Chemical equations — Chemistry, 11–13 years
A chemical equation is a compact way to show which substances react and which new substances form.
A reaction written as a sentence
A chemical equation puts the starting substances on the left and the substances made on the right. An arrow means “changes into” or “produces”. Numbers in front show how many particles or formula units take part, so the equation describes a pattern rather than one single visible event.
Why not just use words?
Words can name the substances, but they hide the exact proportions. Chemists needed a way to compare reactions and calculate how much material was needed. Equations solve this by combining names or formulae with numbers, while still showing that the starting substances and products are different.
Making water
Start with H₂ + O₂ → H₂O. Count oxygen atoms: there are 2 on the left but only 1 on the right, so write 2H₂O. Now the right has 4 hydrogen atoms, so write 2H₂ on the left. The balanced equation is 2H₂ + O₂ → 2H₂O: four hydrogen atoms and two oxygen atoms on each side.
Changing the small number
When balancing, many people change H₂O into H₂O₂ because adding a number seems the quickest fix. That is a reasonable move in ordinary counting, but it changes the substance: H₂O₂ is hydrogen peroxide, not water. Balance by changing large numbers in front, never the small numbers inside a formula.
Planning real amounts
Factories use balanced equations to estimate the ingredients needed to make fertilisers, medicines or useful materials. A water-treatment worker can also predict which amount of a chemical will react with a contaminant. The equation is a guide for proportions; measurements still need checking because real materials are rarely perfectly pure.
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