📚 Evaporation and Distillation: Separating Mixtures | 蒸发与蒸馏：混合物的分离
In KS3 chemistry, you meet many useful mixtures such as salt water, ink and muddy water. This article explains two key physical separation methods: evaporation and distillation. Evaporation removes a liquid from a solution to leave a dissolved solid behind; distillation goes one step further by cooling the vapour back into a liquid. These methods rely on changes of state, not chemical reactions.
在 KS3 化学中，你会遇到许多有用的混合物，例如盐水、墨水和浑水。本文解释两种重要的物理分离方法：蒸发和蒸馏。蒸发从溶液中除去液体，留下溶解的固体；蒸馏则更进一步，将蒸气冷却回液体。这些方法依靠物态变化，而不是化学反应。
1. Mixtures and Separation | 混合物与分离
A mixture contains two or more substances that are not chemically bonded. Each substance keeps its own properties, so we can separate them using physical methods. Filtration separates insoluble solids; evaporation and distillation are used for soluble solids and liquids.
混合物含有两种或两种以上没有化学键结合的物质。每种物质保持自己的性质，因此我们可以用物理方法分离。过滤分离不溶性固体；蒸发和蒸馏用于可溶性固体和液体。
The choice of method depends on what we want to recover. If we want the dissolved solid, evaporation is useful. If we want the pure liquid, distillation is better because it collects the liquid.
方法的选择取决于我们要回收什么。如果要溶解的固体，蒸发很有用。如果要纯液体，蒸馏更好，因为它收集液体。
2. Key Terms: Solute, Solvent, Solution | 关键术语：溶质、溶剂、溶液
A solute is the solid or gas that dissolves. A solvent is the liquid that does the dissolving. A solution forms when the solute spreads evenly through the solvent.
溶质是溶解的固体或气体。溶剂是起溶解作用的液体。当溶质均匀分散在溶剂中时，就形成溶液。
In salt water, salt is the solute and water is the solvent. Evaporation removes the solvent; distillation separates the solvent from the solute and collects the solvent as a pure liquid.
在盐水中，盐是溶质，水是溶剂。蒸发除去溶剂；蒸馏将溶剂从溶质中分离出来，并将溶剂收集为纯液体。
salt water → water vapour ↑ + solid salt
盐水 → 水蒸气 ↑ + 固体盐
3. How Evaporation Works | 蒸发如何起作用
Evaporation happens when particles at the surface of a liquid gain enough energy to escape as gas. Heating speeds up evaporation because more particles have enough energy to break free from the liquid.
当液体表面的粒子获得足够能量以气体形式逸出时，就会发生蒸发。加热加速蒸发，因为更多粒子有足够能量脱离液体。
In a solution, only the solvent evaporates easily at the temperatures used. The dissolved solute remains behind because it usually has a much higher boiling point.
在溶液中，在所用温度下通常只有溶剂容易蒸发。溶解的溶质留在后面，因为它的沸点通常高得多。
This is a physical change: water vapour is still water. No new substance is made, so it is reversible in principle.
这是物理变化：水蒸气仍然是水。没有生成新物质，因此原则上可逆。
4. Evaporation to Recover a Solute | 用蒸发回收溶质
To recover a soluble solid, pour the solution into an evaporating basin and heat gently. Use a water bath or a low Bunsen flame to avoid spitting. As water evaporates, the solution becomes more concentrated.
要回收可溶性固体，将溶液倒入蒸发皿并缓慢加热。使用水浴或小火加热，避免飞溅。随着水分蒸发，溶液变得更浓。
When most water has gone, stop heating and leave the rest to cool. Crystals slowly form. For example, blue copper sulfate crystals can be collected after evaporating water from copper sulfate solution.
当大部分水已经蒸发时，停止加热并让剩余溶液冷却。晶体慢慢形成。例如，蓝色的硫酸铜晶体可以从硫酸铜溶液中蒸发水分后收集。
Do not heat to dryness if you want well-shaped crystals. Strong heating can damage some salts and cause hot solid to spit.
如果想要形状完好的晶体，不要加热至干。强热会破坏某些盐，并导致热固体飞溅。
5. Crystallisation | 结晶
Crystallisation is the process of forming solid crystals from a solution. Slow evaporation gives particles time to arrange into an ordered pattern, producing larger and purer crystals.
结晶是从溶液中形成固体晶体的过程。缓慢蒸发让粒子有时间排列成有序结构，形成更大、更纯的晶体。
A saturated solution contains the maximum amount of solute dissolved at that temperature. Cooling a hot saturated solution often makes crystals appear because solubility decreases with temperature for most solids.
饱和溶液含有该温度下能溶解的最大量溶质。冷却热饱和溶液通常会出现晶体，因为大多数固体的溶解度随温度降低而减小。
6. Simple Distillation: The Principle | 简单蒸馏：原理
Simple distillation combines evaporation and condensation to separate a solvent from a solution. The solution is boiled, the vapour travels away, and then it is cooled back into liquid in a condenser.
简单蒸馏结合蒸发和冷凝，将溶剂从溶液中分离出来。溶液沸腾，蒸气离开，然后在冷凝器中冷却回液体。
Distillation is used when we want to keep the liquid, not just remove it. For example, we can obtain clean water from seawater by simple distillation.
