📚 WJEC Year 8 Chemistry: Quick Reference Handbook of Formulas and Theorems | 威尔士联合考试委员会八年级化学：公式定理速查手册
This handbook compiles the essential formulas, principles and theorems you need to master in Year 8 Chemistry, aligned to the WJEC curriculum. Use it for quick revision, homework help and exam preparation — every rule is explained clearly with paired examples.
本手册整理了 WJEC 八年级化学必须掌握的核心公式、原理和定理，适合快速复习、家庭作业和考试备考——每条规则均配有中英对照的解释和示例。
1. Law of Conservation of Mass | 质量守恒定律
The total mass of reactants equals the total mass of products in a chemical reaction. Matter cannot be created or destroyed, only rearranged.
在化学反应中，反应物的总质量等于生成物的总质量。物质既不能凭空产生，也不能凭空消失，只能重新组合。
- If 4 g of hydrogen reacts with 32 g of oxygen, exactly 36 g of water is formed.
- 如果 4 克氢气与 32 克氧气反应，恰好生成 36 克水。
2. Writing Chemical Formulae – Valency and Crossover | 化学式的书写——化合价与十字交叉法
Use the valency (combining power) of elements and ions to write correct formulae. Swap the valency numbers and simplify if needed.
利用元素和离子的化合价（结合能力）书写正确的化学式。交换化合价数字，必要时约分。
| Ion / 离子 | Valency / 化合价 |
|---|---|
| Na⁺ | 1 |
| Mg²⁺ | 2 |
| O²⁻ | 2 |
| Cl⁻ | 1 |
| SO₄²⁻ | 2 |
or magnesium oxide: Mg²⁺ and O²⁻ → swap valencies (2 and 2) → Mg₂O₂, then simplify to MgO.
以氧化镁为例：Mg²⁺ 和 O²⁻ → 交换化合价（2 和 2）→ Mg₂O₂，化简为 MgO。
3. Balancing Chemical Equations | 化学方程式的配平
Ensure the number of atoms of each element is the same on both sides of the equation. Only change coefficients (large numbers in front), never subscripts.
确保方程式两边每种元素的原子数目相等。只可以改变化学式前的系数，绝不能改动下标数字。
H₂ + O₂ → H₂O (unbalanced / 未配平)
2H₂ + O₂ → 2H₂O (balanced / 已配平)
Count atoms: reactants: H=4, O=2; products: H=4, O=2. Balanced.
原子计数：反应物：H=4, O=2；生成物：H=4, O=2。已配平。
4. Relative Atomic Mass (Aᵣ) and Relative Molecular Mass (Mᵣ) | 相对原子质量与相对分子质量
Relative atomic mass (Aᵣ) is the average mass of an atom compared to 1/12 of carbon-12. Mᵣ is the sum of Aᵣ of all atoms in a formula.
相对原子质量（Aᵣ）是原子的平均质量与碳-12 原子质量的 1/12 相比所得的数值。相对分子质量（Mᵣ）是化学式中所有原子的 Aᵣ 之和。
- Aᵣ of H = 1, C = 12, O = 16, Na = 23, Cl = 35.5
- H 的 Aᵣ = 1，C = 12，O = 16，Na = 23，Cl = 35.5
Mᵣ of H₂O = (2×1) + 16 = 18. Mᵣ of CO₂ = 12 + (2×16) = 44.
H₂O 的 Mᵣ = (2×1) + 16 = 18。CO₂ 的 Mᵣ = 12 + (2×16) = 44。
5. Percentage Composition by Mass | 元素质量百分比
e percentage of an element in a compound: (total Aᵣ of the element ÷ Mᵣ of compound) × 100%.
计算化合物中某元素的质量百分比：(该元素的总 Aᵣ ÷ 化合物的 Mᵣ) × 100%。
%Mg in MgO = (24 ÷ 40) × 100% = 60%
MgO has Mᵣ = 24 + 16 = 40. Mg contributes 24, so 60%; O contributes 40%.
MgO 的 Mᵣ = 24 + 16 = 40。Mg 占 24，故为 60%；O 占 40%。
6. Reactivity Series of Metals | 金属活动性顺序
Metals can be arranged in order of reactivity. A more reactive metal displaces a less reactive metal from its compound. WJEC typically uses the series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au.
