📚 Separating Mixtures – Fractional Distillation | 分离混合物——分馏
When two or more liquids are mixed together, they can look like a single liquid even though they are not chemically combined. Fractional distillation uses differences in boiling point to separate such liquid mixtures into pure components.
当两种或更多液体混合在一起时，虽然它们没有发生化学结合，但看起来可能像一种单一的液体。分馏利用沸点的差异把这类液体混合物分离成较纯的组分。
1. What is fractional distillation? | 什么是分馏？
Fractional distillation is a separation technique used to split a mixture of liquids into its individual parts, or fractions. The liquids must have different boiling points so that they can be separated by heating.
分馏是一种将液体混合物拆分为各个组成部分（即馏分）的分离技术。这些液体的沸点必须不同，这样才可以通过加热将它们分开。
Unlike filtration or evaporation, fractional distillation collects more than one useful liquid product. It is often used when every component of the mixture is valuable.
与过滤或蒸发不同，分馏可以收集不止一种有用的液体产物。当混合物中每一种组分都有价值时，常使用分馏。
2. Key principle: different boiling points | 关键原理：沸点不同
Every pure liquid has a fixed boiling point. For example, pure water boils at 100 °C and ethanol boils at about 78 °C at normal atmospheric pressure.
每种纯液体都有固定的沸点。例如，在标准大气压下，纯水在 100 °C 沸腾，乙醇约在 78 °C 沸腾。
When a mixture is heated, the liquid with the lower boiling point boils first. Its vapour rises, leaving the other components behind.
加热混合物时，沸点较低的液体会先沸腾。它的蒸气上升，其他组分则留在原处。
This means fractional distillation is a physical method based on the property of boiling point, not on chemical reactions.
这意味着分馏是一种基于沸点性质的物理方法，而不是依靠化学反应。
3. Simple distillation and fractional distillation compared | 简单蒸馏与分馏的比较
Simple distillation is used to separate a solvent from a solution, such as pure water from salt water. It works well when the boiling points are very far apart.
简单蒸馏用于从溶液中分离溶剂，例如从盐水中得到纯水。当沸点相差很大时，简单蒸馏效果很好。
Fractional distillation is used for a mixture of two or more liquids that have similar boiling points. A special column helps separate them by repeated evaporation and condensation.
分馏则用于两种或多种沸点相近的液体混合物。一个特殊的分馏柱通过多次蒸发和冷凝帮助分离它们。
In the laboratory, both methods use a flask, a condenser, and a receiving vessel, but fractional distillation has a fractionating column between the flask and condenser.
在实验室中，两种方法都会使用烧瓶、冷凝管和接收容器，但分馏在烧瓶和冷凝管之间多了一根分馏柱。
4. The laboratory apparatus | 实验室装置
The main apparatus includes a round-bottom flask, a fractionating column, a thermometer, a condenser, and several collection tubes or beakers.
主要装置包括圆底烧瓶、分馏柱、温度计、冷凝管以及多个收集试管或烧杯。
The flask holds the liquid mixture and is heated gently. The thermometer is placed at the top of the fractionating column to measure the temperature of the vapour entering the condenser.
烧瓶盛放液体混合物并被缓慢加热。温度计放在分馏柱顶部，用来测量进入冷凝管的蒸气温度。
The condenser has a cold water jacket. Cold water enters at the bottom and leaves at the top so that the inner tube stays fully cool.
冷凝管带有冷水夹套。冷水从下端进入、从上端流出，这样内管可以始终保持冷却。
A fractionating column may be packed with glass beads. These beads increase the surface area for evaporation and condensation.
分馏柱中可以填充玻璃珠。这些玻璃珠增加了蒸发和冷凝的表面积。
5. What happens in the fractionating column? | 分馏柱中发生了什么？
The fractionating column is the key part of this technique. It is cooler at the top and hotter at the bottom.
