📚 Year 9 Edexcel Chemistry: Key Formulas and Principles Quick Reference | Year 9 Edexcel 化学：公式定理速查手册
This quick-reference handbook brings together all the essential formulas, laws, and key facts from the Edexcel Year 9 chemistry course. Use it to reinforce your understanding of atomic structure, chemical reactions, acids and bases, states of matter, and practical tests. Every section pairs a core idea with the precise formula or rule you need to recall for assessments and progress tests.
这本速查手册汇集了 Edexcel Year 9 化学课程中所有必要的公式、定律和关键事实。你可以用它来巩固对原子结构、化学反应、酸与碱、物态变化以及实验检验的理解。每个小节都把核心概念与你需要记住的精确公式或规则配对，方便你用于测评和阶段测验的复习。
1. Law of Conservation of Mass | 质量守恒定律
In any chemical reaction, matter is neither created nor destroyed. This means the total mass of all reactants before the reaction is exactly equal to the total mass of all products formed. When we carry out reactions in a closed system, the reading on the balance does not change. If a reaction appears to lose mass, it is usually because a gas has escaped into the surroundings.
在任何化学反应中，物质既不能被创造也不能被消灭。这意味着反应前所有反应物的总质量恰好等于生成的所有产物的总质量。当我们在一个封闭系统中进行反应时，天平的读数不会改变。如果一个反应看起来质量减少了，通常是因为有气体逸散到了周围环境中。
Total mass of reactants = Total mass of products
2. Writing Chemical Formulae using Valency | 利用化合价书写化学式
Every element has a combining power known as valency. To write the formula of a compound, we use the charges of the ions (or the valency numbers) and criss-cross them to give the correct ratio. The total positive charge must balance the total negative charge, so the overall compound is electrically neutral.
每种元素都有一种结合能力，称为化合价。书写化合物的化学式时，我们利用离子所带电荷（或化合价数值），将它们交叉相乘得到正确的原子个数比。总的正电荷必须与总的负电荷平衡，从而整个化合物呈电中性。
| Ion | Charge | Valency number |
|---|---|---|
| Na⁺ | 1⁺ | 1 |
| Mg²⁺ | 2⁺ | 2 |
| Al³⁺ | 3⁺ | 3 |
| Cl⁻ | 1⁻ | 1 |
| O²⁻ | 2⁻ | 2 |
for the negative ion, and vice versa. For example, aluminium oxide: Al³⁺ and O²⁻ → Al₂O₃.
交叉法则：正电荷的数目成为负离子的下标，反之亦然。例如氧化铝：Al³⁺ 与 O²⁻ → Al₂O₃。
3. Relative Atomic Mass and Relative Formula Mass | 相对原子质量和相对分子质量
The relative atomic mass, symbol Aᵣ, compares the mass of an atom of an element with 1/12 of the mass of a carbon‑12 atom. For molecules and ionic compounds, we use the relative formula mass, symbol Mᵣ. It is calculated by adding together the Aᵣ values of all atoms in the formula.
相对原子质量，符号 Aᵣ，是将一个元素原子的质量与一个碳‑12 原子质量的 1/12 进行比较得到的数值。对于分子和离子化合物，我们使用相对分子质量（式量），符号 Mᵣ。它通过将化学式中所有原子的 Aᵣ 值相加而求得。
Mᵣ = sum of Aᵣ for each atom in the formula
is dimensionless number helps us compare the masses of different substances in a reaction.
示例：对于水，H₂O：Aᵣ(H) = 1，Aᵣ(O) = 16，所以 Mᵣ = 2×1 + 16 = 18。这个无量纲的数值帮助我们在反应中比较不同物质的质量。
4. Balancing Chemical Equations | 化学方程式的配平
A balanced chemical equation shows the same number of each type of atom on both sides of the arrow. We balance equations by placing large numbers (coefficients) in front of the chemical formulae – never by changing the small subscript numbers inside them. This follows directly from the law of conservation of mass.
一个配平了的化学方程式在箭头两边显示出每种原子数目相等。我们通过将大数字（计量系数）放在化学式前头来配平方程式，绝不要改动化学式内部的小下标数字。这直接源自质量守恒定律。
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O
Steps: first write the skeleton equation, count atoms on each side, then add coefficients to balance one element at a time, leaving hydrogen and oxygen until last. Always check your final atom count.
