📚 Practical Assessment in IGCSE Chemistry | IGCSE 化学实验评估
Practical assessment is a core part of IGCSE Chemistry. It tests your ability to plan experiments, use apparatus safely, record observations, handle data and evaluate limitations. Strong practical skills support every theory topic from rates to organic chemistry.
实验评估是IGCSE化学的核心组成部分。它考查你设计实验、安全使用仪器、记录现象、处理数据和评价局限性的能力。扎实的实验技能能帮助你理解从反应速率到有机化学的每一个理论主题。
1. What is Practical Assessment? | 什么是实验评估？
In IGCSE Chemistry, practical assessment may involve written questions based on experiments, teacher-assessed coursework, or a practical test depending on the exam board. You must know how to choose suitable apparatus, describe procedures, record readings to the correct precision, and identify risks.
在IGCSE化学中，实验评估可能包括基于实验的笔试题、教师评分的课程作业或实验考试，具体取决于考试局。你必须知道如何选择合适的仪器、描述操作步骤、按正确精度记录读数，并识别风险。
Marks are often awarded for method, control of variables, repeat readings, and evaluation rather than only for the final answer.
分数通常给在方法、变量控制、重复读数和评价上，而不仅仅是最终答案。
2. Common Apparatus and Measurement | 常见仪器与测量
ample, a burette reads to ±0.05 cm³, while a measuring cylinder may only read to ±0.5 cm³.
你应该认识滴定管、移液管、量筒、气体注射器、温度计、天平、锥形瓶、烧杯、过滤漏斗、蒸发皿和本生灯。每种仪器的精度不同；例如，滴定管读数可到±0.05 cm³，而量筒可能只能读到±0.5 cm³。
When recording volume, read the bottom of the meniscus at eye level to avoid parallax error. Digital balances usually measure mass to 0.01 g. Always use a weighing boat or filter paper and zero the balance before use.
记录体积时，应在与凹液面底部水平的位置读数，以避免视差误差。电子天平通常能测量到0.01 g。使用前要放称量舟或称量纸，并将天平归零。
3. Safety in the Laboratory | 实验室安全
You must wear eye protection throughout practical work. Tie back long hair, avoid loose clothing, and never eat or drink in the lab. Hazard symbols include flammable, corrosive, oxidising, toxic and irritant.
在整个实验过程中必须佩戴护目镜。扎起长发，避免宽松衣物，绝不在实验室饮食。危险标志包括易燃、腐蚀性、氧化性、毒性和刺激性。
When heating a flammable liquid, use a water bath or electric heater rather than a naked flame. When diluting concentrated acid, always add acid to water slowly while stirring, never the reverse.
加热易燃液体时，应使用水浴或电热器，而不是明火。稀释浓酸时，一定要在不断搅拌下将酸缓慢加入水中，绝不能反过来。
Broken glass and chemical spills should be reported immediately. If acid spills on skin, wash with plenty of water.
破碎的玻璃和化学品泄漏应立即报告。如果酸溅到皮肤上，用大量水冲洗。
4. Separation Techniques | 分离技术
Filtration separates an insoluble solid from a liquid, for example sand from a mixture of sand and water. The residue remains in the filter paper and the filtrate passes through.
过滤可将不溶性固体从液体中分离出来，例如从沙和水的混合物中分离出沙。残渣留在滤纸上，滤液通过滤纸。
Simple distillation separates a solvent from a solution, such as pure water from salt water, using evaporation followed by condensation. Fractional distillation separates miscible liquids with different boiling points, such as ethanol and water.
简单蒸馏通过蒸发后冷凝，将溶剂从溶液中分离出来，例如从盐水中得到纯水。分馏则用于分离沸点不同的互溶液体，例如乙醇和水。
Crystallisation obtains a soluble salt from a solution by gentle heating to evaporate some water, then cooling to allow crystals to form. Paper chromatography separates mixtures of coloured or colourless substances.
结晶法通过温和加热蒸发部分水，再冷却使晶体析出，从而从溶液中得到可溶性盐。纸色谱法用于分离有色或无色物质的混合物。
In chromatography, the Rf value is calculated as distance moved by the substance divided by distance moved by the solvent front. Since both distances are measured in mm or cm, Rf has no unit.
