Lesson focus: Metals react with oxygen at different rates to form oxides, providing evidence for the reactivity series and conservation of mass.
Compare metal oxidationA reaction in which a substance gains oxygen (or loses electrons), such as iron rusting., write oxide equations and explain mass change in open and closed systemA container or set-up where no substances can get in or out..
Build on earlier learning
Chemical reactions of metals and non-metalsThe previous lesson explained how metals form positive ions with non-metals. Oxygen forms oxide ions in metal oxides.
Practical
Burning metals in oxygen
- Watch a demonstration of magnesium, iron wool and copper heated in air.
- Compare the brightness of the reaction and the product formed.
Record: Observations and word equations for each oxide formed.
Safety: Do not look directly at burning magnesium.
You will be successful when you can…
- State the metal–oxygen pattern.
- Compare oxidation observations.
- Explain mass gain using oxygen uptake.
- Distinguish open and closed-system measurements.
Words that carry the science
- oxidation
- A reaction involving oxygen, or more generally a loss of electrons.
- metal oxide
- A compound containing metal ions and oxide ions.
- combustion
- A reaction with oxygen that releases energy, often visibly as heating or light.
- tarnish
- A surface layer formed when a metal reacts with substances in its environment.
- passivation
- Protection by a thin, adherent surface layer that slows further reaction.
- closed system
- A system from which matter cannot enter or leave during the measurement.
Use oxidation evidence without losing track of oxygen atoms
01Metal oxides form at different rates
The general pattern is metal + oxygen → metal oxide. Magnesium burns rapidly with bright white light; iron wool glows when heated; copper darkens more slowly to copper oxide.
Rate and conditions matter. Aluminium is reactive but often appears resistant because a compact aluminium oxide layer passivates the surface.
02Mass increases because oxygen joins the metal
When magnesium burns, oxygen atoms from the air become part of solid magnesium oxide, so the measured solid mass increases. The additional mass has not appeared from nowhere.
In a closed systemA container or set-up where no substances can get in or out., total mass remains constant. In an open crucible, oxygen enters the measured solid while smoke or product loss can make practical results lower than the theoretical gain.
03Fair comparisons need matched preparation
Compare equal surface areas or amounts, remove existing oxide consistently and supply oxygen similarly. A lid can reduce product loss while still admitting oxygen when briefly lifted under controlled conditions.
Hot metals and intense magnesium light are hazards. Use small teacher-controlled samples or a simulation, heatproof equipment and appropriate eye protection; do not stare directly at burning magnesium.
Metals and oxygen
When a metal reacts with oxygen it forms a metal oxide. Heat a metal in a crucible with the lid lifted now and then to let air in: the mass goes up, because oxygen atoms from the air join the metal. More reactive metals react faster.
Controls
Final masses come from relative atomic masses (each oxide has one oxygen atom per metal atom). Heating times are illustrative.
Explain a mass increase
A crucible contains 0.60 g magnesium before heating and 0.96 g magnesium oxide afterwards.
- Find the increase: 0.96 − 0.60 = 0.36 g.
- Identify the source: oxygen from the air.
- State that magnesium atoms have combined with oxygen atoms.
- Explain that total mass including the surrounding air is conserved.
How to make the reasoning visible: The product is heavier than the starting magnesium because it includes oxygen that was previously outside the measured solid.
Compare oxidation evidence
The observations assume small, freshly prepared samples under controlled heating.
| Metal | Observation | Relative rate | Surface effect | Cautious conclusion |
|---|---|---|---|---|
| Magnesium | Bright white light; white solid | Very fast | Oxide powder forms | Highly reactive with oxygen |
| Iron wool | Glows; dark solid | Moderate when heated | Fine strands expose area | Reacts less vigorously here |
| Copper | Slow black coating | Slow | Surface darkens | Reacts under heating |
| Aluminium sheet | Little visible change | Surface-limited | Adherent oxide layer | Passivation masks reactivity |
Why is iron wool used instead of an iron nail?
Why should aluminium not be placed below copper using appearance alone?
Check your understanding
Answer from memory, then check against the suggested answer.
State the general equation for metal oxidation.
Why does the solid mass often increase?
What is passivation?
Why might an experimental magnesium oxide mass be too low?
Explain why rusting iron can eventually accelerate after a protective paint layer is scratched.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student heats magnesium in a crucible. The mass of the solid increases.
Explain why the mass of the solid increases.
(3)Suggest two reasons the measured mass gain may be inaccurate.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., account for the atoms first, then evaluate the actual apparatus.