步骤：先写出骨架方程式，数清每侧的原子数目，然后每次添加系数来配平一种元素，把氢和氧留到最后。配平后务必检查最终的原子数目。
5. Factors Affecting the Rate of Reaction | 影响反应速率的因素
The rate of a chemical reaction tells us how quickly reactants are used up or products are made. Four main factors can change the rate: temperature, concentration (or pressure for gases), surface area of solid reactants, and the presence of a catalyst. Each factor increases the frequency of successful collisions between reacting particles.
化学反应的速率表示反应物被消耗或产物生成有多快。四个主要因素可以改变反应速率：温度、浓度（或气体的压强）、固体反应物的表面积，以及催化剂的存在。每个因素都会增加反应微粒之间成功碰撞的频率。
- Temperature: rature → particles move faster → more collisions with energy above the activation barrier.
- Concentration/pressure: more particles per unit volume → collisions more frequent.
- Surface area: finely divided solid → more exposed particles → greater contact between reactants.
- Catalyst: ides an alternative pathway with lower activation energy → more collisions are successful.
温度：升高温度 → 粒子运动更快 → 更多碰撞具有超过活化能垒的能量。浓度/压强：单位体积内粒子更多 → 碰撞更频繁。表面积：固体被分成细小块状 → 暴露更多粒子 → 反应物接触更充分。催化剂：提供活化能较低的替代路径 → 更多碰撞能成功。
6. Acids, Bases and Neutralisation | 酸、碱与中和反应
An acid is a substance that donates hydrogen ions (H⁺) in aqueous solution, while a base (or alkali) accepts H⁺ or releases hydroxide ions (OH⁻). When an acid and a base react together, they neutralise each other to form a salt and water. This is an example of an ionic reaction.
酸是在水溶液中能给出氢离子 (H⁺) 的物质，而碱（或可溶性碱）能接受 H⁺ 或释放氢氧根离子 (OH⁻)。当酸与碱反应时，它们互相中和，生成一种盐和水。这是一个离子反应。
acid + base → salt + water
+ H₂O. The H⁺ from the acid combines with the OH⁻ from the alkali to form water molecules.
示例：盐酸与氢氧化钠反应生成氯化钠和水。HCl + NaOH → NaCl + H₂O。酸中的 H⁺ 与碱中的 OH⁻ 结合生成水分子。
7. The pH Scale | pH 标度
The pH scale is a measure of the acidity or alkalinity of an aqueous solution. It runs from 0 to 14, with 7 being neutral. Acids have a pH less than 7; the lower the number, the more acidic the solution. Alkalis have a pH greater than 7; the higher the number, the more alkaline the solution. Universal indicator or a pH meter can be used to find the pH.
pH 标度是衡量水溶液酸碱性强弱的尺度。其范围从 0 到 14，7 为中性。酸的 pH 小于 7；数值越低，酸性越强。碱的 pH 大于 7；数值越高，碱性越强。可以用通用指示剂或 pH 计来测定 pH 值。
| pH range | Solution type | Colour with universal indicator |
|---|---|---|
| 0–2 | Strong acid | Red |
| 3–6 | Weak acid | Orange/yellow |
| 7 | Neutral | Green |
| 8–11 | Weak alkali | Blue |
| 12–14 | Strong alkali | Purple/violet |
8. States of Matter and Energy Changes | 物态变化与能量变化
Substances can exist as solid, liquid or gas. Changing state requires energy: melting and boiling absorb heat (endothermic), while freezing and condensing release heat (exothermic). The temperature of a pure substance stays constant during a state change because the energy is used to break or form bonds, not to raise the temperature.
物质可以以固态、液态或气态存在。物态变化需要能量：熔化和沸腾吸收热量（吸热），而凝固和冷凝释放热量（放热）。纯物质在物态变化期间温度保持不变，因为此时能量用于打断或形成键，而不是用来升高温度。
Solid ⇌ Liquid ⇌ Gas
ensing: gas → liquid (energy given out). Sublimation is the change from solid directly to gas, e.g. solid carbon dioxide (dry ice).
熔化：固态 → 液态（吸收能量）。凝固：液态 → 固态（放出能量）。沸腾/蒸发：液态 → 气态（吸收能量）。冷凝：气态 → 液态（放出能量）。升华是指固体直接变为气体，例如固态二氧化碳（干冰）。
9. Density Formula | 密度公式
Density is a property that links the mass of a material to the volume it occupies. In chemistry, we sometimes calculate the density of solids, liquids or gases to identify a substance or to work out its mass from its volume. The formula applies to any uniform material.
