Iron wool is burned in a sealed container of oxygen. After the reaction is complete, the container and its contents are reweighed. Explain what the balance reading will show and why, using the law of conservation of mass.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places iron wool and a sealed container of oxygen on a balance and records the starting mass. She then ignites the iron wool inside the sealed container. When the reaction is complete, she reweighs the sealed container.
Model answer (4 marks)
The balance reading will be the same – the total mass will not change.
The law of conservation of mass states that mass (atoms/matter) is neither created nor destroyed in a chemical reaction.
The atoms are simply rearranged – the same atoms are present before and after, just in different substances/products.
The container is sealed so no atoms/matter can enter or leave, meaning the total mass cannot change.
The law of conservation of mass states that mass (atoms/matter) is neither created nor destroyed in a chemical reaction.
The atoms are simply rearranged – the same atoms are present before and after, just in different substances/products.
The container is sealed so no atoms/matter can enter or leave, meaning the total mass cannot change.
Examiner tips
- Use the phrase ‘total mass will not change’ to hit the key point.
- Mention the sealed container to justify why no mass leaves or enters.
- Show the law of conservation of mass explicitly.
- Explain that atoms are rearranged, not created or destroyed.
Common mistakes
- Saying the mass will increase or decrease – ignores conservation of mass.
- Forgetting to mention the sealed container, implying mass could escape.
- Using vague terms like ‘the reaction changes the mass’ without explaining conservation.
Mark scheme (4 marks)
- The balance reading will be the same / the total mass will not change
- The law of conservation of mass states that mass (atoms/matter) is neither created nor destroyed in a chemical reaction
- The atoms are simply rearranged / the same atoms are present before and after (just in different substances/products)
- The container is sealed so no atoms/matter can enter or leave, meaning the total mass cannot change
Key terms in this question
law of conservation of mass · sealed container
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Quantitative chemistry questions
- Explain why the law of conservation of mass means that the total mass of product…
- A student dissolves a small amount of sodium hydroxide pellets in water inside a…
- Magnesium ribbon is burned in a open crucible in air. A student notices that the…
- A student heats copper carbonate powder in a crucible. The solid turns black and…