📚 Year 8 SQA Chemistry: Quick Guide to Formulas & Principles | Year 8 SQA 化学：公式定理速查手册
Welcome to your essential quick-reference handbook for Year 8 SQA Chemistry. This guide brings together the key formulas, laws and principles you need to understand chemical reactions, write correct formulae and tackle calculations with confidence. Use it as a handy revision companion whenever you study atoms, compounds, equations or the pH scale.
欢迎使用这本 Year 8 SQA 化学核心速查手册。本指南汇总了你必须掌握的重要公式、定律和原理，帮助你理解化学反应、书写正确的化学式并自信地完成计算。在学习原子、化合物、化学方程式或 pH 标度时，随时翻阅这本复习伴侣吧。
1. Law of Conservation of Mass | 质量守恒定律
In a chemical reaction, atoms are rearranged but never created or destroyed. This means the total mass of all reactants before the reaction must equal the total mass of all products after the reaction.
Total mass of reactants = Total mass of products
在化学反应中，原子只是重新排列，既不会凭空产生也不会凭空消失。因此，反应前所有反应物的总质量必须等于反应后所有生成物的总质量。
2. Law of Definite Proportions | 定比定律
Any pure compound always contains exactly the same elements combined together in the same fixed proportion by mass. For example, water (H₂O) always consists of hydrogen and oxygen in a mass ratio of 1 : 8, no matter where the water comes from.
任何纯净的化合物总是由完全相同的元素以固定的质量比例结合而成。例如，水 (H₂O) 无论来自何处，永远由氢和氧以 1 : 8 的质量比组成。
3. Writing Chemical Formulae from Valency | 根据化合价书写化学式
To write the formula of an ionic compound, you swap the valencies (or charges) of the positive and negative ions. The number of each ion is written as a subscript. The total positive charge must balance the total negative charge exactly.
书写离子化合物的化学式时，你需要把正、负离子的化合价（或电荷数）交叉互换，作为右下角的下标。总正电荷必须恰好与总负电荷相等。
Example – magnesium chloride: Mg has a charge of 2⁺, Cl has a charge of 1⁻. Swap the numbers to give Mg₁Cl₂, which is written simply as MgCl₂.
例子——氯化镁：Mg 带 2⁺ 电荷，Cl 带 1⁻ 电荷。交叉数字后得到 Mg₁Cl₂，简写为 MgCl₂。
Example – aluminium oxide: Al³⁺ and O²⁻ swap to give Al₂O₃.
例子——氧化铝：Al³⁺ 与 O²⁻ 交叉得到 Al₂O₃。
4. Common Ions and Their Charges | 常见离子及其电荷
The table below lists ions you will frequently use in Year 8 chemistry. Learning these charges will make formula writing much faster.
| Positive Ions (Cations) | Formula | Negative Ions (Anions) | Formula |
|---|---|---|---|
| Sodium | Na⁺ | Chloride | Cl⁻ |
| Potassium | K⁺ | Bromide | Br⁻ |
| Calcium | Ca²⁺ | Oxide | O²⁻ |
| Magnesium | Mg²⁺ | Sulfide | S²⁻ |
| Aluminium | Al³⁺ | Hydroxide | OH⁻ |
| Ammonium | NH₄⁺ | Nitrate | NO₃⁻ |
| Copper(II) | Cu²⁺ | Carbonate | CO₃²⁻ |
| Iron(II) | Fe²⁺ | Sulfate | SO₄²⁻ |
| Iron(III) | Fe³⁺ |
下表列出了 Year 8 化学中常见的离子。记牢这些电荷能让你书写化学式时又快又准。
5. 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 average mass of an atom of an element compared to 1/12 of the mass of a carbon‑12 atom. The relative formula mass (Mr) of a compound is found by adding together the Ar values of all the atoms in its formula.
Mr = Σ (number of atoms × Ar)
相对原子质量 (Ar) 是一个元素原子的平均质量与一个碳-12 原子质量的 1/12 的比值。化合物的相对式量 (Mr) 等于其化学式中所有原子的 Ar 总和。
Worked example – water, H₂O: Ar of H = 1, O = 16. So Mr(H₂O) = (2 × 1) + (1 × 16) = 18.
计算示例——水 H₂O：H 的 Ar = 1，O 的 Ar = 16。因此 Mr(H₂O) = (2 × 1) + (1 × 16) = 18。
Worked example – calcium carbonate, CaCO₃: Ca = 40, C = 12, O = 16. Mr = 40 + 12 + (3 × 16) = 100.
计算示例——碳酸钙 CaCO₃：Ca = 40，C = 12，O = 16。Mr = 40 + 12 + (3 × 16) = 100。
6. Percentage Composition by Mass | 质量百分比组成
To find how much of a compound's mass comes from a certain element, use the percentage composition formula. This is very useful when analysing the purity of a substance.
% by mass of element = (number of atoms of element × Ar / Mr of compound) × 100
要找出某一元素在化合物中所占的质量分数，可以使用质量百分比公式。这在分析物质纯度时非常有用。
Example – percentage of oxygen in H₂O: O atoms = 1, Ar(O) = 16, Mr = 18. %O = (16 / 18) × 100 ≈ 88.9%.
