Chemical changes — AQA GCSE Practice Question 07.4

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Think about the practicalities of an industrial process. High temperatures require a lot of energy, which costs a lot of money. The purpose of adding cryolite is to reduce the costs of the process. How does it achieve this?

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A.The mixture has a lower melting point than pure aluminium oxide.✓ Correct

The extraction of aluminium is done by electrolysing aluminium oxide (Al₂O₃). However, aluminium oxide has an extremely high melting point of over 2000°C. Maintaining such a high temperature would be incredibly expensive due to the vast amount of energy required. To overcome this, the aluminium oxide is dissolved in molten cryolite (Na₃AlF₆). The resulting mixture has a much lower melting point, around 950°C. This significantly reduces the energy requirements and costs of the industrial process.

Common mistakes

✗ Thinking cryolite increases the melting point.\n✗ Confusing the role of cryolite with conductivity (though it does also improve conductivity).

Aluminium is extracted by electrolysis. The electrolyte is a molten mixture of aluminium oxide and cryolite. Why is a mixture used instead of pure aluminium oxide as the electrolyte?

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Practice the full AQA GCSE Chemistry Foundation Tier Paper 1

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