📚 Conventions (e.g. signs, symbols, terminology and nomenclature) | 化学约定规范（符号、术语与命名法）
Chemistry is a universal language. Like any language, it has rules — conventions that allow chemists from Beijing to London to write and understand the same formula, equation or name without ambiguity. For IGCSE Chemistry, mastering these conventions is not just a matter of good practice; it is essential for gaining marks in exams.
化学是一门通用的语言。和任何语言一样，它有自己的规则——这些约定使得从北京到伦敦的化学家都能毫无歧义地书写和理解同一个化学式、方程式或名称。对于IGCSE化学而言，掌握这些约定不仅仅是为了良好的习惯，更是在考试中得分的关键。
1. Chemical Symbols | 化学元素符号
Every element has a unique symbol, usually derived from its English name, but sometimes from its Latin name. The first letter is always capitalised; the second letter, if present, is always lowercase. For example: H (hydrogen), He (helium), Na (sodium from Latin natrium), Fe (iron from Latin ferrum).
每种元素都有唯一的符号，通常源自其英文名称，有时则来自拉丁文名称。首字母始终大写；如果有第二个字母，则必须小写。例如：H（氢）、He（氦）、Na（钠，源自拉丁文 natrium）、Fe（铁，源自拉丁文 ferrum）。
In IGCSE exams, you must write symbols exactly as accepted by IUPAC. Writing “CO” instead of “Co” is a common and costly mistake — “CO” means carbon monoxide, a molecule, while “Co” is cobalt, an element.
在IGCSE考试中，你必须严格按照IUPAC（国际纯粹与应用化学联合会）认可的方式书写符号。将”CO”误写成”Co”是一个常见且代价高昂的错误——”CO”表示一氧化碳，是一种分子，而”Co”是钴，一种元素。
- Symbols with one letter: H, C, N, O, F, P, S, K, V, W, Y, I, U
- 单字母符号：H、C、N、O、F、P、S、K、V、W、Y、I、U
- Symbols with two letters: He, Li, Be, Ne, Na, Mg, Al, Si, Cl, Ar, Ca
- 双字母符号：He、Li、Be、Ne、Na、Mg、Al、Si、Cl、Ar、Ca
2. Chemical Formulas: Empirical and Molecular | 化学式：经验式与分子式
A chemical formula tells us which elements are present and in what ratio. The empirical formula shows the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of each atom in one molecule.
化学式告诉我们化合物中含有哪些元素以及它们的比例。经验式（最简式）表示化合物中原子最简单的整数比，而分子式表示一个分子中每种原子的实际数目。
Glucose: empirical formula CH₂O, molecular formula C₆H₁₂O₆
葡萄糖：经验式 CH₂O，分子式 C₆H₁₂O₆
When writing formulas, subscripts indicate the number of atoms. For example, H₂O shows two hydrogen atoms and one oxygen atom. The subscript 1 is never written. Ions carry superscript charges: Na⁺, Cl⁻, Mg²⁺, O²⁻.
书写化学式时，下标表示原子数目。例如，H₂O表示两个氢原子和一个氧原子。下标”1″从不写出。离子带有上标电荷：Na⁺、Cl⁻、Mg²⁺、O²⁻。
Common polyatomic ions must be memorised:
常见的多原子离子必须牢记：
| Ion | 离子 | Formula | 化学式 | Ion | 离子 | Formula | 化学式 |
| Ammonium | 铵根 | NH₄⁺ | Hydroxide | 氢氧根 | OH⁻ |
| Nitrate | 硝酸根 | NO₃⁻ | Carbonate | 碳酸根 | CO₃²⁻ |
| Sulfate | 硫酸根 | SO₄²⁻ | Phosphate | 磷酸根 | PO₄³⁻ |
3. Valency and Oxidation States | 化合价与氧化态
Valency is the combining power of an element. It tells us how many bonds an atom can form. In ionic compounds, the valency corresponds to the ionic charge. For example, sodium has valency 1 (Na⁺), magnesium has valency 2 (Mg²⁺), and aluminium has valency 3 (Al³⁺).