金属可根据活泼性排序。活泼性强的金属能将活泼性弱的金属从其化合物中置换出来。WJEC 常用顺序：钾、钠、钙、镁、铝、锌、铁、铅、铜、银、金。
Mnemonic: "Please Stop Calling Me A Zebra In The Lunchroom, Come See Gold".
记忆口诀：钾钠钙镁铝，锌铁铅铜银金。
7. Acids, Bases, Alkalis and Neutralisation | 酸、碱、可溶碱与中和反应
se acids; alkalis are soluble bases that release OH⁻ ions. Neutralisation: H⁺ + OH⁻ → H₂O.
酸含有 H⁺ 离子。碱能中和酸；可溶碱（碱溶液）释放 OH⁻ 离子。中和反应：H⁺ + OH⁻ → H₂O。
HCl + NaOH → NaCl + H₂O
General equation: acid + alkali → salt + water.
通式：酸 + 碱 → 盐 + 水。
8. pH Scale and Indicators | pH 标度与指示剂
The pH scale measures how acidic or alkaline a solution is (0–14). pH less than 7 = acidic; pH 7 = neutral; pH greater than 7 = alkaline.
pH 标度用于衡量溶液的酸碱性（0–14）。pH < 7 为酸性；pH = 7 为中性；pH > 7 为碱性。
- Universal indicator: red in strong acid, green in neutral, purple in strong alkali.
- 通用指示剂：强酸中呈红色，中性呈绿色，强碱中呈紫色。
- Litmus: red in acid, blue in alkali.
- 石蕊：酸中红，碱中蓝。
9. Displacement Reactions – Metals and Halogens | 置换反应——金属与卤素
A more reactive element takes the place of a less reactive element in a compound. For metals, the reactivity series dictates; for halogens, reactivity decreases down Group 7 (F > Cl > Br > I).
较活泼的元素能从化合物中置换出较不活泼的元素。对金属而言，依据活动性顺序；对卤素，活泼性自上而下递减（F > Cl > Br > I）。
Zn + CuSO₄ → ZnSO₄ + Cu
Cl₂ + 2KBr → 2KCl + Br₂
Colour change in halogen displacement often confirms the reaction (e.g., orange Br₂ formed).
卤素置换中常伴随颜色变化以确认反应（如生成橙色的 Br₂）。
10. Oxidation and Reduction – Oxygen Transfer | 氧化与还原——氧的得失
Oxidation is gain of oxygen; reduction is loss of oxygen. A reaction where both occur is called a redox reaction.
氧化是指得到氧；还原是指失去氧。两者同时发生的反应称为氧化还原反应。
2Mg + O₂ → 2MgO (Mg is oxidised, O₂ is reduced)
2Mg + O₂ → 2MgO（Mg 被氧化，O₂ 被还原）
→ 2Fe + 3CO₂. Iron oxide loses oxygen (reduced), carbon monoxide gains oxygen (oxidised).
炼铁：Fe₂O₃ + 3CO → 2Fe + 3CO₂。氧化铁失去氧（被还原），一氧化碳得到氧（被氧化）。
11. Factors Affecting Reaction Rate | 影响反应速率的因素
Rate of reaction can be increased by: higher temperature, higher concentration, larger surface area (smaller particles), and use of a catalyst.
提高反应速率的方法：升高温度、增大浓度、增大表面积（更小颗粒）以及使用催化剂。
Collision theory: particles must collide with sufficient energy for a reaction to occur.
碰撞理论：粒子必须发生碰撞且能量足够，反应才能发生。
- Temperature increases: particles move faster → more frequent, energetic collisions.
- 温度升高：粒子运动加快 → 碰撞更频繁、更剧烈。
- Surface area increases: more particles are exposed to react.
- 表面积增大：更多粒子暴露在外可发生反应。
12. Extracting Metals – Reduction with Carbon | 提取金属——用碳还原
Metals below carbon in the reactivity series can be extracted from their oxides by heating with carbon (coke). Carbon acts as the reducing agent, removing the oxygen.
活动性排在碳之后的金属可以通过与碳（焦炭）共热的方法从其氧化物中提取出来。碳作为还原剂，夺走氧。
2PbO + C → 2Pb + CO₂
3C → 2Fe + 3CO (in a blast furnace). Metals above carbon (e.g., Al) require electrolysis.
炼铁应用此原理：Fe₂O₃ + 3C → 2Fe + 3CO（高炉中）。比碳活泼的金属（如铝）需用电解法。
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