📚 KS3 Chemistry Unit 6 Review: Particles, Changes of State and Acids | KS3化学第6单元复习：粒子、状态变化与酸碱
The particle model, changes of state, mixtures and the chemistry of acids and alkalis form the backbone of KS3 Chemistry Unit 6. This review walks through every key idea, pairs each explanation with the language examiners expect, and finishes with a checklist so you can find gaps before a test.
粒子模型、状态变化、混合物以及酸和碱的性质，构成了KS3化学第6单元的主干。这份复习逐条梳理核心概念，把每个解释与考官期待的表达方式对应起来，并在最后给出清单，方便你在测验前找出漏洞。
1. How to Use This Unit 6 Review | 如何使用这份第6单元复习
Unit 6 links three big stories: what matter is made of, how matter changes when it is heated or mixed, and how chemists classify and separate substances. Acids and alkalis bring all three together because they involve particles, energy and reactions at the same time.
第6单元串联三条主线：物质由什么构成、物质在加热或混合时如何变化、化学家如何分类与分离物质。酸和碱把这三条主线合在一起，因为它们同时涉及粒子、能量和反应。
A good way to revise is to move from description to explanation. First check that you can describe what you see (a solid melts, a gas spreads, an indicator turns red). Then check that you can explain it using particles. Finally check that you can predict something new, such as what happens when you add acid to a carbonate.
有效的复习路径是从"描述"走向"解释"。先检查自己能否描述现象（固体熔化、气体扩散、指示剂变红），再检查能否用粒子模型解释这些现象，最后检查能否做出预测，例如把酸加入碳酸盐时会发生什么。
Marks in KS3 assessments usually fall into three groups: recall of facts and definitions, application of ideas to unfamiliar situations, and practical or data skills. If you can only do the first group, your grade will be limited. Spend most of your revision time on the second and third groups.
KS3测评的分数通常分布在三类任务中：事实与定义的记忆、把概念应用到陌生情境、以及实验与数据技能。如果只会第一类，成绩就会受限。把大部分复习时间花在第二类和第三类上。
2. The Particle Model: Core Ideas | 粒子模型的核心思想
The particle model explains the properties of solids, liquids and gases. It says that all matter is made of small particles that are always moving. The model rests on four assumptions: matter is made of tiny particles, there are spaces between the particles, the particles are in constant random motion, and there are forces of attraction between them.
粒子模型解释固体、液体和气体的性质。它指出所有物质都由不断运动的小粒子组成。这个模型建立在四个假设之上：物质由微小粒子构成；粒子之间存在间隙；粒子处于持续的无规则运动中；粒子之间存在吸引力。
Temperature controls the average speed of the particles. Heating transfers energy to the particles, so their average kinetic energy increases and they move faster. Cooling removes energy, so they move more slowly and the forces of attraction hold them together more effectively.
温度决定粒子的平均运动速度。加热把能量传递给粒子，粒子的平均动能增加，运动加快。冷却带走能量，粒子运动减慢，粒子之间的吸引力能更有效地把它们聚在一起。
It is important to remember that the particle model is a model, not a photograph. Real particles are not coloured balls, and the gaps are not empty rooms. The model is useful because it correctly predicts behaviour such as diffusion, gas pressure, expansion on heating and the fixed volume of a liquid.
要记住，粒子模型是模型，不是照片。真实的粒子不是彩色小球，粒子间隙也不是空房间。这个模型之所以有用，是因为它能正确预测扩散、气体压强、受热膨胀以及液体体积固定等行为。
heating → particles gain kinetic energy → move faster → forces overcome → state changes
加热 → 粒子获得动能 → 运动加快 → 克服吸引力 → 状态改变
3. Solids, Liquids and Gases Compared | 固、液、气三态对比
Solids have a fixed shape and volume because their particles are packed closely and only vibrate about fixed positions. Liquids have a fixed volume but take the shape of their container because particles can flow past one another. Gases have no fixed shape or volume because particles are far apart and move rapidly in all directions.
