AQA · Chemistry · 8462
GCSE Chemistry (AQA): Balancing Chemical Equations
Topic 1 — Atomic Structure & the Periodic Table. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Balancing Equations | Writing Word & Symbol Equations (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 1.04 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_1.04.
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What is the balanced symbol equation for the combustion of methane?
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CH4 + 2O2 → CO2 + 2H2O.
Chlorine is written in symbol equations as ? because it exists as a two-atom molecule.
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Cl2
Which elements should you balance first in an equation?
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The least common ones — those appearing in the fewest substances — leaving common elements like hydrogen and oxygen until last.
Why can't you change the small subscript numbers when balancing an equation?
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Changing them changes the substance itself — e.g. O2 becomes O3, ozone, a different chemical.
What does a single arrow in a chemical equation show?
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That the reactants react completely to form the products.
Balancing numbers in front of formulae must always be ? numbers, never fractions.
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whole
When is a chemical equation balanced?
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When there is the same number of each type of atom on both sides of the arrow.
What does 2O2 represent?
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Two molecules of oxygen (four oxygen atoms).
In a chemical equation, what are the substances on the left of the arrow called?
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The reactants — the substances that react together.
Why is oxygen written as O2 rather than O in equations?
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Because oxygen exists as a molecule made of two atoms.
In a chemical equation, what are the substances on the right of the arrow called?
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The products — the substances formed by the reaction.
What do you change to balance a chemical equation?
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The big numbers written in front of elements or compounds, which say how many of that substance there are.
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