化合价是元素的化合能力。它告诉我们一个原子可以形成多少条化学键。在离子化合物中，化合价对应于离子电荷。例如，钠的化合价为1（Na⁺），镁的化合价为2（Mg²⁺），铝的化合价为3（Al³⁺）。
The oxidation state is a formal concept used to track electrons in reactions. In IGCSE, common oxidation states include:
氧化态是一个形式概念，用于追踪反应中的电子。在IGCSE中，常见氧化态包括：
- Group I metals: always +1 | 第Ⅰ族金属：始终为 +1
- Group II metals: always +2 | 第Ⅱ族金属：始终为 +2
- Oxygen: usually −2 (except in H₂O₂ where it is −1) | 氧：通常为 −2（但 H₂O₂ 中为 −1）
- Hydrogen: usually +1 (except in metal hydrides where it is −1) | 氢：通常为 +1（但金属氢化物中为 −1）
- Chlorine: usually −1 | 氯：通常为 −1
The sum of oxidation states in a neutral compound is always zero.
中性化合物中所有原子的氧化态之和始终为零。
4. Balancing Chemical Equations | 配平化学方程式
A chemical equation must obey the law of conservation of mass — the number of atoms of each element must be the same on both sides. Coefficients (large numbers before formulas) are used to balance equations. Subscripts must never be changed to balance an equation.
化学方程式必须遵守质量守恒定律——每种元素的原子数目在方程式两边必须相同。使用系数（化学式前面的数字）来配平方程式。绝不能通过修改下标来配平方程式。
Example: the reaction of magnesium with oxygen:
示例：镁与氧气的反应：
2Mg + O₂ → 2MgO
2Mg + O₂ → 2MgO
Here, the coefficient 2 in front of Mg and MgO ensures two Mg atoms and two O atoms appear on each side. Note the use of the arrow (→), which means “produces” or “reacts to form”.
这里，Mg和MgO前面的系数2确保了两边各有2个镁原子和2个氧原子。注意使用箭头（→），表示”生成”或”反应生成”。
5. State Symbols | 状态符号
State symbols indicate the physical state of each substance in a reaction. They are written in lowercase italics (though in handwriting, simply lower case) in parentheses after each formula.
状态符号表示反应中各物质的物理状态。它们写在每个化学式后的括号中，使用小写字母。
| Symbol | 符号 | Meaning | 含义 | Example | 示例 |
| (s) | Solid | 固体 | NaCl(s) |
| (l) | Liquid | 液体 | H₂O(l) |
| (g) | Gas | 气体 | CO₂(g) |
| (aq) | Aqueous (dissolved in water) | 水溶液（溶于水） | NaOH(aq) |
For example, the reaction between hydrochloric acid and sodium hydroxide:
例如，盐酸与氢氧化钠的反应：
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
6. Reversible Reactions: The ⇌ Symbol | 可逆反应：⇌ 符号
Some reactions can proceed in both directions. The symbol ⇌ is used to show that a reaction is reversible — the products can react to reform the reactants. This is extremely important in industrial processes such as the Haber process and the Contact process.
有些反应可以双向进行。符号⇌用于表示反应是可逆的——生成物可以重新反应生成反应物。这在哈伯法和接触法等工业过程中极为重要。
For example, the Haber process:
例如，哈伯法：
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
In a reversible reaction, the forward and backward reactions occur simultaneously at equilibrium. The double arrow is not optional — writing a single arrow here would imply the reaction only goes one way, which would be incorrect.
在可逆反应中，正向和逆向反应在平衡时同时进行。双向箭头不可省略——如果写成单向箭头，则暗示反应只沿一个方向进行，这是不正确的。
7. Organic Chemistry Nomenclature | 有机化学命名法
Organic compounds are named according to a systematic set of rules. In IGCSE, you need to know the names and formulas of the first few alkanes and alkenes.
