iGCSE Chemistry (0620) Core:6.4 Redox: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
In this reaction, copper(II) oxide (CuO) loses its oxygen to become copper (Cu). Loss of oxygen is the definition of reduction, so CuO is reduced. Carbon (C) gains oxygen to become carbon dioxide (CO₂) – gain of oxygen is oxidation, so carbon is oxidised. CO₂ is a product and is not being oxidised further. Copper is a product of reduction, not being oxidised. So the only correct statement is that CuO is reduced.
▶️ Answer/Explanation
Detailed solution:
Redox reactions involve changes in oxidation states, often visible as one substance gaining oxygen while another loses it. Incomplete combustion of propane involves oxygen adding to carbon and hydrogen atoms (oxidation), and rusting is iron metal being oxidised to iron(III) oxide. So reactions 1 and 2 are definitely redox. Reaction 3 is a straightforward precipitation of silver chloride with no oxidation state changes. Reaction 4 is just calcium carbonate breaking down into calcium oxide and carbon dioxide, which is a thermal decomposition but not a redox reaction. So only 1 and 2 fit the bill.
▶️ Answer/Explanation
Detailed solution:
Reduction can be defined as the loss of oxygen. Here, water ($\text{H}_2\text{O}$) loses its oxygen to carbon, turning into hydrogen ($\text{H}_2$). So, water is reduced. Carbon gains oxygen (becomes CO), so it is oxidised. We look for the substance that loses oxygen, which is $\text{H}_2\text{O}$.