📚 States of Matter and the Particle Model | 物质的状态与粒子模型
In the Cambridge Lower Secondary Science 7 course, one of the most important ideas is the particle model of matter. This model helps you explain why solids, liquids and gases behave differently, and it is used throughout chemistry and physics. By imagining matter as tiny moving particles, you can understand changes of state, diffusion, gas pressure and expansion.
在剑桥初中科学 7 年级课程中，最重要的概念之一就是物质的粒子模型。这个模型帮助你解释为什么固体、液体和气体的行为不同，并且在化学和物理中都会反复用到。把物质想象成微小的运动粒子，你就能理解物态变化、扩散、气体压强和热胀冷缩等现象。
1. The Three States of Matter | 物质的三态
Solid, liquid and gas are the three common states of matter on Earth. A solid has a fixed shape and a fixed volume. A liquid has a fixed volume but takes the shape of its container. A gas has no fixed shape and no fixed volume, so it spreads out to fill the whole container.
固体、液体和气体是地球上物质常见的三种状态。固体有固定的形状和固定的体积；液体有固定的体积，但形状会随容器而变；气体既没有固定的形状，也没有固定的体积，因此会扩散并充满整个容器。
Everyday examples include ice (solid water), liquid water and water vapour (gaseous water). The same substance can exist in all three states if the temperature and pressure are suitable. For instance, water changes from ice to liquid to vapour as it is heated.
日常例子包括冰（固态水）、液态水和水蒸气（气态水）。如果温度和压强合适，同一种物质可以以三种状态存在。例如，水受热时会从冰变成液体，再变成水蒸气。
2. The Particle Model | 粒子模型
The particle model states that all matter is made up of tiny particles that are too small to see. These particles are always moving, and the amount of movement depends on the state and temperature. The model is a scientific way of explaining the behaviour of solids, liquids and gases.
粒子模型认为，所有物质都由小得看不见的粒子组成。这些粒子总是在运动，运动程度取决于物质的状态和温度。该模型是解释固体、液体和气体行为的科学方法。
In a solid, particles are arranged in a regular pattern and vibrate in fixed positions. In a liquid, particles are close together but can slide past each other. In a gas, particles are far apart and move rapidly in all directions.
在固体中，粒子排列规则，只能在固定位置振动；在液体中，粒子彼此靠近，但能相互滑动；在气体中，粒子相距很远，并朝各个方向快速运动。
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes container shape | Fills container |
| Volume | Fixed | Fixed | Not fixed |
| Particle arrangement | Regular, close | Random, close | Random, far apart |
| Particle movement | Vibrate only | Slide past each other | Move rapidly in all directions |
The table above summarises the key differences between solids, liquids and gases in terms of shape, volume, particle arrangement and particle movement. In exams, you will often be asked to use the particle model to explain these macroscopic properties.
上表总结了固体、液体和气体在形状、体积、粒子排列和粒子运动方面的主要区别。考试中常要求你用粒子模型解释这些宏观性质。
3. Melting and Freezing | 熔化与凝固
Melting is the change from solid to liquid. It occurs at the melting point. Freezing is the reverse change, from liquid to solid, and for a pure substance it happens at the same temperature as melting.
熔化是固体变为液体的过程，发生在熔点。凝固是相反的转变，即液体变为固体；对于纯净物，凝固点和熔点相同。
During melting, particles gain energy and vibrate faster. Eventually they break free from their fixed positions and become able to slide past each other. During freezing, particles lose energy and settle into a regular pattern.
熔化时，粒子获得能量，振动加快。最终，粒子摆脱固定位置，可以相互滑动。凝固时，粒子失去能量，重新排列成规则的图案。
A pure ice cube melts at 0 °C, and liquid water freezes at 0 °C. Impurities such as salt lower the melting point, which is why salt is spread on icy roads in winter.
纯冰块在 0 °C 熔化，液态水在 0 °C 凝固。盐等杂质会降低熔点，所以冬天人们会在结冰的路面上撒盐。
4. Boiling and Condensation | 沸腾与凝结
Boiling is the change from liquid to gas. It happens throughout the liquid at the boiling point. Condensation is the change from gas to liquid, and it occurs when a gas is cooled.
沸腾是液体变为气体的过程，在沸点时整个液体内部都会发生。凝结是气体变为液体的过程，当气体冷却时就会发生。
When a liquid boils, particles gain enough energy to overcome the attractive forces holding them together. They escape from the surface and from inside the liquid as bubbles. When a gas condenses, particles slow down and move closer together.
液体沸腾时，粒子获得足够能量，克服了将粒子拉在一起的吸引力。它们从液体表面和内部以气泡的形式逸出。气体凝结时，粒子速度减慢，彼此靠近。
Water boils at 100 °C at normal atmospheric pressure. Water vapour condenses on a cold window because the gas particles lose energy when they touch the cool surface.
在标准大气压下，水在 100 °C 沸腾。水蒸气在冷窗户上凝结，是因为气体粒子接触冷表面时失去能量。
5. Evaporation vs Boiling | 蒸发与沸腾
Evaporation is different from boiling. Evaporation only happens at the surface of a liquid and can occur at any temperature below the boiling point. Boiling happens throughout the liquid at a specific temperature.
