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Lesson 27: What is a Chemical Reaction? Signs of a reaction New substances formed Comparing physical vs chemical change
Year 8 • Chemistry
What Is a Chemical Reaction?
In this lesson, students learn what a chemical reaction is, identify common signs that a reaction has taken place, understand how new substances are formed, and compare chemical changes with physical changes.
Lesson Objectives
- Define a chemical reaction.
- Identify signs of a chemical reaction.
- Explain how new substances are formed.
- Compare physical and chemical changes.
1. What Is a Chemical Reaction?
A chemical reaction happens when substances react together and form new substances with different properties.
[ Image Placeholder – Chemical Reaction Diagram ]
Key idea:
• The starting substances are called reactants
• The new substances formed are called products
• The starting substances are called reactants
• The new substances formed are called products
2. Signs of a Chemical Reaction
There are several clues that tell us a chemical reaction has happened.
[ Image Placeholder – Signs of Reaction ]
Common signs include:
• Colour change
• Gas bubbles
• Temperature change
• Formation of a solid (precipitate)
• Colour change
• Gas bubbles
• Temperature change
• Formation of a solid (precipitate)
3. New Substances Are Formed
In a chemical reaction, the products are different substances from the reactants.
These changes are usually difficult to reverse.
[ Image Placeholder – Before and After Reaction ]
Examples:
• Rust forming on iron
• Burning wood to ash
• Vinegar reacting with baking soda
• Rust forming on iron
• Burning wood to ash
• Vinegar reacting with baking soda
4. Physical vs Chemical Change
Not all changes are chemical reactions.
[ Image Placeholder – Physical vs Chemical Change ]
Physical change:
• No new substance formed
• Often reversible (melting ice)
• No new substance formed
• Often reversible (melting ice)
Chemical change:
• New substance formed
• Usually irreversible
Practice Questions
A. Fill in the Blanks
- A chemical reaction forms __________ substances.
- The starting substances are called __________.
- Bubbles can show a __________ is produced.
- Rusting is a __________ change.
- Melting ice is a __________ change.
B. True or False
- All changes are chemical reactions.
- Colour change can indicate a reaction.
- Physical changes form new substances.
- Chemical changes are often irreversible.
- Gas production is a sign of reaction.
C. Short Answer
- What is a chemical reaction?
- Name two signs of a chemical reaction.
- What is a reactant?
- Give one example of a chemical change.
- Give one example of a physical change.
✅ Show Answer Key
A. Fill in the Blanks
- new
- reactants
- gas
- chemical
- physical
B. True or False
- False
- True
- False
- True
- True
C. Short Answer
- A process where new substances are formed.
- Colour change / gas bubbles.
- A starting substance in a reaction.
- Rusting / burning.
- Melting ice.
© Aviate Learning – Chemical Reactions (Year 8)