固体有固定的形状和体积，因为粒子紧密排列且只能在固定位置附近振动。液体有固定体积但会呈现容器的形状，因为粒子可以相互滑动。气体没有固定形状或体积，因为粒子相距很远并快速向各个方向运动。
| State | Arrangement | Movement | 中文说明 |
|---|---|---|---|
| Solid | Closely packed, regular | Vibrate about fixed positions | 紧密规则排列，在固定位置振动 |
| Liquid | Close, irregular | Slide past each other | 紧密但不规则，相互滑动 |
| Gas | Far apart, random | Move quickly in all directions | 相距很远，随机快速运动 |
The table above is worth memorising in full, because almost every explanation question in Unit 6 begins with arrangement and movement. If a question asks why a gas can be compressed but a liquid cannot, the answer is about the size of the gaps between particles, not about the particles themselves changing size.
上表值得完整记住，因为第6单元几乎每道解释题都从"排列方式"和"运动方式"入手。如果题目问为什么气体可以被压缩而液体不能，答案在于粒子间隙的大小，而不是粒子本身变小了。
- Compressibility: gases are easy to compress because of large gaps; liquids and solids are almost incompressible. | 可压缩性：气体因间隙大而易压缩；液体和固体几乎不可压缩。
- Density: gases are usually least dense; solids are usually most dense, though water is an exception because ice floats. | 密度：气体密度通常最小，固体通常最大，但水是例外，因为冰会浮在水上。
- Diffusion: fastest in gases, slower in liquids, extremely slow in solids. | 扩散：气体中最快，液体中较慢，固体中极其缓慢。
- Flow: liquids and gases flow; solids do not, because their particles cannot change position. | 流动性：液体和气体可以流动，固体不行，因为其粒子无法改变位置。
4. Changes of State and Conservation of Mass | 状态变化与质量守恒
Changes of state include melting, freezing, boiling, evaporating, condensing and subliming. During these changes, the mass stays the same because particles are not created or destroyed. Only the arrangement and the energy of the particles change. This is why a change of state is a physical change, not a chemical one.
状态变化包括熔化、凝固、沸腾、蒸发、冷凝和升华。在这些变化过程中，质量保持不变，因为粒子既没有生成也没有消失，改变的只是粒子的排列方式和能量。这正是状态变化属于物理变化而非化学变化的原因。
| Change | Direction | Energy | 中文说明 |
|---|---|---|---|
| Melting | Solid → Liquid | Energy taken in | 熔化：固→液，吸收能量 |
| Freezing | Liquid → Solid | Energy released | 凝固：液→固，放出能量 |
| Boiling | Liquid → Gas | Energy taken in | 沸腾：液→气，吸收能量 |
| Evaporating | Liquid → Gas | Energy taken in | 蒸发：液→气，吸收能量 |
| Condensing | Gas → Liquid | Energy released | 冷凝：气→液，放出能量 |
| Subliming | Solid → Gas | Energy taken in | 升华：固→气，吸收能量 |
A common exam trap is to confuse boiling with evaporating. Boiling happens throughout the liquid at one fixed temperature, producing bubbles of gas inside the liquid. Evaporation happens only at the surface, at any temperature, and is faster when the temperature is higher, the surface area is larger or there is a draught.
常见陷阱是把沸腾与蒸发混淆。沸腾发生在整个液体内部，只在某一固定温度进行，并在液体内部产生气泡。蒸发只发生在液体表面，可在任何温度下进行；温度越高、表面积越大或有气流时，蒸发越快。
mass before change = mass after change (particles are conserved)
变化前质量 = 变化后质量（粒子守恒）
5. Energy, Heating Curves and Temperature | 能量、加热曲线与温度
A heating curve for water shows temperature plotted against time. The ice warms up, then the temperature stops rising at 0 °C while the ice melts, then the water warms up, then the temperature stops rising again at 100 °C while the water boils.
水的加热曲线以温度为纵轴、时间为横轴。冰先升温；到达0 °C后温度不再上升，冰开始熔化；熔化完成后水继续升温；到达100 °C温度再次停止上升，水开始沸腾。
The flat sections are called plateaus. During a plateau, energy is still being supplied, but it is used to overcome the forces of attraction between particles rather than to increase their average speed. Since temperature measures average kinetic energy, the temperature does not change while the state is changing.
