📚 Separation and Purification of Mixtures (Edexcel Combined Science 2.2/2.3) | 混合物的分离与纯化 (Edexcel 综合科学 2.2/2.3)
Every substance around us either exists as a single pure chemical or as part of a mixture. In the Edexcel Combined Science chemistry syllabus, mastering how to separate and purify mixtures is a fundamental skill. This topic builds on key concepts from atomic structure and states of matter, and it underpins later work in chemical analysis and industrial processes. We will explore the main separation techniques – filtration, crystallisation, distillation (simple and fractional), and chromatography – and learn to select the right method based on the physical properties of the components in a mixture.
我们周围的每一种物质要么是单一纯化学物，要么是混合物的一部分。在Edexcel综合科学化学大纲中，掌握混合物的分离与纯化是一项基本功。这个主题建立在原子结构和物质状态的基础知识之上，同时为后面的化学分析和工业过程打下根基。我们将逐一探讨主要的分离技术——过滤、结晶、蒸馏（简单蒸馏和分馏）以及色谱法——并学会根据混合物组分的物理性质来选择正确的方法。
1. Pure Substances, Mixtures and the Need for Separation | 纯物质、混合物与分离的必要性
In chemistry, a pure substance is a single element or compound that has a fixed melting point and a fixed boiling point. A mixture contains two or more different substances that are not chemically joined, so each keeps its own physical properties. Mixtures melt and boil over a range of temperatures, not at a sharp point. Because the components in a mixture are simply mixed together, we can separate them using physical techniques that exploit differences in solubility, particle size, boiling point or movement through a medium.
在化学中，纯物质是指具有固定熔点和固定沸点的单一元素或化合物。混合物则含有两种或两种以上未通过化学键结合的不同的物质，因此各组分会保留各自的物理性质。混合物的熔化和沸腾发生在一个温度区间内，而不是一个精确的温度点。正因为混合物中的组分只是简单地混合在一起，我们可以利用它们溶解度、颗粒大小、沸点或在介质中移动性的差异，通过物理方法将它们分开。
A key practical skill is to identify whether a sample is pure or a mixture by comparing its measured melting or boiling point with known data values. Pure water boils at exactly 100 °C at standard atmospheric pressure; seawater boils over a range starting slightly above 100 °C, indicating a mixture.
一项关键的实验技能是，通过将测得的熔点或沸点与已知数据值进行比较，判断样品是纯物质还是混合物。纯水在一个标准大气压下恰好在100 °C沸腾；而海水的沸腾温度会略高于100 °C并在一个区间内逐渐蒸发，表明它是一个混合物。
2. Filtration – Separating an Insoluble Solid from a Liquid | 过滤——从不溶性固体与液体的混合物中分离
Filtration is used to separate an insoluble solid from a liquid, for example, sand from water. The mixture is poured through a filter paper held in a funnel. The liquid (the filtrate) passes through the tiny pores of the filter paper, while the solid (the residue) is trapped on the paper. The driving force is gravity, and separation relies on the difference in particle size between the solid particles and the pores of the filter medium.
过滤用于分离不溶性固体与液体，例如沙子与水。将混合物倒入置于漏斗中的滤纸里，液体（滤液）会穿过滤纸的微孔，而固体（残渣）则被截留在滤纸上。分离的驱动力是重力，分离的依据是固体颗粒尺寸与过滤介质孔径之间的差异。
This technique is commonly used in school practicals to purify a muddy water sample before further treatment. It is also an essential step in the preparation of a pure dry sample of a salt when the salt is formed as a precipitate.
这一技术在学校实验中常用于对浑浊水样进行初步净化，以便后续处理。当盐以沉淀形式生成时，过滤也是制备纯净、干燥盐样品过程中必不可少的一步。
3. Crystallisation – Obtaining a Soluble Solid from a Solution | 结晶——从溶液中提取可溶性固体
When a soluble solid is dissolved in a liquid, filtration cannot separate them because the solid particles are broken down to the ionic or molecular level within the solution. Crystallisation is the technique used to recover the pure solid. The solution is gently heated to evaporate some of the solvent, making it more concentrated. When a hot saturated solution cools down, the solubility of the solid decreases, causing pure crystals to form and grow.
