Most metals are extracted from rocks found in the Earth's crust - Edexcel - GCSE Chemistry - Question 4 - 2017 - Paper 1

The correct answer is D: carbon is oxidised and iron oxide is reduced. This is because during the reaction, carbon acts as a reducing agent, converting iron oxide into iron while itself being oxidised.

Gold alloys are often used instead of pure gold due to their strength and durability. For instance, 18-carat gold is an alloy that provides a medium strength, making it less prone to scratches and damage compared to pure 24-carat gold, which is softer and more malleable. Furthermore, 18-carat gold is more cost-effective at £16 per gram compared to the higher cost of 24-carat gold at £22 per gram.

The method used to extract sodium from its ore is electrolysis.

Electrolysis is necessary for the extraction of sodium because sodium is a highly reactive metal that cannot be reduced by carbon or other reducing agents due to its reactivity. This method allows for the direct separation of sodium ions from their ionic compounds.

A shape memory alloy is a metal alloy that can return to a predetermined shape when it is heated above a certain temperature. This unique property allows them to be deformed at lower temperatures and then revert to their original shape when warmed, making them useful in various applications, such as stents in medical procedures.