在色谱法中，Rf值等于物质移动距离除以溶剂前沿移动距离。由于两个距离均以mm或cm为单位，Rf没有单位。
Rf = distance moved by substance ÷ distance moved by solvent front
5. Rates of Reaction Experiments | 反应速率实验
A common method to follow a reaction that produces a gas is to measure the volume of gas collected in a gas syringe over time, or to measure the loss in mass if the gas escapes from a flask on a balance.
研究产生气体的反应时，常用气体注射器测量随时间收集的气体体积；如果气体从烧瓶逸出，也可用天平测量质量损失。
Another method is the disappearing cross experiment, where sodium thiosulfate solution reacts with dilute hydrochloric acid to produce a yellow precipitate of sulfur. The time taken for a cross drawn under the flask to disappear is recorded.
另一种方法是“消失的十字”实验：硫代硫酸钠溶液与稀盐酸反应生成黄色硫沉淀。记录烧瓶下方画的十字消失所需的时间。
The rate can be calculated as 1 ÷ time, or as change in volume ÷ change in time. Repeat experiments at different temperatures or concentrations to investigate factors affecting rate.
速率可计算为1 ÷ 时间，或体积变化 ÷ 时间变化。在不同温度或浓度下重复实验，以研究影响反应速率的因素。
rate = 1 ÷ time
Keep the total volume constant when changing concentration by adding distilled water, so only the concentration of one reactant changes. This is a controlled variable.
改变浓度时，通过加入蒸馏水保持总体积不变，这样只有一种反应物的浓度发生变化。这是一个控制变量。
6. Energetics: Measuring Temperature Changes | 能量学：测量温度变化
Record the initial and final temperature of the solution; the temperature change is ΔT = Tfinal − T_initial.
要测量反应的焓变，可使用称为量热计的隔热杯。记录溶液初始和最终温度；温度变化为ΔT = Tfinal − T_initial。
s the mass of solution, c is the specific heat capacity, and ΔT is the temperature change.
放热反应温度升高；吸热反应温度降低。热量变化可用公式 q = m × c × ΔT 估算，其中m为溶液质量，c为比热容，ΔT为温度变化。
q = m × c × ΔT
In displacement or neutralisation experiments, use a plastic cup with a lid to reduce heat loss to the surroundings. Stir the mixture and record the maximum or minimum temperature reached.
在置换或中和实验中，使用带盖塑料杯以减少向环境的热量损失。搅拌混合物，并记录达到的最高或最低温度。
Plot a graph of temperature against time and extrapolate the cooling curve to find a more accurate maximum temperature.
绘制温度-时间图像，并外推冷却曲线，以获得更准确的最高温度。
7. Acid-Base Titration | 酸碱滴定
Titration is used to find the concentration of an acid or base. Rinse the burette with the solution it will contain, then fill it and record the initial reading. Use a pipette to transfer a known volume of alkali into a conical flask and add a few drops of indicator.
滴定用于测定酸或碱的浓度。用待装溶液润洗滴定管，然后装液并记录初始读数。用移液管将已知体积的碱液转移至锥形瓶中，并加入几滴指示剂。
Add acid from the burette while swirling until the indicator just changes colour. Record the final burette reading. The titre is the difference between final and initial readings.
边摇动边从滴定管加入酸，直到指示剂刚好变色。记录滴定管最终读数。滴定体积为最终读数与初始读数之差。
Repeat the titration until you obtain consistent results within 0.1 cm³ of each other. Use phenolphthalein for strong acid–strong base titrations; it changes from pink to colourless in acid.
重复滴定，直到获得彼此相差不超过0.1 cm³的一致结果。强酸-强碱滴定可使用酚酞；它在酸性条件下由粉红色变为无色。
les ÷ volume. Use the balanced equation ratio to convert moles of acid to moles of alkali.
使用公式浓度 = 物质的量 ÷ 体积进行计算。利用配平方程式中的物质的量比，将酸的物质的量换算为碱的物质的量。
c = n ÷ V
8. Salt Preparation and Purity | 盐的制备与纯度
Soluble salts can be prepared by reacting an acid with an insoluble base, such as copper(II) oxide or copper(II) carbonate, with sulfuric acid. Add the
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