New
New
Year 8
Chemical reactions: decomposition
I can describe what happens during decomposition reactions, and write chemical equations to show these.
New
New
Year 8
Chemical reactions: decomposition
I can describe what happens during decomposition reactions, and write chemical equations to show these.
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Lesson details
Key learning points
- Decomposition reactions involve a single compound breaking down into two or more products.
- Decomposition reactions usually require heat, light, or electricity, to occur.
- When metal carbonates decompose upon heating, they typically form metal oxides and carbon dioxide gas.
Keywords
Decompose - When a substance decomposes it undergoes a decomposition reaction.
Decomposition - A type of reaction in which a reactant compound breaks down into two or more products.
Common misconception
Pupils often struggle to understand that not all reactions consist of two substances mixed together to cause a reaction.
Show the symbol equation and how the reactants are rearranged to form the products.
Pupils can work together to obtain the results, give students a timescale of what constitutes no thermal decomposition, i.e. if there is no change in 3 minutes consider that thermal decomposition has not taken place.
Teacher tip
Equipment
Boiling tubes, metal carbonates, clamp and stand, heat resistant mat, bung with delivery tube, spatula, limewater.
Content guidance
- Risk assessment required - equipment
Supervision
Adult supervision required
Licence
This content is © Oak National Academy Limited (2025), licensed on
Open Government Licence version 3.0
except where otherwise stated. See Oak's terms & conditions (Collection 2).Lesson video
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Starter quiz
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6 Questions
Q1.
What are common signs that a chemical reaction is taking place?
the substance melts
Q2.
In a chemical reaction, what happens to the atoms of the reactants?
they disappear
they change
they become larger
Q3.
What happens to the mass of the products compared to the mass of the reactants in a chemical reaction?
it increases
it decreases
it fluctuates
Q4.
What type of reaction involves a substance combining with oxygen?
decomposition
neutralisation
precipitation
Q5.
To test for carbon dioxide, you would bubble the gas through . If carbon dioxide is present, the solution turns milky (cloudy).
Q6.
Which of the following chemicals are reactants in this reaction?
zinc + hydrochloric acid → zinc chloride + hydrogen
zinc chloride
hydrogen
Exit quiz
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6 Questions
Q1.
What is a decomposition reaction?
A reaction where two compounds combine to form one product.
A reaction where a gas is produced by combining liquids.
A reaction where heat is absorbed.
Q2.
Decomposition reactions usually require , light, or electricity to occur.
Q3.
What gas is commonly produced when metal carbonates decompose?
oxygen
hydrogen
nitrogen
water
Q4.
Which of the following is a decomposition reaction?
2H₂ + O₂ → 2H₂O
NaCl + AgNO₃ → NaNO₃ + AgCl
2K + Cl₂ → 2KCl
Q5.
When metal carbonates decompose upon heating, they typically form metal and carbon dioxide gas.
Q6.
Which of the following reactions represents the decomposition of water?
2H₂ + O₂ → 2H₂O
2H₂O₂ → 2H₂O + O₂
2Li + 2H₂O → 2LiOH + H₂