有机化合物按照一套系统的规则命名。在IGCSE中，你需要知道前几种烷烃和烯烃的名称与化学式。
| Alkane | 烷烃 | Formula | 化学式 | Alkene | 烯烃 | Formula | 化学式 |
| Methane | 甲烷 | CH₄ | Ethene | 乙烯 | C₂H₄ |
| Ethane | 乙烷 | C₂H₆ | Propene | 丙烯 | C₃H₆ |
| Propane | 丙烷 | C₃H₈ | Butene | 丁烯 | C₄H₈ |
| Butane | 丁烷 | C₄H₁₀ | — | — |
The general formula for alkanes is CₙH₂ₙ₊₂; for alkenes, it is CₙH₂ₙ. The suffix “-ane” indicates a single bond (alkane), while “-ene” indicates a double bond (alkene).
烷烃的通式为 CₙH₂ₙ₊₂；烯烃的通式为 CₙH₂ₙ。后缀”-ane”（烷）表示单键（烷烃），而”-ene”（烯）表示双键（烯烃）。
8. Naming Inorganic Compounds | 无机化合物命名
Inorganic compounds follow a binomial naming system. The cation (positive ion) is named first, followed by the anion (negative ion). For example:
无机化合物遵循双词命名系统。先写出阳离子（正离子）的名称，然后是阴离子（负离子）的名称。例如：
- NaCl: sodium chloride | 氯化钠
- MgO: magnesium oxide | 氧化镁
- CaCO₃: calcium carbonate | 碳酸钙
- Fe₂O₃: iron(III) oxide | 氧化铁（Ⅲ）
- CuSO₄: copper(II) sulfate | 硫酸铜（Ⅱ）
Transition metals that can form more than one ion require Roman numerals in parentheses to show their charge. Iron(II) is Fe²⁺; iron(III) is Fe³⁺. This is essential for distinguishing compounds like FeCl₂ (iron(II) chloride) from FeCl₃ (iron(III) chloride).
能形成多种离子的过渡金属需要在括号中加罗马数字表示其电荷。铁（Ⅱ）是 Fe²⁺；铁（Ⅲ）是 Fe³⁺。这对于区分 FeCl₂（氯化亚铁/氯化铁（Ⅱ））和 FeCl₃（氯化铁（Ⅲ））至关重要。
Acids are named based on their anions:
酸的命名基于其阴离子：
| Anion | 阴离子 | Acid | 酸 |
| Cl⁻ (chloride 氯化物) | HCl (hydrochloric acid 盐酸) |
| NO₃⁻ (nitrate 硝酸盐) | HNO₃ (nitric acid 硝酸) |
| SO₄²⁻ (sulfate 硫酸盐) | H₂SO₄ (sulfuric acid 硫酸) |
| CO₃²⁻ (carbonate 碳酸盐) | H₂CO₃ (carbonic acid 碳酸) |
9. Units and Quantities | 单位与物理量
In chemistry, precise measurement is essential. The international system of units (SI) provides standard units for all quantities. For IGCSE chemistry, the most important are:
在化学中，精确测量至关重要。国际单位制（SI）为所有物理量提供了标准单位。对于IGCSE化学，最重要的有：
- Mass: kilograms (kg) or grams (g) | 质量：千克（kg）或克（g）
- Volume: cubic centimetres (cm³) or cubic decimetres (dm³) | 体积：立方厘米（cm³）或立方分米（dm³）
- Amount of substance: moles (mol) | 物质的量：摩尔（mol）
- Concentration: mol/dm³ (often written as mol L⁻¹) | 浓度：mol/dm³（常写作 mol L⁻¹）
- Temperature: degrees Celsius (°C) or kelvin (K) | 温度：摄氏度（°C）或开尔文（K）
- Pressure: kilopascals (kPa) or atmospheres (atm) | 压强：千帕（kPa）或大气压（atm）
1 dm³ = 1000 cm³ = 1 L
1 dm³ = 1000 cm³ = 1 L
Molar concentration is calculated as: concentration (mol/dm³) = amount of solute (mol) ÷ volume of solution (dm³).