蒸发与沸腾不同。蒸发只发生在液体表面，可以在低于沸点的任何温度下进行。沸腾则在整个液体内部发生，并且需要达到特定温度。
Evaporation explains why puddles dry up on a cool day, and why sweating cools your skin. The fastest-moving particles escape from the surface, so the average kinetic energy of the remaining particles decreases, lowering the temperature.
蒸发可以解释为什么凉天里水坑会变干，以及为什么出汗能让皮肤降温。运动最快的粒子从表面逃逸，剩余粒子的平均动能降低，因此温度下降。
Factors that increase evaporation include higher temperature, larger surface area, and moving air. A wet cloth dries faster on a warm, windy day because these factors help particles escape more quickly.
加快蒸发的因素包括温度更高、表面积更大以及空气流动。湿布在温暖有风的日子里干得更快，因为这些因素帮助粒子更快逸出。
6. Sublimation and Deposition | 升华与凝华
Some substances can change directly between solid and gas without becoming liquid. This is called sublimation when a solid turns into a gas, and deposition when a gas turns into a solid.
有些物质可以不经过液态，直接在固态和气态之间转变。固体直接变为气体叫做升华，气体直接变为固体叫做凝华。
Solid carbon dioxide (dry ice) and iodine are common examples. Dry ice sublimes at room temperature, producing a white fog that is often used in theatres. Iodine crystals form a purple vapour when heated gently.
固态二氧化碳（干冰）和碘是常见的例子。干冰在室温下升华，产生舞台常用的白色雾气。碘晶体受微热时会形成紫色蒸气。
Sublimation and deposition are physical changes because the particles themselves do not change chemically. They simply move further apart or closer together.
升华和凝华属于物理变化，因为粒子本身并没有发生化学变化。它们只是间距变大或变小。
7. Diffusion in Liquids and Gases | 液体和气体中的扩散
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. It happens in liquids and gases because particles are free to move and collide with each other.
扩散是粒子从浓度较高的区域向浓度较低的区域运动的过程。它发生在液体和气体中，因为粒子可以自由运动并相互碰撞。
Diffusion is fastest in gases, slower in liquids, and does not happen in solids because the particles cannot change positions. For example, the smell of perfume spreads through a room by diffusion in air.
气体中扩散最快，液体中较慢，固体中不会发生扩散，因为粒子不能改变位置。例如，香水的香味就是通过空气中的扩散在房间里传播的。
The rate of diffusion increases with temperature because particles move faster. It also depends on particle mass: lighter particles diffuse more quickly than heavier ones.
扩散速率随温度升高而加快，因为粒子运动更快。扩散速率还取决于粒子质量：较轻的粒子比较重的粒子扩散得更快。
8. Gas Pressure and the Particle Model | 气体压强与粒子模型
Gas pressure is caused by gas particles colliding with the walls of their container. Each collision exerts a tiny force, and the combined effect of many collisions produces pressure.
气体压强是气体粒子撞击容器壁产生的。每次撞击都会产生一个微小的力，大量撞击的总和就形成了压强。
If the temperature of a gas increases, the particles move faster and collide more often and with greater force, so the pressure increases if the volume stays the same. If the volume is increased, the particles have more space and hit the walls less often, so pressure decreases.
如果气体温度升高，粒子运动加快，碰撞更频繁、更有力，因此若体积不变，压强会增大。如果体积增大，粒子有更多空间，撞击容器壁的次数减少，压强就会减小。
This relationship explains why a balloon gets harder when you squeeze it: you reduce the volume, so particles hit the walls more often. It also explains why a sealed can may burst if heated strongly.
这种关系解释了为什么挤压气球会变硬：你减小了体积，粒子撞击壁面的频率增加。它也解释了为什么密封罐在剧烈加热时可能爆裂。
9. Expansion and Contraction | 热胀冷缩
Most substances expand when heated and contract when cooled. The particle model explains this by changes in particle movement and spacing, not by changes in the size of the particles themselves.
大多数物质受热膨胀、遇冷收缩。粒子模型用粒子运动和间距的变化来解释这一现象，而粒子本身的大小并没有改变。
In solids, particles vibrate more and take up slightly more space when heated. Railways and bridges have expansion gaps to allow for this. In liquids, thermometers rely on the expansion of liquid to measure temperature. In gases, hot air balloons use expansion and lower density to rise.
固体受热时粒子振动更剧烈，占据的空间略大。铁轨和桥梁都留有伸缩缝。液体温度计依靠液体的膨胀来测量温度。气体中，热气球利用气体膨胀和密度降低来上升。
Water is unusual because it expands slightly when it freezes. This is why ice floats on liquid water and why pipes can burst in very cold weather.
水很不寻常，因为它在结冰时会略微膨胀。这就是冰浮在液态水上以及管道在极冷天气中可能爆裂的原因。
10. Physical and Chemical Changes | 物理变化与化学变化
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