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📘 Study MCQs
Q1. What happens when copper powder is heated in air?
• It melts.
• It turns white.
• Its surface gets coated with black copper(II) oxide.
• It evaporates.
Answer: Its surface gets coated with black copper(II) oxide.
On heating, copper reacts with oxygen in the air to form a black coating of copper(II) oxide (CuO).
Q2. In the reaction 2Cu + O₂ → 2CuO, what happens to the copper?
• It is reduced.
• It is oxidised.
• It is displaced.
• It is decomposed.
Answer: It is oxidised.
Copper gains oxygen to form copper oxide. A substance that gains oxygen during a reaction is said to be oxidised.
Q3. What change is observed when hydrogen gas is passed over heated copper(II) oxide?
• The black coating turns blue.
• The black coating turns brown and copper is obtained.
• A white precipitate forms.
• The coating turns green.
Answer: The black coating turns brown and copper is obtained.
The reaction CuO + H₂ → Cu + H₂O reverses the previous change, turning the black CuO back to brown copper.
Q4. In the reaction CuO + H₂ → Cu + H₂O, what happens to copper(II) oxide?
• It is oxidised.
• It is reduced.
• It is decomposed.
• It is displaced.
Answer: It is reduced.
Copper(II) oxide loses oxygen to form copper. A substance that loses oxygen during a reaction is said to be reduced.
Q5. What is a reaction called where one substance is oxidised and another is reduced?
• Displacement reaction
• Combination reaction
• Oxidation-reduction or redox reaction
• Double displacement reaction
Answer: Oxidation-reduction or redox reaction
If one substance gets oxidised while the other gets reduced, the reaction is called an oxidation-reduction or redox reaction.
Q6. In the reaction ZnO + C → Zn + CO, which substance is oxidised?
• Zinc oxide (ZnO)
• Carbon (C)
• Zinc (Zn)
• Carbon monoxide (CO)
Answer: Carbon (C)
Carbon gains oxygen to form carbon monoxide (CO). Therefore, carbon is oxidised.
Q7. In the reaction ZnO + C → Zn + CO, which substance is reduced?
• Carbon (C)
• Zinc (Zn)
• Zinc oxide (ZnO)
• Carbon monoxide (CO)
Answer: Zinc oxide (ZnO)
Zinc oxide loses oxygen to form zinc (Zn). Therefore, zinc oxide is reduced.
Q8. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is reduced?
• Hydrochloric acid (HCl)
• Manganese dioxide (MnO₂)
• Manganese chloride (MnCl₂)
• Chlorine (Cl₂)
Answer: Manganese dioxide (MnO₂)
Manganese dioxide (MnO₂) loses oxygen and is converted to manganese chloride (MnCl₂). Therefore, it is reduced.
Q9. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is oxidised?
• Water (H₂O)
• Manganese dioxide (MnO₂)
• Hydrochloric acid (HCl)
• Manganese chloride (MnCl₂)
Answer: Hydrochloric acid (HCl)
Hydrochloric acid (HCl) loses hydrogen to form chlorine gas (Cl₂). Therefore, it is oxidised.
Q10. According to the text, a substance is oxidised if it:
• Gains oxygen or loses hydrogen.
• Loses oxygen or gains hydrogen.
• Loses electrons only.
• Gains electrons only.
Answer: Gains oxygen or loses hydrogen.
The text states: “if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised.”
Q11. According to the text, a substance is reduced if it:
• Gains oxygen or loses hydrogen.
• Loses oxygen or gains hydrogen.
• Gains electrons only.
• Loses electrons only.
Answer: Loses oxygen or gains hydrogen.
The text states: “if a substance loses oxygen or gains hydrogen during a reaction, it is reduced.”
Q12. When magnesium burns in air to form magnesium oxide, what happens to magnesium?
• It is reduced.
• It is oxidised.
• It is displaced.
• It is decomposed.
Answer: It is oxidised.
Magnesium gains oxygen to form magnesium oxide. Therefore, magnesium is oxidised.
Q13. What is the common term for the reddish-brown coating formed on iron when left exposed?
