📘 Study MCQs
Q1. What happens when copper powder is heated in air? CBSE 2026
A. It melts.
B. It turns white.
C. Its surface gets coated with black copper(II) oxide.
D. It evaporates.
C. Its surface gets coated with black copper(II) oxide.When copper powder is heated in the presence of air (oxygen), it undergoes oxidation to form copper(II) oxide (CuO). This oxide appears as a black coating on the surface of the copper. The reaction is 2Cu + O₂ → 2CuO. This colour change from reddish-brown to black is a clear visual indicator that a chemical reaction has taken place.
Q2. In the reaction 2Cu + O₂ → 2CuO, what happens to the copper? CBSE 2026
A. It is reduced.
B. It is oxidised.
C. It is displaced.
D. It is decomposed.
B. It is oxidised.In this reaction, copper gains oxygen to form copper oxide. According to the classical definition of oxidation, the gain of oxygen by a substance is called oxidation. Therefore, copper is oxidised. The copper atoms lose electrons (another definition of oxidation) to become Cu²⁺ ions, which then combine with oxygen to form CuO.
Q3. What change is observed when hydrogen gas is passed over heated copper(II) oxide? CBSE 2026
A. The black coating turns blue.
B. The black coating turns brown and copper is obtained.
C. A white precipitate forms.
D. The coating turns green.
B. The black coating turns brown and copper is obtained.When hydrogen gas is passed over heated black copper(II) oxide (CuO), the copper(II) oxide is reduced to reddish-brown copper metal. The hydrogen removes oxygen from the copper oxide, forming water and leaving behind copper. The colour change from black to brown indicates that the reduction reaction has occurred.
Q4. In the reaction CuO + H₂ → Cu + H₂O, what happens to copper(II) oxide? CBSE 2026
A. It is oxidised.
B. It is reduced.
C. It is decomposed.
D. It is displaced.
B. It is reduced.In this reaction, copper(II) oxide (CuO) loses oxygen to form copper (Cu). According to the classical definition of reduction, the loss of oxygen from a substance is called reduction. Therefore, copper(II) oxide is reduced. Hydrogen gains oxygen to form water, so hydrogen is oxidised.
Q5. What is a reaction called where one substance is oxidised and another is reduced? CBSE 2026
A. Displacement reaction
B. Combination reaction
C. Oxidation-reduction or redox reaction
D. Double displacement reaction
Oxidation-reduction or redox reaction.A reaction in which one substance is oxidised and another is reduced simultaneously is called an oxidation-reduction reaction, commonly shortened to redox reaction. These reactions always occur together because the electrons lost by the substance being oxidised are gained by the substance being reduced.
Q6. In the reaction ZnO + C → Zn + CO, which substance is oxidised? CBSE 2026
A. Zinc oxide (ZnO)
B. Carbon (C)
C. Zinc (Zn)
D. Carbon monoxide (CO)
Carbon (C).In this reaction, carbon (C) gains oxygen to form carbon monoxide (CO). The gain of oxygen is oxidation, so carbon is oxidised. At the same time, zinc oxide (ZnO) loses oxygen to form zinc (Zn), so zinc oxide is reduced. This is a redox reaction commonly used in the extraction of metals.
Q7. In the reaction ZnO + C → Zn + CO, which substance is reduced? CBSE 2026
A. Carbon (C)
B. Zinc (Zn)
C. Zinc oxide (ZnO)
D. Carbon monoxide (CO)
Zinc oxide (ZnO).In this reaction, zinc oxide (ZnO) loses oxygen to form zinc (Zn). According to the definition of reduction, loss of oxygen is reduction. Therefore, zinc oxide is reduced. Carbon gains oxygen to form carbon monoxide, so carbon is oxidised.