当可溶性固体溶解在液体中时，过滤无法将它们分开，因为固体颗粒在溶液中已经分解到了离子或分子层面。结晶便是用来回收纯固体的技术。将溶液温和加热，蒸发掉部分溶剂，使其变得更浓。当热的饱和溶液冷却时，固体的溶解度下降，纯晶体便会析出并长大。
The key steps are: place the solution in an evaporating basin, heat using a water bath or low Bunsen flame to avoid spitting, stop heating when crystals start to appear (point of crystallisation), then allow the remaining solution to cool slowly. The crystals are collected by filtration and dried between pieces of filter paper or in a low-temperature oven. This process is widely used to purify samples of salts such as copper(II) sulfate or potassium nitrate.
关键步骤包括：将溶液放入蒸发皿中，用水浴或本生灯小火加热以防止飞溅，当晶膜出现时（结晶点）停止加热，然后让剩余溶液缓慢冷却。晶体通过过滤收集，并在滤纸之间压干或在低温烘箱中干燥。这一过程广泛用于纯化硫酸铜或硝酸钾等盐的样品。
4. Simple Distillation – Separating a Liquid from a Solid Solution | 简单蒸馏——从溶解有固体的溶液中分离液体
Simple distillation is ideal for obtaining a pure liquid from a solution in which a solid is dissolved, for instance, pure water from salt water. The solution is heated in a flask. The liquid component boils and turns into vapour, which passes into a condenser. The condenser has a cold water jacket that cools the vapour back into a liquid, which is collected as the distillate. The dissolved solid remains behind in the flask because it has a much higher boiling point and does not vaporise under the conditions used.
简单蒸馏非常适合于从溶有固体的溶液中获取纯液体，例如从盐水中获得纯水。将溶液在烧瓶中加热，液体组分沸腾后变成蒸汽，蒸汽进入冷凝管。冷凝管的水外套将其冷却，蒸汽重新凝结为液体，收集起来即为馏出液。溶解的固体因沸点远高于溶剂而留在烧瓶内，不会在所用条件下汽化。
In the laboratory, anti-bumping granules are added to the flask to ensure smooth boiling. The thermometer should be placed with its bulb just at the side arm of the distillation head to measure the boiling point of the vapour accurately. This technique is used in both the preparation of distilled water for laboratory work and in large-scale desalination plants.
在实验室中，烧瓶内会加入防爆沸小瓷片以确保平稳沸腾。温度计的水银球应正好位于蒸馏头支管口位置，以准确测量蒸汽温度。这一技术既用于实验室制备蒸馏水，也用于大型海水淡化工厂。
5. Fractional Distillation – Separating Miscible Liquids | 分馏——分离互溶液体混合物
When two or more liquids are completely miscible with one another, simple distillation cannot achieve a clean separation if their boiling points are relatively close. Fractional distillation is the answer. A fractionating column is inserted between the flask and the condenser. This column is packed with glass beads or other materials that provide a large surface area for repeated condensation and evaporation cycles.
当两种或多种液体完全互溶且它们的沸点相差不大时，简单蒸馏无法实现干净的分离。分馏正是解决方案。在烧瓶与冷凝管之间插入一根分馏柱，柱内填满玻璃珠或其他能提供大表面积的材料，以实现反复冷凝和蒸发的循环。
The mixture is heated, and the vapour rising up the column progressively enriches in the more volatile component (the one with the lower boiling point). Each mini-distillation plate inside the column increases the purity of the vapour. The most volatile liquid reaches the top of the column first and passes into the condenser, while the less volatile components drain back down. This is the principle behind the industrial separation of crude oil into fractions such as petrol, kerosene and diesel, as well as the production of pure ethanol from fermentation mixtures.
