AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical changes › Reactivity of metals
Practise Metal oxides. 9 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How metals react with oxygen to form metal oxides, and what oxidation and reduction mean in terms of oxygen. Expect word equations, balancing symbol equations and short questions asking which substance is oxidised or reduced, with a reason.
Key facts
metal + oxygen → metal oxide
Oxidation = gain of oxygen
Reduction = loss of oxygen
Redox = oxidation and reduction in the same reaction
2Mg + O2 → 2MgO
4Na + O2 → 2Na2O
2Cu + O2 → 2CuO (black copper(II) oxide)
Oxide ion: O2−
Notes
Metals reacting with oxygen
Many metals react with oxygen (for example in the air) to produce a metal oxide.
Word equation: metal + oxygen → metal oxide, e.g. magnesium + oxygen → magnesium oxide.
Symbol equation: 2Mg + O2 → 2MgO. Oxygen gas is always O2, so you often need a 2 in front of the oxide.
More reactive metals react with oxygen more quickly: freshly cut sodium goes dull within seconds, but copper only reacts noticeably when heated, forming black copper(II) oxide.
Gold does not react with oxygen, which is why it stays shiny.
Oxidation and reduction in terms of oxygen
diagram
Oxidation is the gain of oxygen. A metal that reacts with oxygen is oxidised.
Reduction is the loss of oxygen. A metal oxide that loses its oxygen to form the metal is reduced.
Oxidation and reduction happen together: if one substance gains oxygen, another substance must have lost it. A reaction with both is a redox reaction.
Example: copper(II) oxide + hydrogen → copper + water. Copper(II) oxide loses oxygen, so it is reduced. Hydrogen gains oxygen, so it is oxidised.
Follow the oxygen: the substance that loses it is reduced, the one that gains it is oxidised.
Example: 2PbO + C → 2Pb + CO2. Lead(II) oxide is reduced; carbon is oxidised.
Writing formulae and balancing
The oxide ion is O2−. Balance the charges to get the formula: Na+ gives Na2O, Mg2+ gives MgO, Al3+ gives Al2O3.
Balance an equation by putting numbers in front of formulae. Never change the small numbers inside a formula.
Check by counting every atom of each element on both sides.
How to answer each type of question
Write a word or balanced symbol equation
1 to 3 marksGrade 4
Name the product: metal + oxygen gives the metal oxide.
For a symbol equation, write the correct formulae first (oxygen is O2).
Balance with numbers in front of formulae, then count every atom on both sides.
Example. Calcium burns in oxygen to form calcium oxide. (a) Write a word equation for this reaction. (b) Write a balanced symbol equation for this reaction.
Which substance is reduced (or oxidised)? Give a reason
2 marksGrade 5
Find the substance that loses oxygen: it is reduced.
Find the substance that gains oxygen: it is oxidised.
Give the reason using the words 'loses oxygen' or 'gains oxygen'.
Example. Hot iron(III) oxide reacts with carbon monoxide: Fe2O3 + 3CO → 2Fe + 3CO2 Which substance is reduced? Give a reason for your answer.
Show the model answerHide the model answer
Iron(III) oxide (1) It loses oxygen (to form iron) (1)
Don’t lose marks
Writing oxygen as O instead of O2 in a symbol equation.
Changing a formula to balance an equation, e.g. writing MgO2.
Naming the substance that is reduced but not giving the reason (loses oxygen), which loses the second mark.
Saying the oxide is oxidised: an oxide that becomes a metal has lost oxygen, so it is reduced.
More tips
Memory tricks
Oxidation has 'oxygen' in its name: oxidation = oxygen added.
An oxide that turns into an element has been 'reduced' down to the bare element: it lost oxygen.
Oxygen gas is O2, so metal oxide equations nearly always need a 2 (or 4) somewhere: 2Mg, 4Na, 4Al.
Exam technique
When a question says 'give a reason', always use 'gains oxygen' or 'loses oxygen'.
Name the whole substance, e.g. 'copper(II) oxide is reduced', not just 'copper'.
Only add state symbols if the question asks for them.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the oxide formed by a metalA metal reacting with oxygen forms the metal oxide, e.g. magnesium + oxygen → magnesium oxide.
Grade 4
Define oxidation and reduction using oxygenOxidation is the gain of oxygen and reduction is the loss of oxygen.
Grade 5
Identify what is oxidised or reducedIn an equation, the substance that gains oxygen is oxidised and the substance that loses oxygen is reduced.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Metals react with oxygen to produce metal oxides. Name the product formed when calcium reacts with oxygen.
calcium oxide
Write a balanced symbol equation for the reaction of lithium with oxygen.
4Li + O2 → 2Li2O
Metals react with oxygen to form metal oxides. Name the oxide formed when potassium burns in oxygen.
potassium oxide
A student holds a small piece of magnesium ribbon in tongs and places it into a Bunsen flame. Name the product formed, and give its formula.
magnesium oxide, MgO
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Metals react with oxygen to produce metal oxides.
(a) Name the product formed when calcium reacts with oxygen.[1]
(b) Balance the equation for the reaction of copper with oxygen. ....Cu + O2 → ....CuO[1]
(c) The reaction of a metal with oxygen is an oxidation reaction. Explain why.[1]
(d) Which change is a reduction? Tick (✓) one box.[1]
copper oxide → copper
iron → iron oxide
carbon → carbon dioxide
magnesium → magnesium oxide
Show the answer and mark scheme
(a)Answer: calcium oxide
calcium oxide
(b)Answer: 2Cu + O2 → 2CuO
2Cu + O2 → 2CuO
(c)
the metal gains oxygen
(d)Answer: copper oxide → copper
Question 2Medium5 marks
A student passed hydrogen gas over heated black copper(II) oxide. The powder turned pink-brown and droplets of water formed further along the tube. CuO + H2 → Cu + H2O
(a) Which substance was reduced? Give a reason for your answer.[2]
(b) Which substance was oxidised? Give a reason for your answer.[2]
(c) The student let hydrogen flow through the tube for some time before heating it. Suggest why.[1]
Show the answer and mark scheme
(a)
copper(II) oxide
it lost oxygen
(b)
hydrogen
it gained oxygen (to form water)
(c)
to remove the air, because a mixture of hydrogen and air / oxygen can explode when heated