📚 Classification of Matter | 物质的分类
Understanding how to classify matter is fundamental to chemistry. In IB Chemistry Structure 3: Classification of Matter, students explore the distinction between pure substances and mixtures, learn to identify elements and compounds, and examine how the particle model explains the properties of solids, liquids and gases. This article provides a comprehensive revision guide covering key concepts, separation techniques, and common IB exam questions.
理解物质的分类是化学的基础。在 IB 化学 S3：物质的分类中，学生将探究纯物质与混合物的区别，学会识别元素和化合物，并研究粒子模型如何解释固体、液体和气体的性质。本文提供全面的复习指南，涵盖关键概念、分离技术以及常见的 IB 考题。
1. The Concept of Matter and Its Classification | 物质的概念及其分类
Matter is defined as anything that has mass and occupies space. All materials around us, from the air we breathe to the water we drink, are forms of matter.
物质被定义为任何具有质量并占据空间的东西。我们周围的所有材料，从呼吸的空气到饮用的水，都是物质的形式。
At the macroscopic level, matter can be classified into pure substances and mixtures. Pure substances have a definite and constant composition, while mixtures consist of two or more substances physically combined in variable proportions.
在宏观层面上，物质可分为纯物质和混合物。纯物质具有确定且恒定的组成，而混合物由两种或多种物质以可变的物理方式组合而成。
Pure substances are further divided into elements and compounds. Mixtures can be homogeneous (uniform composition) or heterogeneous (non-uniform composition). This classification is essential for understanding chemical behaviour and separation methods.
纯物质进一步分为元素和化合物。混合物可以是均相的（组成均匀）或非均相的（组成不均匀）。这种分类对于理解化学行为和分离方法至关重要。
2. Pure Substances: Elements | 纯物质：元素
An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is made up of atoms with the same number of protons, defining its atomic number.
元素是一种纯物质，不能通过化学手段分解成更简单的物质。每种元素由质子数相同的原子组成，质子数定义了其原子序数。
Elements are represented by chemical symbols, such as O for oxygen, Fe for iron, and Na for sodium. Some elements exist as single atoms (e.g., helium, neon), while others form diatomic molecules like O₂ or polyatomic structures like S₈.
元素用化学符号表示，例如 O 代表氧，Fe 代表铁，Na 代表钠。有些元素以单原子形式存在（如氦、氖），而另一些则形成双原子分子如 O₂ 或多原子结构如 S₈。
There are about 118 known elements arranged in the periodic table. In IB Chemistry, you need to recall the symbols and names of the first 20 elements, as well as common ones like Br, I, Ag, Au, and Pb.
已知约有 118 种元素排列在周期表中。在 IB 化学中，你需要记住前 20 种元素的符号和名称，以及常见的如 Br、I、Ag、Au 和 Pb。
Elements can be classified as metals, non-metals, and metalloids based on their physical and chemical properties. Metals are typically shiny, conductive, and malleable, whereas non-metals are often brittle and poor conductors.
根据物理和化学性质，元素可分为金属、非金属和类金属。金属通常有光泽、导电且具有延展性，而非金属通常脆且导电性差。
3. Pure Substances: Compounds | 纯物质：化合物
A compound is a pure substance formed when two or more different elements chemically combine in a fixed ratio. The composition of a compound is always the same, as stated by the law of constant composition.
化合物是由两种或多种不同元素以固定比例化学结合而成的纯物质。根据定组成定律，化合物的组成始终相同。
Compounds are represented by chemical formulas, such as H₂O for water, CO₂ for carbon dioxide, and NaCl for sodium chloride. The properties of a compound are entirely different from those of its constituent elements.
化合物用化学式表示，例如水 H₂O、二氧化碳 CO₂ 和氯化钠 NaCl。化合物的性质与其组成元素的性质完全不同。
For example, sodium is a highly reactive metal and chlorine is a poisonous green gas, but sodium chloride is harmless table salt essential for life. Compounds can only be separated into their elements by chemical reactions, not by physical methods.
例如，钠是一种高活性金属，氯是一种有毒的绿色气体，但氯化钠是生命必需的无害食盐。化合物只能通过化学反应分解为元素，不能通过物理方法分离。
Compounds may be molecular (formed from non-metals, held by covalent bonds) or ionic (formed between metals and non-metals, held by ionic bonds). Understanding bonding type is crucial for predicting properties.
化合物可以是分子的（由非金属形成，通过共价键结合）或离子的（由金属和非金属形成，通过离子键结合）。理解键型对于预测性质至关重要。
4. Mixtures: Homogeneous and Heterogeneous | 混合物：均相与非均相
A mixture is a physical combination of two or more substances that retain their individual properties. Unlike compounds, mixtures do not have a fixed composition and can be separated by physical means.
混合物是两种或多种物质的物理组合，这些物质保留其各自的性质。与化合物不同，混合物没有固定的组成，可以通过物理方法分离。
Mixtures are classified as homogeneous or heterogeneous. A homogeneous mixture, also called a solution, has a uniform composition throughout. Examples include saltwater, air, and alloys like steel.