密度是将物质的质量与其所占体积联系起来的一个性质。在化学中，我们有时会计算固体、液体或气体的密度，以鉴别物质或根据体积求算质量。该公式适用于任何均匀材料。
Density = Mass ÷ Volume
s density, m is mass and V is volume. Common units are g/cm³ for solids and liquids, and g/dm³ for gases. When you measure the mass and volume of a regular solid, you can calculate its density to help confirm what the substance is.
用符号表示为：ρ = m / V，其中 ρ 表示密度，m 表示质量，V 表示体积。固体和液体的常用单位是 g/cm³，气体的常用单位是 g/dm³。当你测量规则固体的质量和体积时，就可以计算其密度，以帮助确认该物质是什么。
10. Tests for Common Gases | 常见气体的检验
In the laboratory, we identify gases by simple chemical tests that produce a characteristic observation. These tests are quick, safe and give reliable results if performed with the correct apparatus. Year 9 students must know the tests for hydrogen, oxygen, carbon dioxide and chlorine.
在实验室里，我们通过简单的化学检验来识别气体，这些检验会产生特征性的观察结果。这些检验操作快捷、安全，如果使用正确的仪器，可得出可靠的结果。九年级学生必须掌握氢气、氧气、二氧化碳和氯气的检验方法。
- Hydrogen (H₂): Hold a lit splint at the mouth of a test tube; a squeaky pop is heard.
- Oxygen (O₂): Place a glowing splint into a test tube; the splint relights.
- Carbon dioxide (CO₂): Bubble the gas through limewater (calcium hydroxide solution); the limewater turns milky or cloudy.
- Chlorine (Cl₂): Hold damp blue litmus paper at the mouth of the tube; the paper turns red and then is bleached white.
氢气 (H₂)：在试管口放一根点燃的木条；会听到咝咝的爆鸣声。氧气 (O₂)：将带火星的木条伸入试管；木条复燃。二氧化碳 (CO₂)：将气体通入石灰水（氢氧化钙溶液）；石灰水变浑浊或呈乳白色。氯气 (Cl₂)：将湿润的蓝色石蕊试纸放在试管口；试纸先变红，然后被漂白。
11. Reactivity Series and Displacement Reactions | 金属活动性顺序与置换反应
The reactivity series lists metals in order of how vigorously they react. A more reactive metal can displace a less reactive metal from a solution of its salt. This is called a displacement reaction. It is a redox process: the more reactive metal loses electrons (oxidation) and the less reactive metal ions gain electrons (reduction).
金属活动性顺序按金属反应的剧烈程度排列。活泼性较强的金属能将活泼性较弱的金属从其盐溶液中置换出来。这称为置换反应。它是一种氧化还原过程：较活泼的金属失去电子（氧化），较不活泼的金属离子得到电子（还原）。
| Most reactive | → | → | Least reactive |
|---|---|---|---|
| K, Na, Ca, Mg, Al, | Zn, Fe, Sn, Pb, | Cu, Hg, Ag, Au, Pt |
ve metal + salt of less reactive metal → salt of more reactive metal + less reactive metal
CuSO₄ → ZnSO₄ + Cu. The blue colour of copper sulfate fades as copper metal is deposited.
示例：锌 + 硫酸铜(II) → 硫酸锌 + 铜。Zn + CuSO₄ → ZnSO₄ + Cu。随着铜单质的析出，硫酸铜的蓝色逐渐褪去。
12. Exothermic and Endothermic Reactions | 放热与吸热反应
Chemical reactions always involve an energy change. In exothermic reactions, heat energy is released to the surroundings, causing a temperature rise. Combustion and neutralisation are typical exothermic changes. In endothermic reactions, heat energy is absorbed from the surroundings, leading to a temperature drop. Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are everyday examples.
化学反应总是伴随着能量变化。在放热反应中，热能释放到周围环境中，导致温度上升。燃烧和中和反应是典型的放热过程。在吸热反应中，热能是从周围环境中吸收的，导致温度下降。热分解以及柠檬酸与碳酸氢钠的反应是常见的例子。
Exothermic: Reactants → Products + energy
Endothermic: Reactants + energy → Products
We can monitor the energy change with a thermometer. For a reaction in solution, an exothermic process makes the solution feel warmer; an endothermic one makes it feel colder. Hand‑warmers and self‑cooling packs rely on these principles.
我们可以用温度计监测能量变化。对于溶液中的反应，放热过程会使溶液温度升高，吸热过程则使溶液温度降低。暖手宝和自冷冰袋就是利用这些原理工作的。
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