Chemical equations
Introduced| English | Meaning |
|---|---|
| reactants/rɪˈæktənts/ | The substances consumed or changed during a chemical reaction. |
| products/ˈprɒdʌkts/ | The substances formed by a chemical reaction. |
| chemical equation/ˈkemɪkl ɪˈkweɪʒn/ | chemical equation |
| balanced/ˈbælənst/ | Describing a chemical equation with the same number of atoms of each element on both sides and consistent total charge. |
| conservation of mass/ˌkɒnsəˈveɪʃn ɒv mæs/ | The principle that total mass remains unchanged in an ordinary chemical reaction within a closed system. |
| coefficients/ˌkəʊɪˈfɪʃənts/ | Numerical or constant factors multiplying terms in an expression. |
Writing what happens in a reaction
- A chemical reaction turns reactants 反应物 into products 生成物.
- We record it with a chemical equation 化学方程式 — and it must be balanced 配平.
Equation balancing route
Watch atom counts stay equal on both sides of an equation.
In an equation, the substances on the left of the arrow are the:
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Reactants → products.
Word and symbol equations
- A word equation uses names: methane + oxygen → carbon dioxide + water.
- A symbol equation uses formulae: CH₄ + 2O₂ → CO₂ + 2H₂O.
A balanced symbol equation has the same number of each atom on both sides
A balanced equation must have the same number of each atom on both sides because:
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Conservation of mass: atoms are only rearranged.
A balanced equation obeys the law of conservation of ______.
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No atoms are gained or lost.
Conservation of mass 质量守恒
- Atoms are not created or destroyed in a reaction — they are rearranged.
- So there must be the same number of each atom on both sides. This is balancing.
To balance an equation you change the:
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Only coefficients may be changed, not subscripts.
In the equation 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are on the left?
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2 × H₂ = 4 hydrogen atoms.
How to balance
- Adjust the big numbers (in front of formulae) — never change a formula's subscripts.
- Count each element on both sides and add coefficients 系数 until they match.
Worked example. H₂ + O₂ → H₂O is not balanced (2 O on the left, 1 on the right). Balance it: 2H₂ + O₂ → 2H₂O — now 4 H and 2 O on each side.
You've got it
- a reaction turns reactants → products; a symbol equation uses formulae
- mass is conserved: the same number of each atom on both sides
- balance by changing the big numbers (coefficients), never the subscripts