📚 The Particle Model & States of Matter | 粒子模型与物质状态
The particle model is one of the most powerful ideas in IGCSE Science. It explains why materials behave differently as solids, liquids and gases, and it underpins key ideas from diffusion to gas pressure. This article will guide you through the core definitions, evidence and exam-style applications you need for Edexcel IGCSE Science.
粒子模型是 IGCSE 科学中最强大的概念之一。它能解释为什么物质以固态、液态和气态表现出不同行为，也支撑着从扩散到气体压强的全部内容。本文将带你梳理核心定义、证据以及 Edexcel IGCSE 科学考试中的典型应用。
1. The Three States of Matter | 物质的三种状态
All matter is made of tiny particles called atoms, molecules or ions. In a solid, the particles are packed closely together in a regular arrangement and vibrate about fixed positions. A solid therefore has a definite shape and a definite volume, and it cannot be compressed.
所有物质都由微小的粒子（原子、分子或离子）组成。在固体中，粒子紧密堆积成规则排列，并在固定位置附近振动。因此固体有确定的形状和体积，而且无法被压缩。
In a liquid, the particles are still close together but the forces between them are weaker, so they can slide past one another. A liquid has a definite volume but no fixed shape; it takes the shape of the bottom of its container. Because the particles are almost touching, liquids are also very difficult to compress.
在液体中，粒子仍然靠得很近，但粒子间作用力较弱，因此它们可以相互滑动。液体有确定的体积，但没有固定形状，会随容器底部的形状而变化。由于粒子近乎相互接触，液体同样很难被压缩。
In a gas, the particles are far apart and move rapidly and freely in all directions. The forces between them are negligible. A gas has neither a fixed shape nor a fixed volume; it always fills the whole container available. Gases are easy to compress because there is a lot of empty space between the particles.
在气体中，粒子相距很远，并向各个方向快速自由运动，粒子间作用力可忽略不计。气体既没有固定形状也没有固定体积，总会充满整个可用容器。气体容易压缩，因为粒子之间存在大量空隙。
| Property | Solid 固体 | Liquid 液体 | Gas 气体 |
|---|---|---|---|
| Shape 形状 | Fixed 固定 | No fixed shape 无固定形状 | Fills container 充满容器 |
| Volume 体积 | Fixed 固定 | Fixed 固定 | No fixed volume 无固定体积 |
| Forces between particles 粒子间作用力 | Strong 强 | Weak 弱 | Negligible 可忽略 |
| Density 密度 | High 高 | Moderate 中等 | Very low 很低 |
You must be able to interpret these differences in exam questions. When asked about a property, always link it back to the arrangement, movement and forces of the particles.
你必须能够在考试中解释这些差异。当被问及某种性质时，一定要把它与粒子的排列、运动和作用力联系起来。
2. Using the Particle Model to Explain Evidence | 用粒子模型解释证据
The particle model is only useful if it explains everyday observations. In Edexcel IGCSE Science, you may be asked to use the model to explain an observation, so practise writing answers that name the particles and describe what they do.
粒子模型只有在能解释日常现象时才有价值。在 Edexcel IGCSE 科学考试中，你可能会被要求用该模型解释某个现象，所以要练习写出能点明粒子及其行为的答案。
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Solids are hard to compress because their particles are tightly packed with no empty space between them.
固体难以压缩，因为其粒子紧密堆积，粒子之间没有空隙。
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Liquids can flow and change shape because their particles can slide over one another, although they still stay close together.
液体能够流动并改变形状，是因为粒子虽然仍靠得很近，但能够相互滑过。
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Gases can be compressed and expand because their particles are far apart and the gaps between them can be made smaller or larger.
气体可以被压缩或膨胀，是因为粒子相距很远，粒子之间的空隙可以变小或变大。
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Heating a solid makes it expand slightly because the particles vibrate with greater energy and push slightly further apart. The particles themselves do not get bigger.
加热固体使其轻微膨胀，是因为粒子振动能量更大、彼此略微远离。粒子本身并没有变大。
A common exam mistake is to write that particles “expand” when heated. Never write this. The particles stay the same size; only their kinetic energy, vibration and average spacing increase.
考试中常见的错误是写“粒子受热膨胀”。绝不要这样写。粒子大小不变；改变的只是它们的动能、振动幅度和平均间距。
3. Changes of State | 状态变化
A change of state is a physical change, not a chemical change. No new substance is formed, and the particles themselves remain identical. The changes are explained by gains or losses of kinetic energy and by the forces between particles.
状态变化是物理变化，而不是化学变化。没有新物质生成，粒子本身保持不变。状态变化可以用粒子动能的变化以及粒子间作用力来解释。
| Change 变化 | Name 名称 | Energy 能量 |
|---|---|---|
| Solid → Liquid | Melting 熔化 | Energy absorbed 吸收能量 |
| Liquid → Solid | Freezing 凝固 | Energy released 释放能量 |
| Liquid → Gas | Boiling / Evaporation 沸腾 / 蒸发 | Energy absorbed 吸收能量 |
| Gas → Liquid | Condensation 凝结 | Energy released 释放能量 |
| Solid → Gas | Sublimation 升华 | Energy absorbed 吸收能量 |
| Gas → Solid | Deposition 凝华 | Energy released 释放能量 |
Melting and boiling require energy to overcome the forces holding particles together. Condensation and freezing release energy because particles slow down and the attractive forces pull them closer together. This is why steam condensing on your skin causes a scald: the condensation releases a large amount of energy.