这些水平段称为平台段。在平台段中，能量仍在持续供给，但被用于克服粒子之间的吸引力，而不是提高粒子的平均速度。由于温度反映的是平均动能，所以在状态改变期间温度保持不变。
For extension work, the energy needed to raise the temperature of a substance can be calculated using the equation below, where m is mass in kilograms, c is specific heat capacity in J/kg°C and ΔT is the temperature change in °C.
作为拓展内容，物质升温所需能量可用下面的公式计算，其中 m 是质量（千克），c 是比热容（J/kg°C），ΔT 是温度变化（°C）。
energy transferred = m × c × ΔT
传递的能量 = m × c × ΔT
Note that the melting point and boiling point of a pure substance are sharp and fixed, while an impure substance melts and boils over a range of temperatures. This is one reason why melting point data can be used to test purity.
注意，纯净物的熔点和沸点是明确而固定的，而含有杂质的物质会在一个温度范围内熔化或沸腾。这也是为什么可以用熔点数据来判断物质纯度。
6. Gas Pressure and Diffusion | 气体压强与扩散
Gas pressure is caused by particles colliding with the walls of their container. Every collision exerts a tiny force, and millions of collisions per second add up to a measurable pressure. If the gas is heated, the particles move faster, collide more often and collide harder, so the pressure increases.
气体压强来源于粒子与容器壁的碰撞。每一次碰撞都施加一个微小的力，而每秒数百万次的碰撞累积成可测量的压强。如果给气体加热，粒子运动加快，碰撞更频繁、更猛烈，压强随之增大。
If the container is squeezed to a smaller volume while the temperature stays the same, the particles hit the walls more often, so the pressure rises again. This explains why a sealed syringe is easy to push at first and then becomes very hard to push further.
如果在温度不变的情况下把容器压缩到更小的体积，粒子撞击容器壁的频率增加，压强又会升高。这解释了为什么密封注射器一开始容易推动，越往后越难推动。
Diffusion is the spreading of particles from an area of higher concentration to an area of lower concentration, caused by random motion. A drop of purple potassium manganate(VII) in water spreads through the whole beaker over time without any stirring, which is direct evidence for the particle model.
扩散是粒子由于无规则运动，从高浓度区域向低浓度区域散布的过程。把一滴紫色高锰酸钾加入水中，无需搅拌，颜色最终会扩散到整杯水，这正是粒子模型的直接证据。
- Diffusion is faster at higher temperatures because particles have more kinetic energy. | 温度越高扩散越快，因为粒子动能更大。
- Diffusion is faster for lighter particles and in gases, because there are fewer collisions between particles. | 质量较小的粒子以及气体中的扩散更快，因为粒子间碰撞更少。
- Diffusion happens in liquids and gases, but extremely slowly in solids because particles cannot move from place. | 扩散发生在液体和气体中，在固体中极其缓慢，因为粒子无法移动位置。
7. Pure Substances, Mixtures and Separation | 纯净物、混合物与分离
A pure substance contains only one type of particle, or one compound, and has sharp melting and boiling points. A mixture contains two or more substances that are not chemically joined together, so each substance keeps its own properties and the mixture can be separated by physical methods.
纯净物只含一种粒子或一种化合物，具有明确的熔点和沸点。混合物含有两种或以上未发生化学结合的物质，因此各物质保持自身性质，混合物可以用物理方法分离。
Key vocabulary matters here. A solution is formed when a solute dissolves in a solvent, giving a mixture that looks transparent and does not settle. A saturated solution cannot dissolve any more solute at that temperature. Solubility is usually measured in grams of solute per 100 g of solvent.
