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GCSE Chemistry · key term
Oxygen molecules, O₂, that are not chemically combined with other elements in a compound.
Used by OCR
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
Revise: Development of oxygen-rich atmosphereOnly after major oxygen sinks had become depleted did free oxygen rise substantially; by about 0.6 billion years ago, significant amounts were accumulating in the atmosphere.
Do not suggest that volcanoes supplied the atmospheric oxygen. Volcanic gases provide a possible explanation for the early atmosphere, but contain little or no free oxygen.
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