摩尔浓度的计算公式为：浓度（mol/dm³）= 溶质的物质的量（mol）÷ 溶液的体积（dm³）。
10. Hazard Symbols | 危险警示符号
Safety in the laboratory relies on recognising hazard symbols. The GHS (Globally Harmonized System) symbols are internationally recognised and are required knowledge in IGCSE Chemistry.
实验室安全依赖于识别危险警示符号。全球化学品统一分类和标签制度（GHS）符号在国际上得到认可，是IGCSE化学的必备知识。
| Symbol | 符号 | Meaning | 含义 |
| Flame (火焰) | Flammable — catches fire easily | 易燃——容易着火 |
| Corrosion (腐蚀) | Corrosive — destroys living tissue | 腐蚀性——破坏活体组织 |
| Skull and crossbones (骷髅) | Toxic — can cause death if ingested or inhaled | 有毒——摄入或吸入可致死 |
| Exclamation mark (感叹号) | Harmful or irritant | 有害或刺激物 |
| Gas cylinder (气瓶) | Gas under pressure | 高压气体 |
| Environment (环境) | Dangerous for the environment | 对环境有害 |
11. Common Abbreviations in Chemistry | 化学常用缩写
Chemists use abbreviations to save time. You must understand them and use them correctly in exam answers.
化学家使用缩写来节省时间。你必须理解它们并在考试答案中正确使用。
- RMM or Mr: relative molecular mass | 相对分子质量
- RAM or Ar: relative atomic mass | 相对原子质量
- RTP: room temperature and pressure (20°C, 1 atm) | 室温常压（20°C，1 atm）
- STP: standard temperature and pressure (0°C, 1 atm) | 标准温压（0°C，1 atm）
- Molar volume: the volume of one mole of any gas at RTP, which is 24 dm³ | 摩尔体积：在室温常压下，任何气体1摩尔的体积为24 dm³
At RTP, the molar volume of any gas is 24 dm³. This allows gas volume calculations: moles = volume (dm³) ÷ 24.
在室温常压下，任何气体的摩尔体积为24 dm³。这允许气体体积计算：物质的量（mol）= 体积（dm³）÷ 24。
12. Examining Conventions: Interpretation and Accuracy | 检验约定：解读与准确性
IGCSE examiners frequently penalise students for incorrect convention use. Common errors include writing “na” instead of “Na”, omitting state symbols when asked, using single arrows for reversible reactions, and miswriting ionic charges.
IGCSE考官经常因约定使用不当而扣分。常见错误包括：将”Na”写成”na”、在被要求时漏写状态符号、对可逆反应使用单向箭头、以及错误书写离子电荷。
Always check:
始终检查以下几点：
- Is the first letter of the element symbol capitalised and the second lowercase? | 元素符号首字母是否大写、第二个字母是否小写？
- Are all subscripts and superscripts correctly placed? | 所有下标和上标的位置是否正确？
- Are the charges on ions correct and written as superscripts? | 离子的电荷是否正确且以上标形式写出？
- Is the equation balanced in both atoms and charge? | 方程式在原子数和电荷两方面是否都配平？
- Are the state symbols consistent with the reaction conditions? | 状态符号是否与反应条件一致？
- Is the nomenclature systematic and complete? | 命名是否系统且完整？
Mastering these conventions demonstrates scientific literacy and precision. It separates a student who merely memorises chemistry from one who truly understands and communicates chemistry. In the exam, every mark counts — and convention marks are among the easiest to secure once you practise them.
掌握这些约定体现了科学素养和精确性。它将仅仅死记硬背化学的学生与真正理解并能沟通化学的学生区分开来。在考试中，每一分都很重要——而约定相关的分数，一旦你练习到位，是最容易拿到的。
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