Revision notes for AQA GCSE Chemistry Metal oxides. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Metal oxides
What you'll learn
- What a metal oxide is and how it forms.
- How to write word equations and simple balanced symbol equations for metals reacting with oxygen.
- Why these reactions are examples of oxidation.
- How to explain oxidation and reduction using gain or loss of oxygen.
Starting point: elements, compounds and oxygen
A metal is an element found on the left and middle of the periodic table. Examples include magnesium, iron, copper and zinc.
An element contains only one type of atom. A compound contains atoms of two or more different elements chemically joined together.
Oxygen in the air exists mainly as oxygen molecules, O₂. Each oxygen molecule contains two oxygen atoms joined together.
Definition
Metal oxide
A metal oxide is a compound made when a metal chemically combines with oxygen.
The general word equation is:
metal + oxygen → metal oxide
For example:
magnesium + oxygen → magnesium oxide
As a symbol equation with state symbols:
2Mg(s) + O₂(g) → 2MgO(s)
Here, (s) means solid and (g) means gas.
Metals reacting with oxygen
When metals react with oxygen, the oxygen atoms become chemically joined to the metal atoms. This makes a new substance: a metal oxide.
Some reactions are fast and obvious. For example, magnesium burns with a bright white flame to form white magnesium oxide.
Other reactions are slower. Iron reacts with oxygen from the air over time, forming iron oxides as part of rusting. Copper reacts with oxygen when heated, forming black copper(II) oxide.
Key Idea
The basic reaction pattern
When a metal reacts with oxygen, the product is a metal oxide:
metal + oxygen → metal oxide
Examples you should recognise
| Metal | Reaction with oxygen | Product |
|---|---|---|
| Magnesium | burns strongly | magnesium oxide |
| Copper | reacts when heated | copper(II) oxide |
| Iron | reacts slowly in air, faster when heated | iron oxide |
| Aluminium | reacts quickly at the surface | aluminium oxide layer |
Aluminium is a good one to remember: it forms a thin layer of aluminium oxide on its surface. This layer protects the aluminium underneath from further reaction.
Common Mistake
Thinking no visible change means no reaction
Some metal oxides form as a very thin surface layer, so the change may not be dramatic. A reaction can still have happened even if you cannot easily see it.
Word equations for metal oxides
A word equation shows the reactants and products using names instead of formulae.
Definition
Reactants and products
Reactants are the substances you start with. Products are the new substances made in the reaction.
For metals reacting with oxygen:
- The reactants are the metal and oxygen.
- The product is the metal oxide.
So:
zinc + oxygen → zinc oxide
iron + oxygen → iron oxide
copper + oxygen → copper oxide
You can often build these equations just by spotting the pattern.
Tip
Naming the product
For this topic, if a metal reacts with oxygen, the product name is usually the metal name followed by oxide: magnesium oxide, zinc oxide, copper oxide.
Balanced symbol equations
A symbol equation uses chemical formulae instead of names. A balanced equation has the same number of each type of atom on both sides of the arrow.
For example, magnesium reacting with oxygen is:
2Mg(s) + O₂(g) → 2MgO(s)
This is balanced because:
- Left side: 2 magnesium atoms and 2 oxygen atoms.
- Right side: 2 magnesium atoms and 2 oxygen atoms.
Example
Balancing magnesium reacting with oxygen
Balance the equation for magnesium reacting with oxygen to form magnesium oxide.
-
Start with the correct formulae: magnesium is Mg, oxygen gas is O₂, and magnesium oxide is MgO, so the unbalanced equation is Mg(s) + O₂(g) → MgO(s).
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Compare oxygen atoms. The left side has 2 oxygen atoms in O₂, but the right side has only 1 oxygen atom in MgO. Put a 2 in front of MgO: Mg(s) + O₂(g) → 2MgO(s).
-
Now compare magnesium atoms. The right side has 2 magnesium atoms because there are 2 lots of MgO, so put a 2 in front of Mg: 2Mg(s) + O₂(g) → 2MgO(s).
Oxidation: gain of oxygen
This section uses the oxygen definition of oxidation.
Definition
Oxidation
Oxidation is the gain of oxygen.
When a metal reacts with oxygen, the metal gains oxygen. That means the metal is oxidised.