这一部分的专有名词很关键。溶质溶解在溶剂中形成溶液，外观透明且不会沉降。饱和溶液在该温度下不能再溶解更多溶质。溶解度通常用每100克溶剂中溶解的溶质克数表示。
| Technique | Used to separate | 中文说明 |
|---|---|---|
| Filtration | An insoluble solid from a liquid | 过滤：分离不溶性固体与液体 |
| Evaporation | A dissolved solid from its solution | 蒸发：从溶液中得到溶解的固体 |
| Simple distillation | A solvent from a solution | 简单蒸馏：从溶液中分离溶剂 |
| Fractional distillation | Two or more liquids with different boiling points | 分馏：分离沸点不同的多种液体 |
| Chromatography | Different coloured dyes in an ink or mixture | 色谱：分离墨水或混合物中的不同色素 |
| Magnetism | A magnetic material from a non-magnetic one | 磁性分离：分离磁性物质与非磁性物质 |
When explaining a separation, always name the property that makes it work. Filtration works because one substance is insoluble; distillation works because the substances have different boiling points; chromatography works because the substances have different solubilities in the solvent.
在解释分离方法时，一定要指出所利用的性质。过滤之所以有效，是因为有一种物质不溶；蒸馏之所以有效，是因为各物质沸点不同；色谱之所以有效，是因为各物质在溶剂中的溶解度不同。
8. Atoms, Elements and Compounds | 原子、元素与化合物
An atom is the smallest particle of an element that can exist. An element contains only one type of atom. A compound contains two or more different elements chemically joined together in fixed proportions. A molecule is simply two or more atoms joined together, and it may be an element such as O₂ or a compound such as H₂O.
原子是元素能够存在的最小粒子。元素只含一种原子。化合物由两种或以上不同元素以固定的比例化学结合而成。分子只是两个或多个原子结合在一起，它可以是单质分子（如 O₂），也可以是化合物分子（如 H₂O）。
Chemists use symbols and formulae to write this information compactly. The formula CO₂ tells you that one carbon atom is joined to two oxygen atoms. The subscript number applies only to the atom immediately before it, which is why 2H₂O means four hydrogen atoms and two oxygen atoms in total.
化学家用符号和化学式来简洁表达这些信息。化学式 CO₂ 表示一个碳原子与两个氧原子结合。下标数字只作用于紧挨它前面的原子，所以 2H₂O 表示总共四个氢原子和两个氧原子。
In a chemical reaction, atoms are rearranged but never created or destroyed, which is the basis of conservation of mass in equations. This is why a balanced symbol equation must have the same number of each type of atom on both sides.
在化学反应中，原子只是重新排列，既不会生成也不会消失，这就是化学方程式中质量守恒的基础。因此，配平的符号方程式两边每种原子的数目必须相同。
2H₂ + O₂ → 2H₂O
2H₂ + O₂ → 2H₂O（两边各有4个H和2个O）
9. Acids and Alkalis: Properties and Examples | 酸和碱的性质与实例
Acids are substances that produce hydrogen ions, H⁺, when dissolved in water. Alkalis are soluble bases that produce hydroxide ions, OH⁻, when dissolved in water. This ion-based definition is why all acids behave in similar ways even though they taste and smell differently.
酸是溶于水时产生氢离子 H⁺ 的物质。碱是可溶性碱基，溶于水时产生氢氧根离子 OH⁻。正因为有了这个基于离子的定义，尽管各种酸的味道和气味不同，它们的行为却十分相似。
Common laboratory acids include hydrochloric acid, HCl, sulfuric acid, H₂SO₄, and nitric acid, HNO₃. Common alkalis include sodium hydroxide, NaOH, potassium hydroxide, KOH, and ammonia solution, NH₃(aq). Everyday examples of acids are lemon juice, vinegar and fizzy drinks; everyday alkalis include soap, toothpaste, baking powder and indigestion tablets.
常见实验室酸包括盐酸 HCl、硫酸 H₂SO₄ 和硝酸 HNO₃。常见碱包括氢氧化钠 NaOH、氢氧化钾 KOH 和氨水 NH₃(aq)。生活中的酸有柠檬汁、醋和碳酸饮料；生活中的碱有肥皂、牙膏、发酵粉和助消化药片。
| Property | Acids | Alkalis | 中文说明 |
|---|---|---|---|
| Ion produced | H⁺ | OH⁻ | 酸产生 H⁺，碱产生 OH⁻ |
| pH | style="border:1px solid
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