加热混合物，沿分馏柱上升的蒸汽中，更易挥发的组分（沸点较低者）逐渐富集。柱内的每一级微型蒸馏塔板都会提高蒸汽的纯度。最易挥发的液体首先到达柱顶并进入冷凝管，而较难挥发的组分则回流入烧瓶中。这正是工业上从原油中分离出汽油、煤油和柴油等馏分的原理，也用于从发酵液中生产纯乙醇。
6. Paper Chromatography – Separating Solutes in a Mixture | 纸色谱法——分离混合物中的溶质
Paper chromatography is a powerful analytical technique used to separate small quantities of substances that are dissolved in a solvent. A spot of the mixture is placed on a pencil line near the bottom of a strip of chromatography paper. The paper is then placed upright in a beaker with a shallow layer of solvent. The solvent soaks up the paper by capillary action, carrying the components of the mixture upwards at different rates depending on their relative affinities for the stationary phase (the paper) and the mobile phase (the solvent).
纸色谱法是一种有力的分析技术，用于分离微量溶解在溶剂中的物质。在色谱纸条靠近底部的一条铅笔线上滴上混合物斑点，然后将纸条直立置于盛有浅层展开剂的烧杯中。溶剂因毛细管作用沿纸上升，根据不同组分对固定相（纸）和流动相（溶剂）的相对亲和力差异，各组分以不同速率向上迁移。
The result is a chromatogram – a pattern of spots at different heights. Components that are more soluble in the moving solvent travel further up the paper, while those that are more strongly attracted to the paper stay closer to the baseline. Paper chromatography is commonly used to separate dyes in food colourings, inks or plant pigments, and to identify unknown substances by comparison with known reference samples run in parallel.
结果是一张色谱图——不同高度上的一系列斑点。在流动相中溶解度更高的组分上升得更远，而与滤纸结合更紧密的组分则更靠近底线。纸色谱法常用来分离食用色素、墨水或植物色素中的染料，并可通过平行展开的已知标准品来鉴别未知物质。
7. Interpreting Chromatograms and Calculating Rf Values | 解读色谱图与计算Rf值
To make chromatographic results reproducible and quantitative, chemists calculate the retardation factor, Rf, for each separated substance. The Rf value is defined as the distance travelled by the centre of the spot divided by the distance travelled by the solvent front. Both distances are measured from the origin pencil line.
Rf = distance moved by substance / distance moved by solvent front
为了使色谱结果具有可重复性和定量性，化学家会计算每个分离物质的比移值Rf。Rf值的定义为斑点中心移动的距离除以溶剂前沿移动的距离。此处的距离都从起始铅笔线量起。
Rf = 物质移动距离 / 溶剂前沿移动距离
An Rf value is a dimensionless number always less than 1 (unless the substance moves exactly with the solvent front). Because a given compound has a consistent Rf value under identical conditions (same stationary phase, same mobile phase and same temperature), we can use it to help identify an unknown spot. In an exam, you may be asked to measure distances from a diagram of a chromatogram and calculate Rf values to one or two decimal places.
Rf是一个无量纲数，总是小于1（除非物质随溶剂前沿一起移动）。由于同一化合物在相同条件（相同的固定相、流动相和温度）下具有一致的Rf值，我们可以用它来帮助鉴定未知斑点。在考试中，可能会要求根据色谱图的示意图测量距离并计算Rf值，结果保留一位或两位小数。
A practical limitation is that some spots remain colourless after chromatography. These can be visualised by using a locating agent (such as ninhydrin for amino acids) or by viewing the chromatogram under ultraviolet light. The exact center of each spot must be used for the distance measurement.
实际操作的一个局限是，有些组分在展开后仍为无色斑点。这时可以使用显色剂（例如用于氨基酸的茚三酮）或在紫外光下观察来显示斑点。测量距离时必须以各斑点中心为准。
8. Choosing the Correct Separation Technique – A Decision Framework | 选择正确的分离技术——决策框架
Edexcel Combined Science exam questions frequently present a real-life mixture and ask students to suggest or justify a suitable separation method. The decision hinges on the physical states and properties of the components:
- Undissolved solid + liquid → Filtration
- quid → Crystallisation (to obtain the solid) or Simple distillation (to obtain the liquid)
- Two or more miscible liquids with different boiling points → Fractional distillation
- Small, dissolved coloured or colourless compounds for analysis → Paper chromatography
Edexcel综合科学的考题经常给出一个真实的混合物，要求学生提出并解释合适的分离方法。决策的关键在于组分的物理状态和性质：
- 未溶解的固体 + 液体 → 过滤
- 液体中溶解的固体 → 结晶（以获得固体）或简单蒸馏（以获得液体）
- 两种或多种不同沸点的互溶液体 → 分馏
- 少量溶解的有色或无色化合物用于分析 → 纸色谱法
It is vital to link the choice of method clearly to the property being exploited, such as "separate because of a large difference in boiling point" or "based on different solubility in the solvent and different adhesion to paper". Questions may also ask why a technique is not suitable – for example, filtration cannot separate salt from salty water because the salt is dissolved, forming a homogeneous mixture.
