# 元素、化合物与混合物

> 爱德思国际GCSE化学 · 4CH1 2017
> 来源: https://www.owlsprep.com/zh/study/edexcel-igcse-chemistry-s1-elements-compounds-and-mixtures/

本指南覆盖爱德思IGCSE化学4CH1子主题1(b)要求掌握的核心内容：元素、化合物与混合物的分类，纯度规则，核心分离技术，纸色谱法以及Rf值计算。

**先修:** [基础原子结构](https://www.owlsprep.com/zh/study/edexcel-igcse-chemistry-s1-atomic-structure/); [化学实验室的安全操作规范](https://www.owlsprep.com/zh/study/edexcel-igcse-chemistry-practical-skills/)

## 学习目标

- 将物质分类为元素、化合物或混合物
- 识记纯净物与混合物之间熔沸点的差异
- 为不同混合物选择并描述合适的分离技术
- 解读纸色谱图并计算Rf值以识别混合物组分
- 安全开展纸色谱相关实践探究

## 元素、化合物与混合物的分类

**核心物质分类** — 1. **元素**：仅由一种原子构成的纯净物，无法通过化学方法分解。2. **化合物**：由两种或多种元素以固定比例通过化学键结合形成的纯净物。3. **混合物**：两种或多种未发生化学键合的物质的组合，无固定比例，可通过物理方法分离。

*例:* Element = iron (Fe), Compound = sodium chloride (NaCl), Mixture = salt water

**例题:** 将下列物质分别归类为元素、化合物或混合物：a) 镁条，b) 硫酸铜，c) 空气

1. 步骤1：回忆每一类别的定义
2. 步骤2a：镁仅由镁原子构成，因此属于元素
3. 步骤2b：硫酸铜由铜、硫、氧原子以固定比例化学键合构成，因此属于化合物
4. 步骤2c：空气是氮气、氧气、二氧化碳等多种气体的组合，无固定比例，未发生化学键合，因此属于混合物

## 纯度与熔沸点特性

**纯净物** — 不含其他杂质的单一元素或化合物，拥有固定、明确的熔沸点。混合物（不纯物质）会在一个温度区间内熔化或沸腾。

**例题:** 某乙醇样品的熔化温度区间为-117°C到-114°C，而纯乙醇的熔点恰好为-114°C。该样品是否纯净？解释你的答案。

1. 步骤1：回忆纯净物拥有固定熔点这一性质
2. 步骤2：该样品的熔化温度跨度为3°C，并非在单一固定温度下熔化
3. 步骤3：结论：该样品不纯净（是乙醇与其他物质的混合物）

> **tip**
>
> 如果题目给出的是一个熔点区间，该物质一定是混合物/不纯的。如果给出的单一精确温度与该物质的已知熔点完全匹配，则该物质是纯净的。

## 混合物的分离技术

为应对考试，你必须能够为任意给定的混合物选择正确的分离技术，并描述每种技术的工作原理。下表总结了你需要识记的5种核心技术：

| Technique | Use case | How it works |
| --- | --- | --- |
| Filtration | Separate insoluble solid from liquid | Mixture poured through filter paper; solid residue stays on paper, liquid filtrate passes through |
| Crystallisation | Separate soluble solid from solution | Solution heated to evaporate some solvent, left to cool; pure solid crystals form, can be filtered and dried |
| Simple distillation | Separate solvent from dissolved solute in solution | Solution heated, solvent boils, vapour condenses in cool condenser, collected as pure liquid distillate |
| Fractional distillation | Separate miscible liquids with different boiling points | Mixture heated, vapours rise up fractionating column, liquid with lowest boiling point condenses first and is collected |
| Paper chromatography | Separate mixtures of soluble coloured substances (e.g. inks, food colourings) | Mixture spotted onto chromatography paper, solvent moves up paper, components separate as they move different distances |

**例题:** 请为下列每种混合物写出正确的分离技术：a) 从水中分离沙子，b) 从盐水中获得纯水，c) 混合液中的红色和蓝色食用色素，d) 从水（沸点100°C）中分离乙醇（沸点78°C）

1. 步骤1：将每种混合物匹配到对应的正确技术适用场景
2. 步骤2a：沙子不溶于水 → 过滤
3. 步骤2b：将溶剂（水）与溶解的溶质（盐）分离 → 简单蒸馏
4. 步骤2c：可溶性有色物质 → 纸色谱法
5. 步骤2d：沸点不同的可混溶液体 → 分馏

> **warning**
>
> 不要混淆简单蒸馏和分馏。如果混合物是两种或多种液体，使用分馏；如果是溶解在液体中的固体且你需要得到液体产物，使用简单蒸馏。如果你只想要得到固体产物，则使用结晶法。

## 纸色谱法与比移值（Rf）计算

纸色谱法根据混合物各组分在溶剂中的溶解度不同实现分离。溶解度越高的组分在色谱纸上向上移动的距离越远。色谱图可以判断物质是否纯净（仅出现一个斑点）还是混合物（出现多个斑点）。你可以通过将组分的Rf值与已知参考值比对来识别组分。

