which is essential for organic preparations such as esterification or oxidation.
回流可使反应混合物在沸点下长时间加热而不损失挥发性组分。冷凝管将蒸发的液体送回烧瓶，这在酯化或氧化等有机制备中至关重要。
9. Recrystallization and Melting Point Determination | 重结晶与熔点测定
Recrystallization purifies a solid organic product. The impure solid is dissolved in the minimum volume of a hot solvent, then the hot solution is filtered if necessary, and the filtrate is allowed to cool slowly so crystals form. The crystals are collected by vacuum filtration, washed with cold solvent and dried.
重结晶用于提纯固体有机产物。将不纯固体溶于最少量的热溶剂中，必要时趁热过滤，滤液缓慢冷却析出晶体。晶体经减压过滤分离，用冷溶剂洗涤并干燥。
The chosen solvent should dissolve the solid well when hot and poorly when cold, should not react with the solute, and should dissolve impurities either very well or not at all. The melting point of a pure compound is sharp, usually over a range of 1-2 °C, while impurities lower the melting point and broaden the range.
所选溶剂应在热时溶解固体良好而冷时溶解很少，不与溶质反应，并且对杂质或完全溶解或几乎不溶。纯化合物的熔点敏锐，通常在1-2 °C范围内，而杂质会降低熔点并使熔程变宽。
10. Thin-layer and Paper Chromatography | 薄层色谱与纸色谱
Chromatography separates components of a mixture between a stationary phase and a mobile phase. In paper chromatography, water adsorbed on the paper often acts as the stationary phase, while a solvent such as propanone or ethanol moves through the paper as the mobile phase. In thin-layer chromatography, a thin layer of silica or alumina on a plate is the stationary phase.
色谱法利用固定相与流动相之间的分配来分离混合物各组分。在纸色谱中，吸附在纸上的水常作为固定相，而丙酮或乙醇等溶剂作为流动相穿过纸层。在薄层色谱中，薄板上的一层硅胶或氧化铝为固定相。
Each component is identified by its retention factor Rf value. Spots are located using UV light or a locating agent if they are colourless.
每个组分通过比移值Rf来鉴定。无色斑点需用紫外光或显色剂定位。
Rf = distance moved by spot ÷ distance moved by solvent front
Rf is always less than 1 and is constant for a given compound under the same stationary phase, mobile phase and temperature. It can therefore be compared with known values to identify a substance.
Rf值总小于1，对于同一化合物，在相同固定相、流动相和温度下为常数。因此可与已知值比较以鉴定物质。
11. Drawing Graphs and Analyzing Data | 绘图与数据分析
The independent variable is plotted on the x-axis and the dependent variable on the y-axis. Axes should be labelled with quantity and unit, scales should use at least half the graph grid, and data points should be marked clearly.
自变量画在x轴上，因变量画在y轴上。坐标轴应注明物理量与单位，刻度应至少占用图纸网格的一半，数据点应清晰标记。
Draw a straight line of best fit or a smooth curve through the points, ignoring obvious anomalies. The gradient and intercept of a linear graph can then be used to determine constants from experimental data.
通过各点画最佳拟合直线或平滑曲线，明显异常的点应忽略。线性图的斜率和截距可用于从实验数据确定常数。
gradient = Δy ÷ Δx
For example, in an Arrhenius plot of ln k against 1/T, the gradient is -Ea/R and the intercept is ln A. Careful graph work is therefore part of quantitative practical analysis.
例如，在ln k对1/T的阿伦尼乌斯图中，斜率为-Ea/R，截距为ln A。因此精确作图是定量实验分析的一部分。
12. Evaluating Errors and Improvements | 误差评估与改进
Systematic errors affect all measurements in the same direction and are not reduced by repetition. Examples include a wrongly calibrated balance, a contaminated reagent, or heat loss in a calorimeter. Random errors vary unpredictably and can be reduced by repeating measurements and averaging.
系统误差使所有测量值朝同一方向偏移，且不能通过重复来减小。例如天平校准错误、试剂污染或量热计热量损失。随机误差不可预测地变化，可通过重复测量取平均值来减小。
Percentage difference between an experimental value and an accepted value indicates accuracy, while spread between repeated titres indicates precision. Good experimental design controls variables, uses clean apparatus and includes a sensible blank or control where relevant.
实验值与公认值之间的百分差反映准确度，而重复滴定之间的离散程度反映精密度。良好的实验设计应控制变量、使用洁净仪器，并在适当情况下设置空白或对照实验。
Common improvements include insulating the calorimeter with a lid, using a more accurate balance, wiping off excess liquid from the pipette tip, using a white tile under the flask to see the end-point clearly, and repeating measurements to identify outliers.
常见的改进包括给量热计加盖保温、使用更精密的天平、擦去移液管尖外多余的液体、在锥形瓶下放白瓷板以便清楚观察终点，以及重复测量以识别异常值。
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