Rocks older than about 2.4 billion years contain grains of a mineral that were rolled along by rivers without changing. This mineral reacts quickly with oxygen. Rocks younger than this that formed at the surface contain red iron oxide instead.
Which conclusion is best supported by this evidence?
Oxygen was present at first but had all been used up by 2.4 billion years ago
Photosynthesis by primitive plants first began exactly 2.4 billion years ago
There was little or no oxygen in the air until about 2.4 billion years ago
Volcanoes began to release large amounts of oxygen about 2.4 billion years ago
97 exam-style questions on Edexcel GCSE Chemistry 9.2 Earth and atmospheric science, covering 9.2.1 The Earth's early atmosphere, 9.2.2 How the atmosphere evolved, 9.2.3 Chemical test for oxygen, and 9.2.4 The greenhouse effect and climate change. Each one has a worked solution and a mark scheme showing where the marks go.