**比移值（Rf值）** — 溶质斑点移动的距离（测量至斑点中心）与溶剂前沿（溶剂在纸上到达的最远位置）移动的距离之比。Rf没有单位，取值始终在0到1之间。

*记法:* $R_f$

$$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$

**例题:** 某食用色素混合液的色谱图中，黄色斑点移动了4.2 cm，溶剂前沿距离原点线移动了12.0 cm。计算黄色染料的Rf值，结果保留2位有效数字。

1. 步骤1：写出Rf的计算公式
2. $$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$$
3. 步骤2：代入给出的数值
4. $$R_f = \frac{4.2}{12.0} = 0.35$$
5. 步骤3：确认数值在0到1之间且无单位。最终答案 = 0.35

> **tip**
>
> 如果斑点停留在原点线上（移动距离为0），其Rf值为0，说明它不溶于所用的溶剂。

*计算器:* allowed

## 纸色谱法实践探究

1. Draw a pencil origin line 1 cm from the bottom of the chromatography paper (pencil does not dissolve in solvent, so it won't move)
2. Spot the mixture and known reference substances onto the origin line, leave to dry
3. Pour a small volume of solvent into a beaker, make sure solvent level is below the origin line (so spots don't dissolve into the solvent in the beaker)
4. Place the chromatography paper into the beaker, cover with a lid to saturate the atmosphere with solvent vapour and prevent solvent evaporating too quickly
5. Remove the paper when the solvent front is near the top of the paper, mark the solvent front with a pencil immediately before it dries
6. Leave the paper to dry, then analyse the spots to identify components by matching Rf values or position to reference substances

**概念自测**

1. Why do you use a pencil to draw the origin line?

   - Pencil marks are darker than pen
   - Pencil graphite does not dissolve in solvent
   - Pencil is easier to erase if you make a mistake

   *答案:* Pencil graphite does not dissolve in solvent

   *解析:* Pen ink would dissolve in the solvent and move up the paper, ruining the chromatogram. Pencil graphite is insoluble so it stays in place.

2. Why must the solvent level be below the origin line?

   - To stop the spots dissolving into the solvent in the beaker
   - To make the solvent move faster up the paper
   - To prevent the solvent from evaporating

   *答案:* To stop the spots dissolving into the solvent in the beaker

   *解析:* If the solvent covers the origin line, the spotted mixture will wash off the paper into the beaker, and no separation will occur.

## 常见错误

- **错误做法:** Classifying a substance with a fixed melting point as a mixture
  - 原因: Pure elements and compounds have fixed melting/boiling points, only mixtures have ranges
  - 正确做法: Only classify substances with a melting/boiling range as impure/mixtures
- **错误做法:** Using simple distillation to separate ethanol and water
  - 原因: Simple distillation is for separating solvent from dissolved solute, not two miscible liquids
  - 正确做法: Use fractional distillation for mixtures of miscible liquids with different boiling points
- **错误做法:** Calculating Rf as distance of solvent front divided by distance of spot
  - 原因: Reverse calculation gives values >1 which are invalid, Rf formula requires spot distance first
  - 正确做法: Always divide the distance the spot moved by the distance the solvent front moved, measure to the centre of the spot
- **错误做法:** Using pen to draw the origin line on chromatography paper
  - 原因: Pen ink dissolves in the solvent and moves up the paper, contaminating the chromatogram
  - 正确做法: Always use a pencil to draw the origin line and mark the solvent front
- **错误做法:** Stating that Rf values have units of cm or m
  - 原因: Rf is a ratio of two distances, so units cancel out
  - 正确做法: Always give Rf values as a decimal between 0 and 1 with no units

## 速查表

| Key Concept | Key Fact / Formula |
| --- | --- |
| Element | Only one type of atom, pure, fixed melting point |
| Compound | Two+ elements chemically bonded, pure, fixed melting point |
| Mixture | Two+ substances not bonded, impure, melting/boiling range |
| Filtration | Insoluble solid + liquid separation |
| Crystallisation | Soluble solid + solution separation |
| Simple distillation | Solvent + dissolved solute separation |
| Fractional distillation | Miscible liquids with different boiling points separation |
| Paper chromatography | Soluble coloured substances separation |
| Rf Formula | $R_f = \frac{\text{spot distance}}{\text{solvent front distance}}$, no units, 0 ≤ Rf ≤ 1 |

## 下一步

Now that you have mastered elements, compounds and mixtures, you can move on to more core principles of chemistry content. Next, you will learn about atomic structure, which explains how elements form compounds via chemical bonding, and chemical formulae and equations, which use the fixed proportions of compounds to describe reactions. You will also apply separation techniques later when you study crude oil fractional distillation in the organic chemistry unit, and water purification in the inorganic chemistry section.

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来自 [OwlsPrep](https://www.owlsprep.com) —— A-Level / IB / AP / IGCSE 免费学习指南，依据官方考纲编写。原页面：https://www.owlsprep.com/zh/study/edexcel-igcse-chemistry-s1-elements-compounds-and-mixtures/