AQA · Chemistry · 8462
GCSE Chemistry: Conservation of mass and balanced chemical equations
Quantitative chemistry · AQA GCSE Chemistry (8462), specification point 4.3.1.1. Cards are generated from the exam board's own specification for "Conservation of mass and balanced chemical equations", and every card links back to the page it came from. Specification © AQA — used with attribution.
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State the law of conservation of mass.
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No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
The law of conservation of mass states that no atoms are ? during a chemical reaction.
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lost or made
How does the total mass of the products compare with the total mass of the reactants in a chemical reaction?
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They are equal — no atoms are created or destroyed, only rearranged.
Why can chemical reactions be represented by balanced symbol equations?
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Because mass is conserved — no atoms are lost or made — so the number of atoms of each element must be the same on both sides.
What must be equal on both sides of a balanced symbol equation?
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The number of atoms of each element.
A symbol equation is balanced when the numbers of ? are the same on both sides.
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atoms of each element
In a chemical equation, what does a large number written before a formula (e.g. the 2 in 2H₂O) represent?
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A multiplier: the number of that whole molecule or formula unit. 2H₂O means two water molecules.
In a chemical formula, what does a subscript number (e.g. the 2 in H₂O) represent?
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The number of atoms of the element immediately before it within one molecule — H₂O has two hydrogen atoms.
Which number can be changed to balance a symbol equation: the multiplier before a formula or the subscript within it?
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Only the multiplier before the formula. Changing a subscript would change the substance itself.
How many hydrogen atoms are represented by 2H₂O?
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4 — the multiplier 2 applies to the whole molecule, and each H₂O contains 2 hydrogen atoms.
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