# A student is investigating the composition of the universe and the Earth's crust. The student learns that the most abundant element in the universe is hydrogen, whilst the most abundant element in the Earth's crust is oxygen. Describe how elements are formed in stars and explain why the composition of the universe differs so greatly from the composition of the Earth's crust.

> OCR A-Level Chemistry B: Salters (H433) — EL Elements of life · Describe and Explain · 5 marks

> Hydrogen makes up approximately 75% of all normal matter in the universe by mass. Oxygen makes up approximately 46% of the Earth's crust by mass.

## Mark scheme (5 marks)

1. Hydrogen and helium were formed in the Big Bang / shortly after the Big Bang
2. Heavier / larger elements are formed by nuclear fusion in stars
3. Elements heavier than iron are formed when stars explode / in supernovae
4. The universe is mostly hydrogen because hydrogen was the starting material / most abundant element formed in the Big Bang and has not all been converted by fusion
5. The Earth's crust is rich in oxygen / heavier elements because it is made from the remains of exploded stars / stellar debris, so heavier elements became concentrated in planets

## Key terms

- [elements](https://www.gradenine.co.uk/glossary/elements)
- [universe](https://www.gradenine.co.uk/glossary/universe)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-composition-26506fe4) · Published by Druglandscape Ltd.