• Tarnishing
• Galvanisation
• Rusting
• Rancidity
Answer: Rusting
The process where iron gets coated with a reddish-brown powder is commonly known as rusting.
Q14. What is the general term for the process where a metal is attacked by substances like moisture and acids?
• Reduction
• Oxidation
• Corrosion
• Displacement
Answer: Corrosion
When a metal is attacked by substances around it (moisture, acids), it is said to corrode, and the process is called corrosion.
Q15. What are examples of corrosion on other metals besides iron?
• Black coating on silver and green coating on copper
• White coating on zinc and blue coating on aluminium
• Red coating on gold and yellow coating on lead
• Brown coating on magnesium and grey coating on sodium
Answer: Black coating on silver and green coating on copper
The text gives the examples of the black coating on silver and the green coating on copper as other examples of corrosion.
Q16. What is a major negative effect of corrosion?
• It makes metals stronger.
• It causes damage to metal objects like bridges and ships.
• It prevents metals from reacting.
• It improves the appearance of metals.
Answer: It causes damage to metal objects like bridges and ships.
Corrosion causes damage to car bodies, bridges, iron railings, ships, and all objects made of metals, especially iron.
Q17. What happens when fats and oils in food are oxidised?
• They become more nutritious.
• They become rancid, changing their smell and taste.
• They turn into proteins.
• They become sweeter.
Answer: They become rancid, changing their smell and taste.
Oxidation of fats and oils makes them rancid, which is characterised by a change in their smell and taste.
Q18. What are substances called that are added to food to prevent oxidation?
• Preservatives
• Antioxidants
• Catalysts
• Reducing agents
Answer: Antioxidants
Substances which prevent oxidation are added to foods containing fats and oils. These are called antioxidants.
Q19. How does keeping food in airtight containers help?
• It makes food cook faster.
• It slows down oxidation.
• It increases the fat content.
• It speeds up bacterial growth.
Answer: It slows down oxidation.
Keeping food in airtight containers helps to slow down oxidation by limiting contact with air (oxygen).
Q20. Why do chips manufacturers flush bags of chips with nitrogen gas?
• To make the chips heavier.
• To add flavour.
• To prevent the chips from getting oxidised.
• To change their colour.
Answer: To prevent the chips from getting oxidised.
Flushing with nitrogen, an inert gas, prevents oxygen from reacting with the fats in the chips, thus preventing oxidation and rancidity.
Q21. In the redox reaction CuO + H₂ → Cu + H₂O, which substance is the reducing agent?
• Copper oxide (CuO)
• Water (H₂O)
• Hydrogen (H₂)
• Copper (Cu)
Answer: Hydrogen (H₂)
Hydrogen (H₂) causes copper oxide to lose oxygen (i.e., reduces it) by itself gaining oxygen to form water. Therefore, hydrogen is the reducing agent.
Q22. In the redox reaction CuO + H₂ → Cu + H₂O, which substance is the oxidising agent?
• Hydrogen (H₂)
• Water (H₂O)
• Copper (Cu)
• Copper oxide (CuO)
Answer: Copper oxide (CuO)
Copper oxide (CuO) causes hydrogen to gain oxygen (i.e., oxidises it) by itself losing oxygen. Therefore, copper oxide is the oxidising agent.
Q23. What is the colour change when hydrogen reduces hot copper(II) oxide?
• White to grey
• Black to brown
• Blue to green
• Red to black
Answer: Black to brown
The black coating of copper(II) oxide (CuO) turns brown as it is reduced back to metallic copper (Cu).
Q24. Which of the following is NOT a characteristic of a redox reaction?
• Involves gain of oxygen
• Involves loss of oxygen
• Only one substance changes
• Both oxidation and reduction occur
Answer: Only one substance changes
In a redox reaction, two processes happen simultaneously: one substance is oxidised and another is reduced.
Q25. The process of rusting of iron involves which type of reaction?
• Displacement
• Combination
• Oxidation
• Decomposition
Answer: Oxidation
Rusting is an oxidation reaction where iron gains oxygen from the air in the presence of moisture to form iron oxide.
