📚 KS3 WJEC Chemistry: Formula and Theorem Quick-Reference Handbook | KS3 WJEC 化学：公式定理速查手册
Welcome to your KS3 WJEC Chemistry quick-reference handbook. This resource gathers the essential formulas, key definitions and foundational laws you will meet when studying quantitative chemistry and chemical patterns. Use it to revise, practise and strengthen your understanding of the calculations and principles behind the reactions.
欢迎使用你的 KS3 WJEC 化学速查手册。这份资料汇集了你在学习定量化学和化学规律时会遇到的核心公式、关键定义和基本定律。用它来复习、练习并加深你对化学反应背后计算和原理的理解。
1. Law of Conservation of Mass | 质量守恒定律
In every chemical reaction, the total mass of the reactants is exactly equal to the total mass of the products. Atoms are rearranged but never created or destroyed.
在每一个化学反应中，反应物的总质量总是精确等于生成物的总质量。原子只是重新排列，既不会凭空产生也不会消失。
If a reaction seems to lose mass, it is usually because a gas has escaped. In a sealed container the mass stays unchanged, proving the law holds true.
如果反应看起来“失去”了质量，通常是因为有气体逸出。在密封容器中总质量保持不变，从而证明这条定律始终成立。
2. Relative Atomic Mass (Aᵣ) and Relative Formula Mass (Mᵣ) | 相对原子质量与相对式量
The relative atomic mass (Aᵣ) is the number of times one atom of an element is heavier than 1/12 of the mass of a carbon‑12 atom. It is an average that takes account of different isotopes found naturally.
相对原子质量（Aᵣ）是指一个该元素原子比一个碳‑12 原子质量的 1/12 重多少倍。它是一个平均值，已经考虑了自然界中存在的不同同位素。
l the atoms shown in its chemical formula. For example, water H₂O: Mᵣ = (2 × 1) + 16 = 18.
化合物的相对式量（Mᵣ）等于其化学式中所有原子的 Aᵣ 总和。例如水 H₂O：Mᵣ = (2×1) + 16 = 18。
Mᵣ = Σ Aᵣ of all atoms in the formula
3. Using Valency to Write Chemical Formulae | 利用化合价书写化学式
The valency (or combining power) of an ion is often the same as the charge on the ion. When writing a formula, the positive and negative charges must balance to give an overall neutral compound.
离子的化合价（或结合能力）通常与其所带电荷数相同。书写化学式时，必须使正负电荷总数相等，得到电中性的化合物。
Use the criss‑cross method: swap the charge numbers and drop the signs. Example: calcium ion Ca²⁺ and chloride ion Cl⁻ give Ca₁Cl₂, written as CaCl₂.
使用交叉法：交换电荷的数值并略去符号。例如钙离子 Ca²⁺ 与氯离子 Cl⁻ 得到 Ca₁Cl₂，写作 CaCl₂。
- Common cations (positive ions): Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺.
- 常见阳离子： Na⁺、K⁺、Mg²⁺、Ca²⁺、Al³⁺。
- Common anions (negative ions): Cl⁻, Br⁻, O²⁻, OH⁻, SO₄²⁻, CO₃²⁻.
- 常见阴离子： Cl⁻、Br⁻、O²⁻、OH⁻、SO₄²⁻、CO₃²⁻。
4. Balancing Chemical Equations | 配平化学方程式
A balanced equation has the same number of each type of atom on both sides. Only ever change the big numbers (coefficients) in front of substances, never alter the small numbers inside a formula.
配平的方程式两边各种原子的个数完全相等。只能改动物质前面的系数（大数字），绝不能改动化学式内部的小数字。
Work through atoms step by step: balance metals first, then non‑metals, oxygen and finally hydrogen. Check that all atoms match after each adjustment.
逐步平衡原子：先配平金属原子，然后非金属，再配氧，最后配氢。每次调整后都要立即检查所有原子的总数是否相等。
Example: H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O, with 4 hydrogen atoms and 2 oxygen atoms on each side.
示例： H₂ + O₂ → H₂O 调整为 2H₂ + O₂ → 2H₂O，两边各有 4 个氢原子和 2 个氧原子。
5. Mass Relationships from Equations | 根据方程式计算质量关系
+ O₂ → 2H₂O the mass ratio (H₂ : O₂ : H₂O) is 4 : 32 : 36, which simplifies to 1 : 8 : 9.