For example:
2Cu(s) + O₂(g) → 2CuO(s)
Copper starts as copper metal, Cu. After reacting, it is in copper(II) oxide, CuO. The copper has gained oxygen, so copper has been oxidised.
Key Idea
Metal oxide reactions are oxidation reactions
Metals reacting with oxygen are oxidation reactions because the metal gains oxygen.
More examples
2Zn(s) + O₂(g) → 2ZnO(s)
Zinc gains oxygen, so zinc is oxidised.
4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)
Iron gains oxygen, so iron is oxidised.
Common Mistake
Saying oxygen is oxidised
In metal + oxygen reactions, the metal is oxidised because the metal gains oxygen. Oxygen itself is not “gaining oxygen” in this explanation.
Reduction: loss of oxygen
Reduction is the opposite of oxidation when you are using the oxygen definition.
Definition
Reduction
Reduction is the loss of oxygen.
A metal oxide can be reduced if oxygen is removed from it. For example:
CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Copper(II) oxide loses oxygen to become copper. So copper(II) oxide is reduced.
At the same time, hydrogen gains oxygen to become water, so hydrogen is oxidised.
Example
Identifying oxidation and reduction
In the reaction CuO(s) + H₂(g) → Cu(s) + H₂O(l), identify what is oxidised and what is reduced.
-
Track the oxygen in copper(II) oxide. CuO contains oxygen, but Cu does not, so copper(II) oxide has lost oxygen. It has been reduced.
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Track the hydrogen. H₂ does not contain oxygen, but H₂O does, so hydrogen has gained oxygen. It has been oxidised.
-
State both parts clearly: copper(II) oxide is reduced and hydrogen is oxidised.
Oxidation and reduction often happen together
In many reactions, one substance gains oxygen while another loses oxygen. This means oxidation and reduction can happen at the same time.
A helpful pair to remember is:
- Oxidation = gain of oxygen.
- Reduction = loss of oxygen.
Tip
Oxygen memory trick
Use OIL just for this topic’s oxygen definition:
Oxidation Is oxygen gained, and reduction is the reverse.
Later in chemistry, you may also meet oxidation and reduction in terms of electrons. For this sub-topic, if the question says “in terms of oxygen”, use gain of oxygen and loss of oxygen.
Common Mistake
Use the definition the question asks for
If a question asks you to explain oxidation or reduction “in terms of oxygen”, do not answer using electrons. Say gain of oxygen or loss of oxygen.
How to answer common GCSE questions
If the question asks: “What is made when a metal reacts with oxygen?”
Answer: a metal oxide.
For example:
calcium + oxygen → calcium oxide
If the question asks: “Why is this an oxidation reaction?”
You need to link the reaction to oxygen gain.
A strong answer would be:
“The metal gains oxygen, so the metal is oxidised.”
If the question asks: “Which substance is reduced?”
Look for the substance that loses oxygen during the reaction.
For example:
ZnO(s) + C(s) → Zn(s) + CO(g)
Zinc oxide loses oxygen to become zinc, so zinc oxide is reduced. Carbon gains oxygen to become carbon monoxide, so carbon is oxidised.
Example
Explaining oxygen transfer
In ZnO(s) + C(s) → Zn(s) + CO(g), explain which substance is oxidised and which is reduced.
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Compare ZnO with Zn. ZnO contains oxygen, but Zn does not, so zinc oxide has lost oxygen and has been reduced.
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Compare C with CO. C does not contain oxygen, but CO does, so carbon has gained oxygen and has been oxidised.
-
Link your answer to the definitions: reduction is loss of oxygen, and oxidation is gain of oxygen.
Key summary
- Metals react with oxygen to form metal oxides.
- The general equation is metal + oxygen → metal oxide.
- These reactions are oxidation reactions because the metal gains oxygen.
- Oxidation means gain of oxygen.
- Reduction means loss of oxygen.
- In oxygen-transfer reactions, one substance may be oxidised while another is reduced.
Exam technique
In the exam
- For “metal + oxygen” questions, immediately think: metal oxide is the product.
- If asked why a metal has been oxidised, write: it has gained oxygen.
- For reduction questions, compare reactant and product formulae and identify which substance has lost oxygen.
Self review
Check yourself
- What is the product when magnesium reacts with oxygen?
- In terms of oxygen, what does oxidation mean?
- In CuO(s) + H₂(g) → Cu(s) + H₂O(l), which substance is reduced?
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