AQA · Chemistry · 8462
GCSE Chemistry (AQA): Conservation of Mass
Topic 3 — Quantitative Chemistry. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Conservation of Mass (2027/28 exams)"; each card deep-links to the moment in the video it came from. Lesson 3.04 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_3.04.
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If the measured mass of a reaction seems to decrease, one of the ? is probably a gas.
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products
How can you prevent an apparent change in mass in a reaction involving gases?
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Carry out the reaction in a sealed container, so all gaseous reactants and products are weighed.
Why does magnesium appear to gain mass when heated in air?
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It reacts with oxygen from the air to form magnesium oxide; the oxygen adds mass but was never weighed on the balance.
State the law of conservation of mass.
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In a chemical reaction, mass is always conserved — no atoms are created or destroyed, only the bonds between them change.
Why must chemical equations be balanced?
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Because mass is conserved — the same number of each type of atom must appear on both sides of the equation.
What is the balanced equation for sodium reacting with chlorine?
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2Na + Cl₂ → 2NaCl. Chlorine exists as a diatomic molecule, so two sodiums are needed.
Why can the measured mass appear to change during a reaction involving a gas?
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A gaseous reactant can come from the air, or a gaseous product can escape into the air, so the balance does not weigh it.
Why does the mass appear to decrease when calcium carbonate is decomposed in an open container?
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Carbon dioxide gas is produced and escapes into the air, so it is not included in the measured mass of the products.
If the measured mass of a reaction seems to increase, one of the ? is probably a gas.
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reactants
If 2.3 g of sodium reacts completely with 3.5 g of chlorine, what mass of sodium chloride forms?
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5.8 g — the total mass of the products equals the total mass of the reactants.
Roughly what mass of magnesium oxide is formed from 1 g of magnesium?
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About 1.6 g.
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