📚 8 Writing Symbol Equations | 书写符号方程式
In KS3 Chemistry, a symbol equation is a quick and accurate way to show what happens in a chemical reaction. Instead of writing full names, you use chemical formulae, state symbols and balanced coefficients to describe the substances and the conservation of mass.
在KS3化学中，符号方程式是一种快速而准确地表示化学反应的方法。你不用写出完整名称，而是使用化学式、状态符号和配平系数来描述反应物、生成物以及质量守恒。
1. Why Symbol Equations Matter | 为什么符号方程式很重要
A word equation such as 'hydrogen + oxygen → water' on such as 2H₂ + O₂ → 2H₂O tells us the exact particles and the ratio in which they react.
文字方程式如“氢气 + 氧气 → 水”只告诉我们反应物和生成物的名称。像 2H₂ + O₂ → 2H₂O 这样的符号方程式则告诉我们参与反应的微粒以及它们反应的精确比例。
Using symbols is faster and is the same in every language, so scientists all over the world can read the same equation.
使用符号更快捷，而且在所有语言中都相同，因此世界各地的科学家都能看懂同一个方程式。
2. From Word Equations to Symbol Equations | 从文字方程式到符号方程式
Start by writing the word equation. Then replace each chemical name with its formula, using the correct symbols from the periodic table.
先写出文字方程式。然后用正确的化学式替换每种物质名称，使用元素周期表中的元素符号。
For example: sodium + chlorine → sodium chloride becomes Na + Cl₂ → NaCl.
例如：钠 + 氯 → 氯化钠 变成 Na + Cl₂ → NaCl。
The reactants go on the left of the arrow, and the products go on the right.
反应物写在箭头的左边，生成物写在箭头的右边。
sodium + chlorine → sodium chloride
2Na + Cl₂ → 2NaCl
3. Chemical Formulae You Must Know | 必须掌握的化学式
You should be able to recall the formulae of common compounds. These are built from symbols and small subscript numbers that show how many atoms of each element are present.
你应该熟记常见化合物的化学式。这些化学式由元素符号和表示每种元素原子个数的小下标数字组成。
| Substance | Formula | 中文名称 |
|---|---|---|
| Water | H₂O | 水 |
| Carbon dioxide | CO₂ | 二氧化碳 |
| Methane | CH₄ | 甲烷 |
| Ammonia | NH₃ | 氨 |
| Hydrochloric acid | HCl | 盐酸 |
| Sulfuric acid | H₂SO₄ | 硫酸 |
| Sodium hydroxide | NaOH | 氢氧化钠 |
| Sodium chloride | NaCl | 氯化钠 |
| Calcium carbonate | CaCO₃ | 碳酸钙 |
| Magnesium oxide | MgO | 氧化镁 |
Remember: a subscript only applies to the element immediately before it. In H₂O there are two hydrogen atoms and one oxygen atom.
记住：下标只对它前面的那个元素起作用。在 H₂O 中，有两个氢原子和一个氧原子。
4. Diatomic Molecules and Their Formulae | 双原子分子及其化学式
Some elements exist naturally as pairs of atoms, called diatomic molecules. You must write these as X₂, not as a single atom.
有些元素天然以双原子分子形式存在，称为双原子分子。你必须把它们写作 X₂，而不是单个原子。
The common diatomic elements are hydrogen H₂, oxygen O₂, nitrogen N₂, fluorine F₂, chlorine Cl₂, bromine Br₂ and iodine I₂.
常见的双原子元素有：氢气 H₂、氧气 O₂、氮气 N₂、氟气 F₂、氯气 Cl₂、溴 Br₂ 和碘 I₂。
Forgetting the subscript 2 is one of the most common errors when writing symbol equations.
忘记下标 2 是书写符号方程式时最常见的错误之一。
5. What a Symbol Equation Shows | 符号方程式说明什么
A balanced symbol equation shows the chemical formulae of all reactants and products, their physical states, and the ratio between the particles that react and are produced.
配平的符号方程式显示所有反应物和生成物的化学式、它们的物理状态以及参加反应和生成的微粒之间的比例。
icients 2, 1 and 2 tell us that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.
在 2H₂(g) + O₂(g) → 2H₂O(l) 中，系数 2、1 和 2 告诉我们，两个氢分子与一个氧分子反应生成两个水分子。
The arrow means 'reacts to form' or 'produces'.
箭头表示“反应生成”或“产生”。
6. The Law of Conservation of Mass | 质量守恒定律
Atoms are not created or destroyed in a chemical reaction. Therefore, the number of atoms of each element must be the same on both sides of the equation.
在化学反应中，原子既不会凭空产生，也不会凭空消失。因此，每种元素的原子数在方程式两边必须相等。
This is why we balance equations by adding large numbers in front of formulae, never by changing the small subscript numbers.
这就是为什么我们通过在化学式前面加大数字来配平方程式，而绝不能改动小下标数字。
If you change H₂O to H₂O₂, you have changed the substance. If you
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