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Q1. What happens when copper powder is heated in air?
Its surface gets coated with black copper(II) oxide.When copper powder is heated in air (oxygen), it undergoes oxidation. The reddish-brown copper metal reacts with oxygen to form black copper(II) oxide according to the equation: 2Cu + O₂ → 2CuO. This is a combination reaction as two substances combine to form a single product.
Q2. In the reaction 2Cu + O₂ → 2CuO, what happens to the copper?
It is oxidised.Copper (Cu) gains oxygen to form copper(II) oxide (CuO). Gaining oxygen is the classical definition of oxidation. Therefore, copper is oxidized. At the same time, oxygen is reduced (gains electrons from copper), making this a redox reaction even though the question focuses only on copper.
Q3. What change is observed when hydrogen gas is passed over heated copper(II) oxide?
The black coating turns brown and copper is obtained.In this reaction, CuO (black) + H₂ → Cu (brown/reddish) + H₂O. Hydrogen removes oxygen from copper(II) oxide, reducing it to copper metal. The color change from black to reddish-brown is clearly visible and indicates that reduction has occurred.
Q4. In the reaction CuO + H₂ → Cu + H₂O, what happens to copper(II) oxide?
It is reduced.Copper(II) oxide (CuO) loses oxygen to become copper metal (Cu). Losing oxygen is the classical definition of reduction. Therefore, copper(II) oxide is reduced. Hydrogen gains oxygen to become water, so hydrogen is oxidized.
Q5. What is a reaction called where one substance is oxidised and another is reduced?
Oxidation-reduction or redox reactionRedox is a contraction of reduction-oxidation. In such reactions, oxidation and reduction always occur simultaneously — one substance loses electrons (is oxidized) and another gains those electrons (is reduced). You cannot have one without the other.
Q6. In the reaction ZnO + C → Zn + CO, which substance is oxidised?
Carbon (C)Carbon (C) gains oxygen to form carbon monoxide (CO). Gaining oxygen is oxidation. Therefore, carbon is oxidized. This reaction is used in the extraction of zinc from zinc oxide (zinc ore). Carbon acts as a reducing agent.
Q7. In the reaction ZnO + C → Zn + CO, which substance is reduced?
Zinc oxide (ZnO)Zinc oxide (ZnO) loses oxygen to become zinc metal (Zn). Losing oxygen is reduction. Therefore, zinc oxide is reduced. The carbon (reducing agent) removes oxygen from ZnO, allowing zinc to be obtained in its elemental form.
Q8. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is reduced?
Manganese dioxide (MnO₂)In MnO₂, manganese is in +4 oxidation state. In MnCl₂, manganese is in +2 oxidation state. Manganese gains electrons (reduction) as its oxidation state decreases from +4 to +2. So MnO₂ is reduced. This reaction is used in the laboratory preparation of chlorine gas.
Q9. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is oxidised?
Hydrochloric acid (HCl)In HCl, chlorine is in -1 oxidation state. In Cl₂, chlorine is in 0 oxidation state. Chlorine loses electrons (oxidation) as its oxidation state increases from -1 to 0. HCl is oxidized to Cl₂. Specifically, the chloride ions (Cl⁻) are oxidized to chlorine gas.
Q10. A substance is oxidised if it:
Gains oxygen or loses hydrogen.This is the classical definition of oxidation. For example: gaining oxygen — 2Mg + O₂ → 2MgO (Mg is oxidized). Losing hydrogen — H₂S → S + H₂ (H₂S loses hydrogen, so sulphur is oxidized). The modern definition is loss of electrons, but the classical definition is still widely used.
Q11. A substance is reduced if it:
Loses oxygen or gains hydrogen.This is the classical definition of reduction. For example: losing oxygen — CuO → Cu (CuO loses oxygen, so it is reduced). Gaining hydrogen — S + H₂ → H₂S (sulphur gains hydrogen, so it is reduced). The modern definition is gain of electrons.
Q12. When magnesium burns in air to form magnesium oxide, what happens to magnesium?
It is oxidised.2Mg + O₂ → 2MgO. Magnesium gains oxygen to form magnesium oxide. Gaining oxygen is oxidation. So magnesium is oxidized. This reaction produces a dazzling white light and a lot of heat, making it an exothermic combination reaction as well.
Q13. What is the common term for the reddish-brown coating formed on iron when left exposed?
RustingRusting is the specific term for the corrosion of iron. Rust is hydrated iron(III) oxide with the approximate formula Fe₂O₃·xH₂O. It is reddish-brown, flaky, and weakens the iron structure. This process requires both oxygen and water.
Q14. What is the general term for the process where a metal is attacked by substances like moisture and acids?
CorrosionCorrosion is the gradual destruction of materials (usually metals) by chemical reactions with their environment. This includes attack by moisture, oxygen, acids, bases, salts, and other chemicals. Rusting of iron, tarnishing of silver, and patination of copper are all examples of corrosion.
