The combustion of elements in oxygen

2 marks · Hard

Explain why a strip of iron wool burns more vigorously in pure oxygen than in air.

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Must show: reference to higher oxygen content in pure oxygen, connected explanation of effect on combustion

Also accept: Pure oxygen provides more oxygen molecules to react with the iron, making the combustion faster and brighter than in air.; In pure oxygen, combustion is more rapid because iron gets more oxygen to react with than in air, which only has about 21% oxygen.

Common mistakes: Just saying 'more oxygen' without explanation of effect.; Ignoring the percentage of oxygen in air.

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Marking points

B1 · 1 mark · States that pure oxygen contains a higher proportion of oxygen than air (air is only 21% oxygen)

B1 · 1 mark · Links the increased oxygen to a more vigorous (faster/brighter) combustion reaction

Worked solution

Pure oxygen contains a much higher percentage of oxygen than air, which is only about 21% oxygen.

This higher concentration of oxygen results in a more vigorous (faster and brighter) burning of the iron wool.