示例——水中氧的质量百分比：O 原子数 = 1，Ar(O) = 16，Mr = 18。%O = (16 / 18) × 100 ≈ 88.9%。
7. Balancing Chemical Equations | 化学方程式的配平
A balanced equation has the same number of each type of atom on both sides. You balance an equation by placing large numbers (coefficients) in front of the chemical formulae – never change the small subscript numbers inside a formula.
配平后的方程式两边每一种原子的数目相等。配平的方法是给化学式前面加上大数字（系数），绝不能改动化学式内部的下标数字。
Step-by-step example: H₂ + O₂ → H₂O
分步示例：H₂ + O₂ → H₂O
Count atoms initially: left – H=2, O=2; right – H=2, O=1. To balance O, place a '2' in front of H₂O: H₂ + O₂ → 2H₂O. Now right has H=4, O=2. Balance H by placing a '2' in front of H₂: 2H₂ + O₂ → 2H₂O. Final check: left H=4, O=2; right H=4, O=2. Balanced!
先数原子数：左边 H=2，O=2；右边 H=2，O=1。为使 O 守恒，在 H₂O 前写“2”：H₂ + O₂ → 2H₂O。此时右边 H=4，O=2。再在 H₂ 前写“2”使 H 守恒：2H₂ + O₂ → 2H₂O。最终检查：左边 H=4，O=2，右边 H=4，O=2。配平完毕！
Key rule: Atoms are conserved, so the number of atoms of each element must be the same before and after the reaction.
核心原则：原子守恒，因此反应前后每一种元素的原子总数必须相等。
8. The pH Scale and Neutralisation | pH 标度与中和反应
The pH scale runs from 0 to 14 and tells us how acidic or alkaline a solution is. Acids have a pH less than 7, alkalis have a pH greater than 7, and a pH of 7 is neutral. Indicators such as litmus or universal indicator are used to estimate pH.
pH 标度从 0 到 14，用于表示溶液的酸碱度。酸的 pH 小于 7，碱的 pH 大于 7，pH 等于 7 为中性。我们可以用石蕊或通用指示剂来测定 pH。
Neutralisation: When an acid reacts with an alkali, they cancel each other out and produce a salt plus water. The essential ionic equation for neutralisation is always the same.
H⁺(aq) + OH⁻(aq) → H₂O(l)
中和反应：酸与碱发生反应时，彼此的酸性（或碱性）被抵消，生成盐和水。中和反应的离子方程式始终相同。
Example – hydrochloric acid + sodium hydroxide: HCl + NaOH → NaCl + H₂O.
示例——盐酸与氢氧化钠：HCl + NaOH → NaCl + H₂O。
9. Reactivity Series of Metals | 金属活动性顺序
The reactivity series lists metals in order of how vigorously they react. A more reactive metal can displace a less reactive metal from its compound. This series helps predict the outcomes of displacement reactions and the extraction of metals.
金属活动性顺序根据金属反应的剧烈程度排列。活泼性强的金属能把活泼性弱的金属从其化合物中置换出来。这一顺序有助于预测置换反应的结果以及金属的冶炼方法。
Common reactivity order (most reactive first):
Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminium (Al) > Zinc (Zn) > Iron (Fe) > Lead (Pb) > Copper (Cu) > Silver (Ag) > Gold (Au)
常见活动性顺序（活泼性由强到弱）：
钾 (K) > 钠 (Na) > 钙 (Ca) > 镁 (Mg) > 铝 (Al) > 锌 (Zn) > 铁 (Fe) > 铅 (Pb) > 铜 (Cu) > 银 (Ag) > 金 (Au)
Displacement example: + copper(II) sulfate → zinc sulfate + copper. Zn displaces Cu because Zn is more reactive.
置换示例：锌 + 硫酸铜 → 硫酸锌 + 铜。锌比铜活泼，因此将铜置换出来。
10. Rate of Reaction | 反应速率
The rate of a chemical reaction tells us how quickly reactants are used up or products are formed. It can be calculated by measuring the change in quantity (mass, volume of gas, etc.) over time.
Average rate = Change in quantity / Time taken
化学反应速率表示反应物消耗或生成物产生的快慢。可通过测定一定时间内某物理量（质量、气体体积等）的变化来求得。
Example – gas produced: of hydrogen gas is collected in 10 seconds, the average rate = 40 cm³ / 10 s = 4.0 cm³/s.
示例——气体生成：若 10 秒内收集到 40 cm³ 氢气，平均速率 = 40 cm³ / 10 s = 4.0 cm³/s。
Factors that increase rate include higher temperature, greater concentration of reactants, larger surface area of solids, and the use of a catalyst.
能增大反应速率的因素包括：升高温度、增大反应物浓度、增加固体表面积以及使用催化剂。
11. Endothermic and Exothermic Reactions | 吸热与放热反应
Chemical reactions always involve energy changes. An exothermic Published by TutorHao | Year 8 Chemistry Revision Series | aleveler.com
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