Q26. What is the main economic impact of corrosion mentioned in the text?
• It creates jobs.
• An enormous amount of money is spent to replace damaged iron.
• It makes metals more valuable.
• It has no economic impact.
Answer: An enormous amount of money is spent to replace damaged iron.
The text highlights that a serious problem of corrosion is the enormous cost spent every year to replace damaged iron structures.
Q27. What gas is used to flush chip bags to prevent rancidity?
• Oxygen
• Carbon dioxide
• Hydrogen
• Nitrogen
Answer: Nitrogen
Nitrogen gas is flushed into bags of chips to displace oxygen and prevent the oxidation (rancidity) of the fats.
Q28. Based on the text, which of these is a method to prevent rancidity?
• Adding water
• Keeping food exposed to air
• Adding antioxidants
• Heating food continuously
Answer: Adding antioxidants
Adding antioxidants, which prevent oxidation, is one way to prevent fats and oils from becoming rancid.
Q29. In the reaction where copper gains oxygen, the copper is said to be:
• Reduced
• Oxidised
• Decomposed
• Precipitated
Answer: Oxidised
Gaining oxygen is defined as oxidation. Therefore, copper is oxidised to copper oxide.
Q30. What is the term for the reaction that is the opposite of oxidation?
• Combustion
• Reduction
• Decomposition
• Displacement
Answer: Reduction
Reduction is the process where a substance loses oxygen (or gains hydrogen), which is the opposite of oxidation.
Q31. Which of the following processes is an example of oxidation in everyday life?
• Cutting an apple and it turns brown
• Melting of butter
• Freezing of water
• Mixing salt in water
Answer: Cutting an apple and it turns brown
While not explicitly stated in this text, the browning of a cut apple is a common example of oxidation. The text’s focus on oxidation aligns with such processes where substances react with oxygen.
Q32. What is the visual sign that a redox reaction has occurred in the copper heating experiment?
• Formation of a gas
• Change in colour of the metal
• Formation of a precipitate
• Change in temperature
Answer: Change in colour of the metal
The colour change from reddish-brown copper to black copper oxide indicates oxidation. The reverse colour change indicates reduction.
Q33. According to the broader definition given, HCl is oxidised to Cl₂ because it:
• Gains oxygen.
• Loses hydrogen.
• Loses oxygen.
• Gains hydrogen.
Answer: Loses hydrogen.
The text states HCl is oxidised in the reaction with MnO₂. Oxidation can involve losing hydrogen. HCl loses hydrogen to form Cl₂.
Q34. What is the primary cause of the rancidity of fatty foods?
• Reduction reaction
• Oxidation reaction
• Double displacement
• Decomposition by heat
Answer: Oxidation reaction
Rancidity is caused by the oxidation of fats and oils present in food when exposed to air.
Q35. Why is corrosion considered a redox process?
• Because it involves only reduction.
• Because it involves only decomposition.
• Because it involves both oxidation of the metal and reduction of oxygen.
• Because it involves displacement.
Answer: Because it involves both oxidation of the metal and reduction of oxygen.
Corrosion involves the metal (e.g., iron) losing electrons (oxidised) and oxygen gaining electrons (reduced). It is a redox process, though the text emphasises the oxidation aspect.
Q36. In the reaction CuO + H₂ → Cu + H₂O, hydrogen:
• Is oxidised to water.
• Is reduced to water.
• Is decomposed.
• Acts as an oxidising agent.
Answer: Is oxidised to water.
Hydrogen gains oxygen to form water (H₂O). Therefore, hydrogen itself is oxidised in this reaction.
Q37. What is the common element involved in both corrosion and rancidity?
• Nitrogen
• Hydrogen
• Oxygen
• Carbon
Answer: Oxygen
Both processes involve reaction with oxygen from the air: metals corrode by oxidising, and fats become rancid by oxidising.
Q38. What is the main purpose of the reaction where hydrogen gas is passed over heated copper oxide?
• To produce water.
• To demonstrate a displacement reaction.
• To demonstrate reduction of a metal oxide.
