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The law of conservation of mass states that atoms are not created or destroyed, so reactant and product masses match. Free notes and 9 questions answered.
The Law of Conservation of Mass is part of Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations in GCSE AQA Higher Chemistry.
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The law of conservation of mass states that atoms are not created or destroyed, so reactant and product masses match. Free notes and 9 questions answered.
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Use this lesson to stay inside Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from The Law of Conservation of Mass before testing yourself from memory.
Imagine you're baking a cake. You start with 500g of ingredients - flour, eggs, sugar, butter. After baking, do you expect the cake to weigh 500g? Probably not, because water evaporates. But what if nothing could escape?
Chemical reactions follow the same logic. In any chemical reaction, the total mass of atoms is conserved. A sealed container simply makes this easy to observe directly because no gas can escape. This isn't just a neat trick; it's one of the fundamental laws of chemistry.
Think of atoms like Lego bricks. When you build something new, you don't create or destroy bricks - you just rearrange them. Chemical reactions work exactly the same way. Atoms get rearranged into new compounds, but the total number of each type of atom stays constant.
This means if you start with 10g of reactants, you'll always end up with exactly 10g of products. Always. The mass is conserved because the atoms are conserved.
Let's look at a simple reaction: magnesium burning in oxygen to form magnesium oxide.
Count the atoms on each side. Left side: 2 magnesium atoms, 2 oxygen atoms. Right side: 2 magnesium atoms, 2 oxygen atoms. The atoms balance, so the mass must balance too.
But here's where students often get confused. If you burn magnesium in an open flame, the white powder (magnesium oxide) seems to weigh more than the original magnesium ribbon. Has mass been created?
Conservation of mass becomes a powerful calculation tool. If you know the masses of some reactants and products, you can find the missing values. This is where examiners love to test you.
Worked example: Finding Missing Mass in a Reaction
Iron reacts with sulfur to form iron sulfide. 5.6g of iron reacts with sulfur to produce 8.8g of iron sulfide. What mass of sulfur reacted? [3 marks]
Answer: 3.2g of sulfur reacted
Sometimes mass appears to change during reactions. This catches many students out, but there's always a logical explanation. The key is understanding whether you're dealing with a closed or open system.
In a closed system, mass is always conserved. But in an open system, gases can escape or enter, making it look like mass has changed. The classic example is heating calcium carbonate.
If you heat limestone in an open container, the solid residue weighs less than the original limestone. Students often think mass has been destroyed. Actually, the 'missing' mass has escaped as carbon dioxide gas.
Conservation of mass questions are mark-winners if you know the technique. Examiners test this concept regularly because it combines understanding with calculation skills.
The most common mistake is forgetting that gases have mass too. Just because you can't see carbon dioxide doesn't mean it's massless. In exam questions, if the mass doesn't balance, ask yourself: 'What gas might have escaped?'
Exam precision note: In The Law of Conservation of Mass - Conservation of mass and balanced chemical equations - Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
The law of conservation of mass is about atoms, not about whether a balance reading always looks the same in an open beaker. If a gas escapes from an open vessel, the measured mass can fall even though no atoms have been destroyed. A sealed system prevents that loss, so the measurement reflects the real conservation of atoms.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants. Atoms are only rearranged into new compounds.
Gases have mass, even though they often can't be seen. Forgetting this is the most common mistake in conservation of mass questions: if a measured mass doesn't balance, it is worth asking what gas might have escaped, such as carbon dioxide from heated limestone.
Mass is conserved in a chemical reaction because atoms are not created or destroyed, only rearranged into new compounds. The total number of each type of atom stays constant, so the total mass of atoms stays the same.
To find a missing mass, use mass of reactants = mass of products. If 5.6g of iron reacts with sulfur to make 8.8g of iron sulfide, the mass of sulfur that reacted is 8.8g - 5.6g = 3.2g.
A closed system is a reaction container where no substances can enter or leave during the reaction, so mass is always conserved. In an open system, gases can escape or enter, so the measured mass can appear to change.
A sealed container shows conservation of mass because no gas can escape, so the balance reading reflects the real conservation of atoms. In an open beaker, a gas escaping can make the measured mass fall even though no atoms have been destroyed.
Atoms are never created or destroyed in a chemical reaction; they are only rearranged into new compounds, like Lego bricks. The total number of each type of atom stays constant, which is why the mass of the products equals the mass of the reactants.
Magnesium burning in oxygen to form magnesium oxide shows conservation of mass when atoms are counted. The left side has 2 magnesium atoms and 2 oxygen atoms, and the right side also has 2 magnesium atoms and 2 oxygen atoms, so the mass must balance too.
Measured mass might not balance in a reaction because it is an open system, where gases can escape or enter. Heating limestone in an open container leaves a residue that weighs less because carbon dioxide escapes, but no atoms have been destroyed; a sealed system prevents this loss.