混合物分为均相混合物和非均相混合物。均相混合物又称溶液，其整体组成均匀。例子包括盐水、空气和钢等合金。
In a homogeneous mixture, the particles are evenly distributed at the molecular level, and you cannot distinguish the components visually. Air is a homogeneous mixture of nitrogen, oxygen, and other gases.
在均相混合物中，粒子在分子水平上均匀分布，肉眼无法区分各组分。空气是氮气、氧气和其他气体的均相混合物。
A heterogeneous mixture has a non-uniform composition, meaning the different components can often be seen and separated by simple physical methods. Examples include sand in water, oil and water, and salad.
非均相混合物组成不均匀，意味着不同的组分通常可以被看到，并可通过简单的物理方法分离。例子包括水中的沙子、油水混合物和沙拉。
5. States of Matter and the Particle Model | 物质状态与粒子模型
Matter exists in three primary states: solid, liquid, and gas. The state depends on the arrangement and motion of particles, which is explained by the kinetic particle theory.
物质以三种主要状态存在：固态、液态和气态。状态取决于粒子的排列和运动，这由动力学粒子理论解释。
In solids, particles are closely packed in a regular arrangement and can only vibrate about fixed positions. This gives solids a definite shape and volume.
在固态中，粒子紧密排列成规则结构，只能在固定位置振动。这赋予固体确定的形状和体积。
In liquids, particles are still close together but can move past one another, allowing liquids to flow and take the shape of their container while having a fixed volume.
在液态中，粒子仍然紧密，但可以彼此滑动通过，使液体能够流动并呈现容器形状，同时保持固定体积。
In gases, particles are far apart and move randomly at high speeds. Gases have no fixed shape or volume and are easily compressed because of the large spaces between particles.
在气态中，粒子相距很远并以高速随机运动。气体没有固定的形状或体积，并且由于粒子间间隔大，容易压缩。
The average kinetic energy of particles is directly proportional to the absolute temperature, expressed by the equation:
KE = ½ mv²
. This relationship is fundamental to understanding changes of state.粒子的平均动能与绝对温度成正比，用方程式表示为：
KE = ½ mv²
。这一关系对于理解状态变化至关重要。6. Physical Properties and Chemical Properties | 物理性质与化学性质
Physical properties are characteristics that can be observed or measured without changing the chemical identity of the substance. These include melting point, boiling point, density, colour, hardness, and electrical conductivity.
物理性质是无需改变物质化学身份即可观察或测量的特性。包括熔点、沸点、密度、颜色、硬度和导电性。
Chemical properties describe how a substance reacts or changes into a new substance. Flammability, reactivity with acids, and oxidation states are all chemical properties.
化学性质描述物质如何反应或转化为新物质。可燃性、与酸的反应性以及氧化态都属于化学性质。
A physical change, such as melting ice, does not produce a new substance and is usually reversible. A chemical change, like burning magnesium, produces new substances and involves energy changes.
物理变化，如冰融化，不会产生新物质，通常是可逆的。化学变化，如镁燃烧，产生新物质并涉及能量变化。
In IB exams, you may be asked to distinguish between physical and chemical properties based on experimental observations. For example, boiling water is physical, while effervescence during a reaction indicates a chemical change.
在 IB 考试中，你可能会被要求根据实验观察区分物理性质和化学性质。例如，水沸腾是物理变化，而反应过程中产生气泡则表明化学变化。
7. Purity and How to Determine It | 纯度及其测定
A pure substance, whether an element or compound, has fixed melting and boiling points under constant pressure. This is a direct consequence of the uniform forces between its particles.
纯物质，无论是元素还是化合物，在恒定压力下具有固定的熔点和沸点。这是其粒子间均匀作用力的直接结果。
The presence of impurities lowers the melting point and broadens the melting range. Instead of melting sharply at one temperature, an impure substance melts over a range of temperatures.
杂质的存在会降低熔点并扩大熔程。不纯物质不会在某一个温度下突然熔化，而是在一个温度范围内熔化。
Melting point determination is a common technique to assess purity. Pure aspirin, for instance, melts sharply at 135 °C. An impure sample will start melting earlier and finish later.
熔点测定是评估纯度的常用技术。例如，纯阿司匹林在 135 °C 会急剧熔化。不纯样品会提前开始熔化并延迟完成。
A heating or cooling curve of a pure substance shows a flat plateau at the melting or freezing point, where the temperature remains constant while the state changes. Mixtures do not exhibit this sharp plateau.
纯物质的加热或冷却曲线在熔点或凝固点显示一个平台，温度在状态变化时保持不变。混合物不会显示这种尖锐的平台。
IB exam questions often ask you to analyse such curves and identify whether a sample is pure or a mixture, and to explain the effect of impurities on melting point.
IB 考题经常要求分析此类曲线，判断样品是纯物质还是混合物，并解释杂质对熔点的影响。
8. Separation Techniques for Mixtures | 混合物的分离技术
Since mixtures are physical combinations, they can be separated by exploiting differences in physical properties. The choice of technique depends on the type of mixture and the properties of its components.
由于混合物是物理组合，可以利用物理性质的差异将其分离。技术的选择取决于混合物的类型及其组分的性质。
The table below summarises the main separation techniques required for IB Chemistry S3, including the principle and a typical application.
