The combustion of elements in oxygen

3 marks · Normal

Sodium burns with a bright yellow flame when heated in oxygen. Explain, with reference to the reaction, why the flame is brighter and the reaction is faster when sodium is heated in pure oxygen, compared to heating it in air.

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Must show: Reference to higher oxygen concentration in pure oxygen, Faster reaction due to more oxygen particles available for collision, Brighter flame as a result of faster/more complete combustion

Also accept: Burns more brightly/vigorously because there is more oxygen present; Pure oxygen allows faster reaction compared to air (which has only 21% oxygen)

Common mistakes: Saying air has more oxygen than pure oxygen; Ignoring concentration of oxygen

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Marking points

B1 · 1 mark · States/infers that air contains less oxygen (about 21%) than pure oxygen.

M1 · 1 mark · Explains higher oxygen concentration results in faster/more vigorous reaction.

A1 · 1 mark · Links the brighter flame to faster or more complete combustion in pure oxygen.

Worked solution

Air contains about 21% oxygen, whereas pure oxygen is 100% oxygen. When sodium is heated in pure oxygen, it has more oxygen particles available, so the reaction occurs faster because there are more collisions between sodium and oxygen particles. This results in a brighter flame and a faster reaction compared to burning in air, where less oxygen is present.