熔化和沸腾需要能量来克服粒子间的作用力。凝结和凝固会释放能量，因为粒子减速并且吸引力把粒子拉得更近。这就是为什么水蒸气在皮肤上凝结会造成烫伤：凝结过程会释放大量能量。
Boiling and evaporation are not the same. Boiling occurs throughout the whole liquid at the boiling point, producing bubbles. Evaporation happens only at the surface of a liquid, at any temperature, and cools the liquid because the fastest particles escape.
沸腾和蒸发并不相同。沸腾发生在整个液体内部，在沸点温度进行并产生气泡；蒸发只发生在液体表面，在任何温度下都能进行，而且由于最快的粒子逸出，蒸发会使液体冷却。
4. Heating and Cooling Curves | 加热曲线与冷却曲线
When a pure substance is heated steadily, a temperature–time graph shows characteristic flat regions. These horizontal sections appear during melting and during boiling. The temperature stays constant because the energy being supplied is used to overcome the forces between particles, increasing their potential energy, rather than increasing their kinetic energy.
当纯净物质被持续加热时，温度–时间图像会出现特征性的水平段。这些水平段出现在熔化过程和沸腾过程中。温度保持不变，是因为外界供给的能量用于克服粒子间作用力、增加粒子的势能，而不是增加粒子的动能。
For example, when ice is heated, the temperature rises until it reaches 0 °C. During melting, the temperature stays at 0 °C until all the ice has become water. Then the temperature rises again until 100 °C, where it stays flat again while the water boils into steam.
例如，加热冰块时，温度会上升直至达到 0 °C。在熔化期间，温度保持在 0 °C，直到所有冰都变成水。随后温度继续上升到 100 °C，在沸腾变成水蒸气的过程中再次保持水平。
A cooling curve shows the reverse. When a gas cools, its temperature falls until condensation begins; the temperature then stays constant while the gas changes into a liquid, because energy is released to the surroundings. The same flat section appears during freezing.
冷却曲线则相反。气体冷却时温度下降，直到凝结开始；随后温度保持恒定，直到气体完全变成液体，因为能量持续释放到环境中。凝固过程中也会出现同样的水平段。
On a graph, remember: a sloping region means the kinetic energy of the particles is changing; a flat region means a change of state is happening and pure substances have sharp melting and boiling points.
在读图时请记住：倾斜段表示粒子动能正在改变；水平段表示正在发生状态变化，并且纯净物质的熔点和沸点是固定的。
5. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random motion. It happens in liquids and gases because their particles are free to move. Diffusion does not happen in solids because particles can only vibrate about fixed positions.
扩散是粒子由于其无规则运动而从高浓度区域向低浓度区域发生的净移动。扩散发生在液体和气体中，因为它们的粒子可以自由移动。固体中不发生扩散，因为粒子只能在固定位置附近振动。
A classic Edexcel experiment uses cotton wool soaked in ammonia solution and concentrated hydrochloric acid in opposite ends of a glass tube. Ammonia particles diffuse faster because ammonia molecules have a smaller mass. A white ring of ammonium chloride forms closer to the hydrochloric acid end. This proves that lighter particles diffuse faster.
Edexcel 的一个经典实验是在玻璃管两端分别放置浸有氨水和浓盐酸的棉花。氨粒子扩散得更快，因为氨分子质量较小。氯化铵白环会出现在靠近盐酸端的位置，这证明质量越小的粒子扩散越快。
Factors that speed up diffusion include: a higher temperature (particles have more kinetic energy), a steeper concentration gradient, and smaller particles. In a gas, diffusion is very fast; in liquids, it is much slower but still observable, such as the spreading of a dye in water.
加快扩散的因素包括：温度更高（粒子动能更大）、浓度梯度更大、以及粒子更小。气体中扩散非常快；液体中扩散慢得多，但仍然可以观察到，例如染料在水中扩散。
6. Gas Pressure | 气体压强
Gas pressure is caused by particles of a gas colliding with the walls of their container. Each collision exerts a tiny force, but billions of collisions every second combine to produce the pressure that instruments measure.
气体压强是由气体粒子与容器壁碰撞引起的。每次碰撞产生一个极小的力，但每秒钟数十亿次碰撞叠加起来，就形成了仪器测到的压强。
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If the temperature of a fixed mass of gas increases, the particles move faster, so they collide with the walls harder and more often. The pressure therefore increases.
如果固定质量气体的温度升高，粒子运动得更快，撞击器壁更猛烈、更频繁，因此压强增大。
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If the volume is reduced, the particles are closer together and collide with the walls more frequently. The pressure therefore increases. This explains how bicycle pumps and aerosols work.
