Conservation of Mass and Balancing Chemical Equations
Conservation of Mass
This GCSE chemistry lesson explores Conservation of Mass and Balancing Chemical Equations. Students apply conservation of mass to reactions, balance symbol equations and explain apparent mass changes in open and closed systems. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Lesson duration is: 55 minItems in this lesson
Conservation of Mass
This GCSE chemistry lesson explores Conservation of Mass and Balancing Chemical Equations. Students apply conservation of mass to reactions, balance symbol equations and explain apparent mass changes in open and closed systems. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Conservation of Mass, KS4 Chemistry, GCSE Chemistry, Law of Conservation of Mass, Chemical Reactions, Reactants, Products, Balanced Chemical Equations, Closed Systems, Non-Enclosed Systems, Relative Formula Mass, Stoichiometry, Precipitate Reactions, Gas Loss, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
Learning goal
I can explain the law of conservation of mass and apply it to chemical reactions in closed and open systems.
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What specific substances are left inside a sealed glass jar after a piece of wood finishes burning inside it?
Use this to surface prior ideas such as atoms, gas, closed system, burning, balance, same mass, or common misconceptions such as mass disappearing.
The law of conservation of mass
In a chemical reaction, atoms are rearranged
Atoms are not created and not destroyed
So the total mass stays the same
This is true when all substances are kept in the system
Stress that this is about chemical reactions, not nuclear reactions. Students often confuse visible change with loss of matter.
What happens to the atoms?
Reactant particles break and form bonds in new ways. The atoms present at the start are the same atoms present at the end, just joined differently in the products.
Use a simple example such as hydrogen reacting with oxygen to make water, focusing on the same atoms before and after rather than the reaction details.
Closed and open systems
A closed system does not let substances enter or leave
An open system can exchange substances with the surroundings
Mass is easiest to compare in a closed system
In an open system, the measured mass can appear to change
Keep the distinction physical and practical: lid on or off, gas trapped or allowed to escape. This will support later examples.
Why closed systems matter
No particles escape
No particles enter
Measured mass before reaction = measured mass after reaction
Link the measurement result directly to the particle idea: if every atom stays inside, the balance reading must stay the same.
Burning magnesium
Magnesium reacts with oxygen to form magnesium oxide
In a closed container, the total mass stays the same
Magnesium atoms and oxygen atoms are both still present after the reaction
Students may know that magnesium gains mass when burned in air. Clarify that this is because oxygen from the air is added to the magnesium.
Why does total mass stay the same in a closed system?
Atoms are rearranged
Energy becomes mass
Products weigh less
Bonds add atoms
Checks the core idea from slides 2 to 5. Distractors target common confusion between visible change, bond change and atom conservation.
Complete the summary
In a chemical reaction, atoms are rearranged and not created or destroyed. In a closed system, no substances enter or leave, so the total mass stays the same before and after the reaction.
Echoes the exact language of slides 2 to 6 and reinforces the link between particle model and measurement.
Why can burning magnesium seem to gain mass?
Expected elements: magnesium can seem to gain mass because oxygen from the air joins the magnesium in an open system. Mass is not created; oxygen has entered from outside the metal sample.
Why magnesium can seem to gain mass
In an open system, burning magnesium can give a larger final mass for the solid because oxygen from the air has joined the magnesium. The extra mass was not created: it came from outside the metal sample.
This is a key misconception point. The increase is real for the solid measured on its own, but not for the whole system including the air.
Acids and carbonates
Acid + carbonate produces a salt, water and carbon dioxide
Carbon dioxide is a gas
If the gas escapes, the measured mass can decrease
A familiar example is hydrochloric acid with calcium carbonate. Keep the focus on gas leaving the flask, not on naming every product.
Apparent mass loss in an open system
Reaction produces carbon dioxide → gas escapes to the air → balance reading falls
The mass has not been destroyed. It is no longer all inside the container being measured.
Use the arrow chain because the visible change in balance reading is the core idea here.
The same reaction in a closed system
Trap the carbon dioxide
Keep all reactants and products together
Total mass before and after is the same
A balloon over a flask is a useful demonstration because students can see gas produced without losing it from the system.
Why can mass seem to decrease in an acid-carbonate reaction?
Water is destroyed
Solid turns lighter
Gas escapes away
Atoms disappear
Based on slides 8 to 10. The correct idea is that carbon dioxide leaves the measured container in an open system.
Complete the explanation
An acid reacting with a carbonate can make carbon dioxide. In an open system, this gas may escape, so the balance reading can decrease. In a closed system, the gas is trapped and the total mass stays the same.
Reinforces the measurement chain from slides 8 to 10 and keeps the focus on what is inside the system being measured.
Balancing equations as evidence
Chemical equations show the atoms in reactants and products
A balanced equation has the same number of each type of atom on both sides
This matches the law of conservation of mass
Emphasise that balancing changes coefficients, not formulas. Students must not alter the substances themselves.
Example: magnesium and oxygen
`2Mg + O2 -> 2MgO`
Left side: 2 magnesium atoms, 2 oxygen atoms
Right side: 2 magnesium atoms, 2 oxygen atoms
The atoms are conserved
Point out that oxygen is diatomic, which is why O2 appears in the reactants.
Example: acid and carbonate
`2HCl + CaCO3 -> CaCl2 + H2O + CO2`
Count each type of atom on both sides
The numbers match when the equation is balanced
The gas formed is still part of the products
This example helps students connect balancing with the mass changes they observe when carbon dioxide escapes.
Interpreting mass measurements
Ask what was placed on the balance
Ask whether the system was closed or open
Include gases as matter, even if they cannot be seen
Compare the mass of the whole system, not just one part
Students often compare only the solid left in a container. Reframe the question as: what matter was actually measured?
Reading results correctly
A lower or higher balance reading does not automatically mean mass was destroyed or created. It usually means some substance left the system, or entered it from the air, during the reaction.
This is the landing point of the lesson: observations must be interpreted using the particle model and system boundaries.
What can you change when balancing an equation?
Atom symbols
Formulae
Coefficients
Products
Checks the warning on slide 11 before students overgeneralise balancing as rewriting substances.
Complete the equation idea
A balanced equation has the same number of each type of atom on both sides. For magnesium and oxygen, 2Mg + O2 -> 2MgO shows that atoms are conserved. To interpret mass results, compare the whole system, including any gases.
Brings together slides 11 to 15: balancing as evidence, and interpreting what was actually measured.
What should you check before judging a mass change?
Expected elements: students should mention checking what was on the balance, whether the system was open or closed, and whether gases entered or left. They may also note that a changed reading does not mean mass was created or destroyed.
How confident do you now feel in your understanding of the conservation of mass
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Glossary
law of conservation of mass - the idea that mass stays the same during a chemical reaction if no substances enter or leave
chemical reaction - a process in which atoms are rearranged to form new substances
atom - the smallest particle of an element that takes part in chemical reactions
reactant - a starting substance in a chemical reaction
product - a substance formed in a chemical reaction
closed system - a system where substances cannot enter or leave
open system - a system where substances can enter or leave
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Glossary
balanced equation - a chemical equation with the same number of each type of atom on both sides
coefficient - a number placed before a formula to show how many particles are involved
carbonate - a compound containing the carbonate ion that can react with acids to release carbon dioxide
carbon dioxide - a gas often made when acids react with carbonates
magnesium oxide - the compound formed when magnesium reacts with oxygen
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