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Lesson 28: Conservation of Mass (Chemical Reactions) Law of conservation of mass Why mass seems to change Closed systems
Year 8 • Chemistry
Conservation of Mass (Chemical Reactions)
In this lesson, students learn about the law of conservation of mass, understand why mass sometimes appears to change in chemical reactions, and explore the importance of closed systems.
Lesson Objectives
- State the law of conservation of mass.
- Explain why mass appears to change in some reactions.
- Understand the meaning of a closed system.
- Apply the idea to simple experiments.
1. Law of Conservation of Mass
The law of conservation of mass states that mass is never created or destroyed in a chemical reaction.
[ Image Placeholder – Mass Before and After Reaction ]
Key idea:
• Total mass of reactants = total mass of products
• Total mass of reactants = total mass of products
2. Why Mass Seems to Change
Sometimes it looks like mass has changed, but this is usually because gases escape into the air.
[ Image Placeholder – Gas Escaping Reaction ]
Example:
• A gas produced during a reaction escapes
• The measured mass appears to decrease
• A gas produced during a reaction escapes
• The measured mass appears to decrease
3. Closed Systems
A closed system is one where no substances can enter or leave during a reaction.
[ Image Placeholder – Closed Container Experiment ]
In a closed system:
• Gases cannot escape
• Mass before = mass after
• Gases cannot escape
• Mass before = mass after
4. Why Closed Systems Matter
Closed systems help scientists correctly measure mass and prove the conservation law.
[ Image Placeholder – Accurate Mass Measurement ]
Without a closed system:
• Mass may appear to change
• Results can be misleading
• Mass may appear to change
• Results can be misleading
Practice Questions
A. Fill in the Blanks
- Mass is __________ created or destroyed.
- The total mass before and after a reaction is __________.
- Mass may seem to change if a __________ escapes.
- A closed system does not allow __________ to escape.
- The conservation of mass applies to __________ reactions.
B. True or False
- Mass can disappear during reactions.
- Gas escape can affect mass readings.
- Closed systems keep mass constant.
- Mass always appears unchanged.
- The law applies to chemical reactions.
C. Short Answer
- What does the law of conservation of mass state?
- Why does mass sometimes appear to change?
- What is a closed system?
- Why are closed systems important?
- Give one example of a reaction where gas is produced.
✅ Show Answer Key
A. Fill in the Blanks
- never
- the same
- gas
- matter
- chemical
B. True or False
- False
- True
- True
- False
- True
C. Short Answer
- Mass is not created or destroyed.
- Because gas escapes.
- A system where nothing enters or leaves.
- They give accurate mass results.
- Acid reacting with metal.
© Aviate Learning – Conservation of Mass (Year 8)