至关重要的是，要将方法的选择明确地与被利用的性质联系起来，例如“由于沸点差异大而分离”或“基于在溶剂中的溶解度以及对纸的吸附力不同”。题目也可能问为什么某种技术不适用——例如，过滤不能将盐从盐水中分离出来，因为盐已经溶解，形成了均一混合物。
9. Purifying Water – A Core Practical Application | 水的净化——核心实验应用
Within the Edexcel Combined Science specification, students are required to describe how to analyse and purify a water sample. The practical may involve measuring pH, determining the mass of dissolved solids, and using distillation to obtain pure water. A measured volume of water sample is placed in a weighed evaporating basin, heated gently to dryness without spitting, and the mass of the remaining solid residue is recorded. The mass of dissolved solids per unit volume can then be calculated, giving an indication of water hardness or purity.
在Edexcel综合科学的考纲中，要求学生描述如何分析和纯化水样。实验可能包括测量pH值、测定溶解固体的质量以及用蒸馏法获得纯水。将一定体积的水样置于已称重的蒸发皿中，小火加热至干且无飞溅，记录剩余固体残渣的质量，即可计算出单位体积中溶解固体的质量，从而反映水的硬度或纯度。
Though simple distillation effectively removes dissolved solids, it does not remove dissolved gases or substances with boiling points close to water. Nevertheless, the distillate collected is significantly purer and can be used to demonstrate that the original sample was a mixture by comparing boiling points. This hands-on context helps reinforce the choice between filtration, evaporation and distillation for water treatment.
虽然简单蒸馏能有效去除溶解的固体，但不能去除溶解的气体或沸点与水接近的物质。尽管如此，收集到的馏出液明显更纯，并可通过比较沸点来证明原样品是混合物。这个动手实践有助于巩固在水处理中过滤、蒸发和蒸馏之间的选择。
10. Linking Separation to Purity and Real-World Uses | 将分离与纯度及实际应用联系起来
Separation techniques are not just isolated laboratory skills; they are central to quality control in the pharmaceutical, food and petroleum industries. The purity of medicines must be verified by chromatography and melting point analysis. Drinking water must meet strict legal limits for dissolved solids, bacteria and heavy metals. Fractional distillation of liquid air separates oxygen, nitrogen and argon for medical and industrial use. Being able to connect a separation method to the properties of the starting mixture and the desired product is a higher-order skill that Edexcel examiners love to test.
分离技术并非孤立的实验室技能；它们在制药、食品和石油工业的质量控制中处于核心地位。药品的纯度必须通过色谱法和熔点分析来验证。饮用水必须符合严格的法规限值，包括溶解固体、细菌和重金属含量。液态空气的分馏可分离出供医疗和工业使用的氧气、氮气和氩气。能够将分离方法与起始混合物的性质及目标产物联系起来，是Edexcel考官喜欢考察的高阶技能。
Whether you are designing a purification protocol for a newly synthesised compound or simply explaining how to obtain salt from rock salt in a six-mark question, a clear understanding of the underlying physical principles will always be your best tool. Remember: the art of separation lies in looking at what is different between the components and using that difference to pull them apart.
无论你是在为一个新合成的化合物设计纯化方案，还是在六分题中解释如何从岩盐中提取食盐，对背后物理原理的透彻理解永远是你最好的工具。请记住：分离的艺术在于寻找组分之间的差异，并利用这些差异将它们分开。
Published by TutorHao | Combined Science Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图