配平的方程式显示了物质的量之比，在 KS3 阶段我们可以通过比较 Mᵣ 值来使用质量比。对于反应 2H₂ + O₂ → 2H₂O，质量比 (H₂ : O₂ : H₂O) 为 4 : 32 : 36，化简为 1 : 8 : 9。
This means 1 g of hydrogen always needs 8 g of oxygen to react completely and will produce 9 g of water. The proportion stays constant no matter what starting mass you use.
这意味着 1 g 氢气总需要 8 g 氧气才能完全反应，并生成 9 g 水。无论起始质量是多少，这个比例始终保持不变。
6. Concentration of Solutions (g/dm³) | 溶液浓度 (克每立方分米)
Concentration tells you how much solute is dissolved in a certain volume of solvent. The most common units at KS3 are grams per cubic decimetre (g/dm³).
浓度表示在一定体积的溶剂中溶解了多少溶质。KS3 阶段最常用的单位是克每立方分米 (g/dm³)。
concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³)
1 dm³ = 1000 cm³. Always convert cm³ to dm³ by dividing by 1000 before using the formula.
记住：1 dm³ = 1000 cm³。使用公式前必须先将 cm³ 除以 1000 转化为 dm³。
7. Average Rate of Reaction | 平均反应速率
The rate of a reaction is a measure of how quickly reactants are used up or products are formed. The average rate can be calculated from the change in quantity over time.
反应速率是衡量反应物消耗或产物生成快慢的量度。平均速率可以通过某段时间内量的变化来计算。
average rate = change in quantity ÷ time taken
Quantity could be mass (g), volume of gas (cm³) or concentration (g/dm³). The unit of rate will be [quantity] per second (s⁻¹), per minute (min⁻¹) etc.
“量”可以是质量 (g)、气体体积 (cm³) 或浓度 (g/dm³)。速率的单位相应为 [量] 每秒 (s⁻¹)、每分钟 (min⁻¹) 等。
8. Density and the Particle Model | 密度与粒子模型
Density links the mass of a substance to the space it occupies. It helps explain why solids are usually denser than liquids and gases because of how closely particles are packed.
密度将物质的质量与其占据的空间联系起来。它可以解释为什么固体通常比液体和气体密度更大，这取决于粒子堆积的紧密程度。
density (g/cm³) = mass (g) ÷ volume (cm³)
Particles in solids are arranged in fixed, regular patterns with very little space between them, giving high density. Gases have large gaps between particles, so their density is very low.
固体中的粒子排列成固定的规则结构，粒子间几乎没有空隙，因此密度高。气体粒子间存在很大的间隙，因此密度非常低。
9. Neutralisation Reaction | 中和反应
Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water. The general equation sums up this important pattern.
中和是酸与碱（或可溶性碱）反应生成盐和水的过程。其通式概括了这一重要规律。
Acid + Base → Salt + Water
For example: hydrochloric acid + sodium hydroxide → sodium chloride + water.
HCl + NaOH → NaCl + H₂O
例如：盐酸 + 氢氧化钠 → 氯化钠 + 水。
HCl + NaOH → NaCl + H₂O
10. Common Formulae of Acids, Bases and Salts | 酸碱盐的常见化学式
Learning the formulas of important laboratory acids and alkalis will save you time when writing equations. Here are the ones you will use most often in KS3 WJEC Chemistry.
熟记重要实验用酸和碱的化学式，将为你书写方程式节省大量时间。以下是你在 KS3 WJEC 化学中最常用到的几种。
- Acids: Hydrochloric acid HCl, Sulfuric acid H₂SO₄, Nitric acid HNO₃
- 酸： 盐酸 HCl、硫酸 H₂SO₄、硝酸 HNO₃
- Alkalis: Sodium hydroxide NaOH, Potassium hydroxide KOH, Calcium hydroxide Ca(OH)₂
- 碱： 氢氧化钠 NaOH、氢氧化钾 KOH、氢氧化钙 Ca(OH)₂
- Some common salts: Sodium chloride NaCl, Copper sulfate CuSO₄, Potassium nitrate KNO₃, Calcium carbonate CaCO₃
- 一些常见盐： 氯化钠 NaCl、硫酸铜 CuSO₄、硝酸钾 KNO₃、碳酸钙 CaCO₃
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