当我们想保留液体而不仅仅是除去它时，使用蒸馏。例如，我们可以通过简单蒸馏从海水中获得洁净的水。
The solute stays in the flask because it does not evaporate at the boiling point of the solvent. This means the collected liquid is much purer than the original mixture.
溶质留在烧瓶中，因为它在溶剂沸点不蒸发。这意味着收集到的液体比原混合物纯净得多。
7. Simple Distillation Apparatus | 简单蒸馏装置
The main parts of simple distillation apparatus are a round-bottom flask, a thermometer, a condenser with a water jacket, and a receiving beaker or flask. Anti-bumping granules are added to make boiling smooth.
简单蒸馏装置的主要部分有圆底烧瓶、温度计、带水套的冷凝器和接收烧杯或烧瓶。加入防暴沸颗粒使沸腾平稳。
The thermometer bulb should be placed at the neck of the flask, near the side arm. This measures the temperature of the vapour entering the condenser, not the temperature of the liquid.
温度计球应放在烧瓶颈部、支管附近。这测量进入冷凝器的蒸气温度，而不是液体温度。
The condenser has an inner tube carrying hot vapour and an outer jacket carrying cold water. The cold water enters at the bottom and leaves at the top so the condenser stays full and cooling is efficient.
冷凝器有一根内管输送热蒸气，和一个外夹套输送冷水。冷水从底部进入、从顶部流出，使冷凝器保持充满，冷却效率高。
8. The Distillation Process Step by Step | 蒸馏步骤
First, set up the apparatus safely: flask clamped securely, condenser supported, and joints connected. Add anti-bumping granules to the solution.
首先，安全安装装置：烧瓶夹稳，冷凝器支撑好，接头连接好。向溶液中加入防暴沸颗粒。
Second, turn on the condenser water before heating. This ensures vapour is cooled as soon as it reaches the condenser.
第二，在加热前先打开冷凝器水。这确保蒸气一到达冷凝器就被冷却。
Third, heat the solution gently. The liquid boils, vapour rises into the condenser, cools back into liquid, and drips into the receiving flask as distillate.
第三，缓慢加热溶液。液体沸腾，蒸气上升进入冷凝器，冷却回液体，滴入接收烧瓶成为馏出液。
Finally, record the steady temperature. For pure water at normal pressure, the boiling point should be about 100 °C. Stop heating before the flask dries out completely.
最后，记录稳定温度。对于常压下的纯水，沸点应约为 100 °C。在烧瓶完全蒸干前停止加热。
9. Evaporation vs Distillation | 蒸发与蒸馏的对比
Evaporation and distillation both use the idea that a liquid can turn into vapour. However, evaporation usually loses the liquid into the air, while distillation captures the vapour and condenses it into a separate container.
蒸发和蒸馏都利用液体可以变成蒸气的原理。然而，蒸发通常让液体散失到空气中，而蒸馏捕获蒸气并将其冷凝到另一个容器中。
| Feature / 特征 | Evaporation / 蒸发 | Distillation / 蒸馏 |
|---|---|---|
| Purpose / 目的 | Recover dissolved solid / 回收溶解固体 | Recover pure liquid / 回收纯液体 |
| State change / 物态变化 | Liquid to gas only / 液体到气体 | Liquid to gas to liquid / 液体到气体到液体 |
| Apparatus / 装置 | Evaporating basin / 蒸发皿 | Flask, condenser, receiver / 烧瓶、冷凝器、接收器 |
10. Fractional Distillation: Separating Liquids | 分馏：分离液体
Simple distillation works well when separating a liquid from a dissolved solid. When two liquids are mixed and both can boil, simple distillation may not separate them cleanly. Fractional distillation is used instead.
简单蒸馏在从溶解的固体中分离液体时效果很好。当两种液体混合且都能沸腾时，简单蒸馏可能无法完全分离。这时使用分馏。
Fractional distillation uses a fractionating column packed with glass beads or plates. The column is hot at the bottom and cooler at the top. Different liquids condense at different heights and can be collected separately.
分馏使用填充玻璃珠或塔板的分馏柱。柱底温度高、柱顶温度低。不同液体在不同高度冷凝，可以分别收集。
An important example is separating ethanol and water. Ethanol boils at about 78 °C, lower than water. The fractionating column helps keep water vapour from passing over too easily.
一个重要例子是分离乙醇和水。乙醇沸点约为 78 °C，低于水。分馏柱有助于防止水蒸气太容易通过。
11. Real-World Applications | 实际应用
Desalination plants use distillation to produce fresh water from seawater in dry regions. Although evaporation by the Sun is cheaper, distillation is useful when energy is available.
海水淡化厂利用蒸馏在干旱地区从海水中生产淡水。虽然太阳蒸发更便宜，但有能源时蒸馏很有用。
The oil industry separates crude oil into petrol, diesel, kerosene and other fractions by fractional distillation. Each fraction contains molecules with similar boiling points.
石油工业通过分馏将原油分离为汽油、柴油、煤油和其他馏分。每种馏分含有沸点相近的分子。
In laboratories, distillation is used to purify water or recover a solvent from a reaction mixture.
在实验室中，蒸馏用于纯化水或从反应混合物中回收溶剂。
12. Common Misconceptions and Safety | 常见误区与安全
Misconception 1: ‘Distillation is the same as boiling.’ In fact, distillation includes condensation and collection. Boiling alone only sends vapour into the air.
误区 1：「蒸馏和沸腾是一样的
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