分馏柱是这项技术的关键部分。它的顶部温度较低，底部温度较高。
Hot vapour rises from the flask. When it meets cooler glass beads or surfaces, it condenses back into a liquid.
热蒸气从烧瓶上升。当它遇到较冷的玻璃珠或表面时，会冷凝回液体。
As more hot vapour rises, it re-heats this condensed liquid, causing it to evaporate again. Each evaporation–condensation cycle makes the vapour richer in the lower-boiling liquid.
随着更多热蒸气上升，它会重新加热这些冷凝液体，使其再次蒸发。每一次蒸发–冷凝循环都使蒸气中低沸点液体的含量更高。
By the time vapour reaches the top of the column, it is almost entirely the lowest-boiling component.
当蒸气到达分馏柱顶部时，它几乎完全是沸点最低的组分。
6. Step-by-step separation process | 分步分离过程
First, pour the liquid mixture into the flask and add a few anti-bumping granules to make boiling smooth.
首先，将液体混合物倒入烧瓶，并加入几粒防暴沸颗粒，使沸腾更平稳。
Second, heat the flask gently. The liquid with the lowest boiling point boils first, and its vapour rises through the fractionating column.
第二，缓慢加热烧瓶。沸点最低的液体先沸腾，其蒸气上升通过分馏柱。
Third, when the thermometer reading stays constant near the first boiling point, collect the drops of distilled liquid in a clean container.
第三，当温度计读数稳定在第一个沸点附近时，用干净的容器收集蒸馏出的液体。
Fourth, when the first fraction has been collected, raise the temperature gradually to boil the next component and collect it separately.
第四，收集完第一个馏分后，逐渐升高温度使下一个组分沸腾，并单独收集它。
7. Reading the thermometer and changing fractions | 温度计读数与更换馏分
The thermometer should show a steady temperature while a pure fraction is being collected. A sudden rise suggests the next substance has started to distil.
收集纯馏分时，温度计应当显示稳定的温度。温度突然上升说明下一种物质已经开始蒸馏。
If the temperature changes quickly, the receiving tube should be changed so different fractions do not mix.
如果温度变化很快，就应更换接收管，以免不同馏分混合。
For an ethanol–water mixture, the first fraction is collected at around 78 °C. The fraction collected near 100 °C is mainly water.
对于乙醇和水的混合物，第一个馏分在约 78 °C 收集。在 100 °C 附近收集的馏分主要是水。
8. Collecting fractions and checking purity | 收集馏分与检验纯度
Each liquid collected over a narrow temperature range is called a fraction. If the boiling point range is narrow, the fraction is likely to be purer.
在一个较窄温度范围内收集到的液体称为馏分。如果沸点范围很窄，该馏分通常更纯。
In school experiments, fractions can be tested by measuring their boiling points, checking density, or observing how easily they evaporate.
在学校实验中，可以通过测量沸点、检查密度或观察蒸发的难易程度来检验馏分。
Do not drink any collected liquid, even if it looks like water, because it may contain harmful impurities.
不要饮用任何收集到的液体，即使它看起来像水，因为其中可能含有有害杂质。
9. Fractional distillation of crude oil | 原油的分馏
Crude oil is a complex mixture of many different hydrocarbons. It is useless in its raw form, so it is separated into useful fractions in an oil refinery.
原油是许多不同碳氢化合物的复杂混合物。它未经加工时用途不大，因此要在炼油厂中分离成有用的馏分。
In industry, crude oil is heated in a furnace and pumped into a huge fractionating tower. The tower is hot at the bottom and cooler towards the top.
在工业上，原油在炉中加热，然后被送入巨大的分馏塔。塔的底部温度高，越往上温度越低。
Fractions with long molecules and high boiling points condense near the bottom. Short-chain fractions with low boiling points rise higher before condensing.
分子较长、沸点较高的馏分在接近塔底处冷凝。短链、沸点低的馏分会升得更高后再冷凝。
Published by TutorHao | KS3 Chemistry Revision Series | aleveler.com
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