下表总结了 IB 化学 S3 要求的主要分离技术，包括原理和典型应用。
| Technique | 分离技术 | Principle & Example (English) | 原理与示例 (中文) |
|---|---|---|---|
| Filtration | 过滤 | Separates an insoluble solid from a liquid using filter paper; e.g. sand from water. | 利用滤纸将不溶性固体与液体分离；例如沙子与水。 |
| Evaporation / Crystallisation | 蒸发 / 结晶 | Obtains a dissolved solid from a solution by evaporating the solvent; e.g. salt from seawater. | 通过蒸发溶剂从溶液中获取溶解的固体；例如从海水中提取盐。 |
| Simple Distillation | 简单蒸馏 | Separates a liquid from a soluble solid or from another liquid with widely different boiling points; e.g. pure water from ink. | 将液体与可溶性固体或沸点差异较大的另一种液体分离；例如从墨水中提取纯水。 |
| Fractional Distillation | 分馏 | Separates miscible liquids with close boiling points using a fractionating column; e.g. ethanol and water, crude oil components. | 使用分馏柱分离沸点相近的互溶液体；例如乙醇和水、原油组分。 |
| Paper Chromatography | 纸层析 | Separates small amounts of substances based on solubility and adsorption; used for dyes, amino acids, and forensic analysis. | 基于溶解度和吸附性分离少量物质；用于染料、氨基酸和法医分析。 |
| Magnetic Separation | 磁选 | Uses a magnet to remove magnetic materials from non-magnetic ones; e.g. iron filing from sulfur powder. | 利用磁铁将磁性材料从非磁性材料中分离；例如从硫粉中分离铁屑。 |
In IB experiments, you may need to design a separation procedure for a given mixture. Always justify your choice based on the physical properties involved.
在 IB 实验中，你可能需要为给定的混合物设计分离流程。务必根据所涉及的物理性质来证明你的选择。
9. Distinguishing Elements, Compounds and Mixtures at the Particle Level | 在粒子层面区分元素、化合物和混合物
IB Chemistry often tests your ability to interpret particle diagrams where circles represent atoms. A diagram showing only one type of atom (same circles) represents an element, which could be monatomic or diatomic.
IB 化学经常测试你解读粒子图的能力，图中用圆圈表示原子。只显示一种原子（相同的圆圈）的图表示元素，可以是单原子的也可以双原子的。
If the diagram shows two different types of atoms chemically bonded together in a fixed ratio throughout, it indicates a compound. Water, for example, would be shown as two hydrogen circles bonded to an oxygen circle repeated uniformly.
如果图显示两种不同类型的原子以固定比例化学键合在一起并均匀分布，则表明是化合物。例如，水会表示为两个氢圆与一个氧圆键合，并均匀重复出现。
A mixture is represented by a diagram where two or more different types of particles are not chemically combined. This can be a mix of element atoms and compound molecules, or different elements not bonded to each other.
混合物由两种或多种不同类型粒子未化学结合的图表示。这可以是元素原子和化合物分子的混合，或者不同的元素之间没有键合。
Common exam questions provide a box containing several circles and ask you to identify if the sample is an element, compound, or mixture. Remember: a pure substance contains only one type of particle, whereas a mixture contains two or more.
常见的考题提供一个包含多个圆圈的方框，要求你识别该样品是元素、化合物还是混合物。记住：纯物质只含有一种类型的粒子，而混合物含有两种或多种。
10. Exam Tips and Common Misconceptions | 考试提示与常见误区
Many students wrongly assume that all solutions are liquids. A solution can be solid (brass, an alloy), liquid (seawater), or gas (air). The key is the uniform distribution of components.
许多学生错误地认为所有溶液都是液体。溶液可以是固体（黄铜，一种合金）、液体（海水）或气体（空气）。关键在于组分的均匀分布。
Alloys are homogeneous mixtures of metals, not compounds, because the metals are not chemically combined in a fixed ratio. Steel is a mixture of iron and carbon.
合金是金属的均相混合物，不是化合物，因为金属并未以固定比例化学结合。钢是铁和碳的混合物。
Air is a mixture, not a compound, despite having a roughly constant composition in dry conditions. Its components can be separated by physical means, such as fractional distillation of liquid air.
空气是混合物，不是化合物，尽管在干燥条件下组成大致恒定。其组分可以通过物理方法分离，例如液态空气的分馏。
When a substance dissolves, it is a physical change, not a chemical one. The dissolved particles are simply dispersed, and the original substance can be recovered by evaporating the solvent.
物质溶解时是物理变化，不是化学变化。溶解的粒子只是被分散，原物质可以通过蒸发溶剂回收。
In exam answers, always use precise terminology: say 'fixed composition' for compounds, 'variable composition' for mixtures, and link separation techniques to differences in physical properties. Practice drawing and labelling diagrams of distillation and filtration.
在考试答案中，务必使用精确术语：对于化合物说“固定组成”，对于混合物说“可变的组成”，并将分离技术与物理性质的差异联系起来。练习绘制并标注蒸馏和过滤的示意图。
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