元素、化合物与混合物
爱德思国际GCSE化学· 1.8–1.13 (子主题1(b))· 15 分钟阅读
1. 元素、化合物与混合物的分类★☆☆☆☆⏱ 3 min
核心物质分类
- 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
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
2. 纯度与熔沸点特性★★☆☆☆⏱ 2 min
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。
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）
3. 混合物的分离技术★★★☆☆⏱ 4 min
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 |
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
4. 纸色谱法与比移值（Rf）计算★★★☆☆⏱ 4 min
✓ 计算器
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。
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
- 3
步骤2：代入给出的数值
- 4
- 5
步骤3：确认数值在0到1之间且无单位。最终答案 = 0.35
5. 纸色谱法实践探究★★☆☆☆⏱ 2 min
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)
Spot the mixture and known reference substances onto the origin line, leave to dry
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)
Place the chromatography paper into the beaker, cover with a lid to saturate the atmosphere with solvent vapour and prevent solvent evaporating too quickly
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
Leave the paper to dry, then analyse the spots to identify components by matching Rf values or position to reference substances
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
显示答案
1 —Pen ink would dissolve in the solvent and move up the paper, ruining the chromatogram. Pencil graphite is insoluble so it stays in place.
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
6. 常见陷阱
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
7. 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 | , no units, 0 ≤ Rf ≤ 1 |
8. 常见问题
我如何通过熔点判断一种物质是否纯净？
纯净物在单一固定温度下熔化，而混合物会在一个温度区间内熔化。例如，纯水的熔点恰好为0°C，但盐水的熔点会根据盐浓度在-5°C到0°C之间变化。
考试时我需要牢记Rf的计算公式吗？
是的，你必须熟记Rf的计算公式：。Rf值没有单位，取值始终在0到1之间。
简单蒸馏和分馏的区别是什么？
简单蒸馏用于将溶剂从溶解的溶质中分离出来（例如从盐水中分离出水）。分馏则使用分馏柱分离沸点不同的互溶液体（例如乙醇和水）。
深入阅读
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.