• To demonstrate decomposition.
Answer: To demonstrate reduction of a metal oxide.
The experiment shows that a metal oxide (CuO) can be reduced back to the metal (Cu) by using hydrogen as a reducing agent.
Q39. Which of the following is a consequence of rancidity?
• Improved food texture
• Change in smell and taste of food
• Increase in nutritional value
• Food becomes softer
Answer: Change in smell and taste of food
When fats and oils become rancid, their smell and taste change, making the food unpleasant.
Q40. From the examples, which substance can act as a reducing agent?
• CuO
• MnO₂
• H₂
• HCl
Answer: H₂
Hydrogen (H₂) reduces CuO to Cu by taking away its oxygen, acting as a reducing agent.
Q41. The green coating on copper is a result of:
• Reduction
• Corrosion
• Rancidity
• Displacement
Answer: Corrosion
The green coating (like on old statues) is an example of corrosion, where copper reacts with substances like carbon dioxide and moisture.
Q42. In a redox reaction, the total amount of oxygen lost by one substance is:
• Equal to the oxygen gained by another.
• More than the oxygen gained.
• Less than the oxygen gained.
• Unrelated to the oxygen gained.
Answer: Equal to the oxygen gained by another.
In a balanced redox reaction, the oxygen atoms lost by the substance being reduced are gained by the substance being oxidised.
Q43. What common household practice can slow down food spoilage due to rancidity?
• Storing in open containers
• Refrigeration
• Adding more oil
• Storing in airtight containers
Answer: Storing in airtight containers
Storing food in airtight containers limits its exposure to air (oxygen), thereby slowing down the oxidation that causes rancidity.
Q44. What is the chemical nature of the “black substance” formed on heating copper?
• Copper carbonate
• Copper oxide
• Copper chloride
• Copper sulphate
Answer: Copper oxide
The black substance is specifically copper(II) oxide, formed by the addition of oxygen to copper.
Q45. The conversion of ZnO to Zn in the reaction with carbon is an example of:
• Oxidation of zinc
• Reduction of zinc oxide
• Reduction of carbon
• Displacement of oxygen
Answer: Reduction of zinc oxide
Zinc oxide (ZnO) loses oxygen to form zinc (Zn). Therefore, zinc oxide is reduced.
Q46. Which of the following is a problem caused by oxidation reactions?
• Formation of alloys
• Corrosion of metals
• Production of salts
• Neutralisation
Answer: Corrosion of metals
Corrosion, which damages metals, is cited as a major problem caused by oxidation reactions in everyday life.
Q47. In the context of food preservation, flushing with nitrogen is effective because nitrogen:
• Is an oxidising agent.
• Is a reducing agent.
• Is chemically unreactive (inert).
• Adds flavour.
Answer: Is chemically unreactive (inert).
Nitrogen is an inert gas that does not readily react. It displaces oxygen, preventing the oxidation of food.
Q48. The reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ demonstrates that oxidation can involve:
• Only gain of oxygen.
• Only loss of hydrogen.
• Both gain of oxygen and loss of hydrogen.
• Neither gain of oxygen nor loss of hydrogen.
Answer: Loss of hydrogen.
In this reaction, HCl is oxidised to Cl₂. According to the text’s broader definition, HCl loses hydrogen, which is a form of oxidation.
Q49. What is the scientific term for the “reverse reaction” mentioned when hydrogen is passed over CuO?
• Displacement
• Decomposition
• Redox
• Reduction
Answer: Redox
The reaction CuO + H₂ → Cu + H₂O is the reverse of 2Cu + O₂ → 2CuO in terms of oxygen transfer. It is a redox reaction where CuO is reduced and H₂ is oxidised.
Q50. The fundamental idea behind the concept of redox reactions is the:
• Exchange of ions.
• Transfer of oxygen or hydrogen between substances.
• Formation of a precipitate.
• Absorption of heat.
Answer: Transfer of oxygen or hydrogen between substances.
As defined in the text, redox reactions involve one substance gaining oxygen/losing hydrogen (oxidation) and another losing oxygen/gaining hydrogen (reduction).