Conservation of Mass — Interactive Lesson (KS3 Chemistry)

Where does the missing mass go in an open system? This 20-slide interactive lesson explains conservation of mass, a law that can seem to be broken until you understand it properly.

Mastering Conservation of Mass in KS3 Chemistry

Students learn to explain the law of conservation of mass, understand that mass is conserved in chemical reactions, and practise balancing and analysing chemical equations. They learn that matter cannot be created or destroyed, a law proven by Antoine Lavoisier, so the total mass of reactants always equals the total mass of products. In a closed system mass is clearly conserved, while in an open system gases may escape and make it seem like mass is lost, when it has simply moved out of the system; atoms are only rearranged during a reaction, never gained or lost.

What’s Included

• 20-slide interactive lesson on conservation of mass
• Learning objectives on the law and balancing chemical equations
• Reactants versus products explained clearly
• Open systems versus closed systems, and where ‘lost’ mass really goes
• Fill-in-the-blank and sentence-writing activities with instant feedback

Key Benefits for Students

• Explains a law that can seem counterintuitive at first
• Clarifies the open-system misconception directly
• Links the law to reading and balancing equations
• Instant feedback on answer questions

Why Teachers Love It

• No prep — ready for whole-class teaching on an interactive whiteboard
• A structured lesson that can be split across sessions
• Self-marking questions support independent practice
• Suits teaching, revision and homework
• A dependable foundation before balancing equations in depth

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A dependable addition to a KS3 chemistry topic, giving students a clear explanation for a law that regularly confuses learners at first. Understanding conservation of mass here is essential before tackling balancing equations and stoichiometry.

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FAQs

What year group is this for?
KS3 — Year 8 (ages 12–13).

Does it explain open versus closed systems?
Yes — including why mass can seem to disappear.

Is it self-marking?
Yes, with instant feedback on the questions.