如果体积减小，粒子之间更靠近，撞击器壁更频繁，因此压强增大。这就是打气筒和气溶胶的工作原理。
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If the temperature is constant and the volume doubles, the pressure halves, because particles hit each unit area of wall only half as often.
如果温度恒定且体积加倍，压强减半，因为粒子撞击单位面积器壁的频率只有原来的一半。
Exam answers should always say “collisions with the container walls”, not vague phrases such as “particles push out”. The force of one collision is tiny, so always connect pressure to the combined effect of very many collisions.
考试答案一定要写“与容器壁的碰撞”，不要写模糊的说法，如“粒子向外推”。单次碰撞的力非常小，因此要把压强与大量碰撞的累积效应联系起来。
7. Density | 密度
Density is a measure of how much mass is packed into a given volume. The equation used in Edexcel IGCSE Science is:
Density = Mass ÷ Volume → ρ = m ÷ V
Mass is usually measured in kilograms (kg) or grams (g), and volume in cubic metres (m³) or cubic centimetres (cm³). Density is therefore quoted in kg/m³ or g/cm³. Remember that 1 g/cm³ is equal to 1000 kg/m³, so water has a density of 1.0 g/cm³ or 1000 kg/m³.
质量通常用千克（kg）或克（g）表示，体积用立方米（m³）或立方厘米（cm³）表示，因此密度常用 kg/m³ 或 g/cm³ 表示。请记住 1 g/cm³ 等于 1000 kg/m³，所以水的密度是 1.0 g/cm³ 或 1000 kg/m³。
To measure the density of a regular solid, find its mass with a balance and its volume by measuring the sides and multiplying length × width × height. For an irregular solid, use a eureka (displacement) can: the volume of water displaced equals the volume of the object.
测量规则固体密度时，用天平称质量，再测量各边长度并用长 × 宽 × 高计算体积。测量不规则固体时，可使用溢水杯（排水罐）：排出的水体积等于物体体积。
Worked example: a block has a mass of 200 g and a volume of 50 cm³. Using ρ = m ÷ V, density = 200 ÷ 50 = 4 g/cm³. Converting to kg/m³ gives 4 × 1000 = 4000 kg/m³.
示例：一个物块质量为 200 g，体积为 50 cm³。由 ρ = m ÷ V，密度 = 200 ÷ 50 = 4 g/cm³。换算成 kg/m³ 为 4 × 1000 = 4000 kg/m³。
Ice floats on water because ice is less dense than liquid water, which is unusual. Most substances are denser as solids, but water expands slightly on freezing, increasing its volume and lowering its density.
冰漂浮在水面上，是因为冰的密度小于液态水，这很不寻常。大多数物质固态时密度更大，但水在结冰时会略微膨胀，体积增大，密度减小。
8. Brownian Motion | 布朗运动
Brownian motion is the continuous, random movement of tiny particles suspended in a fluid. It provides direct experimental evidence that particles of matter exist and are in constant motion.
布朗运动是指悬浮在流体中的微小粒子持续进行的无规则运动。它为物质粒子存在且永不停息地运动提供了直接的实验证据。
In the classic experiment, smoke particles in air are viewed under a microscope. The smoke particles are seen to dart about randomly, changing direction constantly. Why do they move? Because invisible air particles are colliding with the smoke particles from all directions at high speed. At any instant, one side receives a slightly stronger push, and the smoke particle is knocked off course. When the next collision comes from another direction, it moves again.
在经典实验中，用显微镜观察空气中的烟粒。可以看到烟粒四处乱窜、不断改变方向。为什么它们会移动？因为看不见的空气粒子正高速从各个方向撞击烟粒。在某一瞬间，某一侧受到的撞击稍强，烟粒就被撞偏方向；当下一次碰撞从另一方向到来时，它又向新方向移动。
The same effect can be seen with pollen grains in water; the larger visible particles jiggle because invisible water molecules are bombarding them. Brownian motion supports the kinetic theory of matter and explains why diffusion occurs even without stirring.
从水中花粉粒上也能看到同样的效果：可见的较大颗粒上下抖动，是因为看不见的水分子不断撞击它们。布朗运动支持了物质微粒理论，也解释了为什么不需要搅拌也会发生扩散。
9. Exam Tips and Common Mistakes | 考试技巧与常见错误
These are the points examiners repeatedly highlight in Edexcel IGCSE Science mark schemes. Practise using them in every particle-model answer.
以下是 Edexcel IGCSE 科学评分标准中考官反复强调的要点。请在每道粒子模型题中练习使用这些要点。
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Never write that particles “expand” when heated. They vibrate more and move further apart, but the particle size is unchanged.
绝不能写粒子受热“膨胀”。粒子振动加剧、间距增大，但粒子本身大小不变。
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Always use the word “kinetic energy” when explaining temperature changes. Temperature depends on the average kinetic energy of the particles.
解释温度变化时一定要用“动能”一词。温度取决于粒子的平均动能。
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Do not confuse melting with dissolving. Melting is a change of state of one pure substance; dissolving mixes a solute into a solvent.
不要把熔化与溶解混为一谈。熔化是单一纯净物质的状态变化；溶解是溶质混入溶剂中。
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In calculation questions, convert units carefully: 1 cm³ =
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