Q15. What are examples of corrosion on other metals besides iron?
Black coating on silver and green coating on copperSilver tarnishes black due to formation of silver sulphide (Ag₂S). Copper develops a green coating of basic copper carbonate (Cu₂(OH)₂CO₃) when exposed to moist air containing CO₂. These are classic examples of corrosion on metals other than iron.
Q16. What is a major negative effect of corrosion?
It causes damage to metal objects like bridges and ships.Corrosion weakens metal structures, leading to potential failure of bridges, ships, pipelines, car bodies, railings, and industrial equipment. The economic cost of corrosion runs into billions of dollars annually for inspection, maintenance, repair, and replacement.
Q17. What happens when fats and oils in food are oxidised?
They become rancid, changing their smell and taste.Oxidation of unsaturated fats and oils produces volatile aldehydes, ketones, and carboxylic acids. These compounds cause a sharp, unpleasant odour (rancid smell) and a bitter or metallic taste, making the food unpalatable and potentially harmful.
Q18. What are substances called that are added to food to prevent oxidation?
AntioxidantsAntioxidants are compounds that prevent or slow down oxidation reactions. They work by being oxidized themselves instead of the food. Common food antioxidants include vitamin C (ascorbic acid), vitamin E (tocopherols), BHA, BHT, and TBHQ.
Q19. How does keeping food in airtight containers help?
It slows down oxidation.Airtight containers prevent oxygen from reaching the food. Since oxidation of fats and oils requires oxygen, removing oxygen greatly slows down the rancidity process. This is why many food packages are sealed airtight.
Q20. Why do chips manufacturers flush bags of chips with nitrogen gas?
To prevent the chips from getting oxidised.Nitrogen is an inert gas that does not react with food. When bags are flushed with nitrogen, oxygen is displaced, creating an oxygen-free environment. This prevents oxidation of the fats in chips, thus preventing rancidity and extending shelf life.
Q21. In the redox reaction CuO + H₂ → Cu + H₂O, which substance is the reducing agent?
Hydrogen (H₂)A reducing agent is a substance that reduces another substance (causes it to gain electrons) and is itself oxidized. Hydrogen (H₂) causes CuO to be reduced (lose oxygen) and is itself oxidized to H₂O. Therefore, hydrogen is the reducing agent.
Q22. In the redox reaction CuO + H₂ → Cu + H₂O, which substance is the oxidising agent?
Copper oxide (CuO)An oxidising agent is a substance that oxidizes another substance (causes it to lose electrons) and is itself reduced. CuO causes hydrogen to be oxidized (gain oxygen) and is itself reduced to Cu. Therefore, copper oxide is the oxidising agent.
Q23. What is the colour change when hydrogen reduces hot copper(II) oxide?
Black to brownCopper(II) oxide is black. When hydrogen reduces it, the black CuO loses oxygen and forms reddish-brown copper metal. This colour change from black to brown is a clear visual indicator that reduction has taken place.
Q24. Which of the following is NOT a characteristic of a redox reaction?
Only one substance changesIn a redox reaction, at least two substances change: one is oxidized (loses electrons/gains oxygen/loses hydrogen) and another is reduced (gains electrons/loses oxygen/gains hydrogen). Only one substance never changes — both processes always occur simultaneously.
Q25. The process of rusting of iron involves which type of reaction?
OxidationRusting is primarily an oxidation reaction where iron loses electrons and reacts with oxygen to form iron oxides. However, it is also a redox reaction (the oxygen is reduced). But among the given options, “oxidation” is the most direct description of what happens to iron.
Q26. What is the main economic impact of corrosion?
An enormous amount of money is spent to replace damaged iron.Corrosion causes billions of dollars in economic losses worldwide annually. This includes costs for replacement of rusted structures, preventive measures (painting, galvanization), maintenance, and lost productivity due to equipment failure.
Q27. What gas is used to flush chip bags to prevent rancidity?
NitrogenNitrogen is an inert, non-reactive gas. When chip bags are flushed with nitrogen, it displaces the oxygen that would otherwise cause oxidation of fats. This prevents rancidity and keeps chips fresh for a longer period.
Q28. Which of these is a method to prevent rancidity?
Adding antioxidantsAntioxidants prevent oxidation of fats and oils. Other methods include storing in airtight containers, refrigeration, flushing with inert gases (nitrogen), and avoiding exposure to light and heat. Adding water would actually promote spoilage, not prevent rancidity.
Q29. In the reaction where copper gains oxygen, the copper is said to be:
OxidisedAccording to the classical definition, gaining oxygen is oxidation. Therefore, when copper gains oxygen to form copper oxide (2Cu + O₂ → 2CuO), copper is oxidized. The opposite process (loss of oxygen) is reduction.
Q30. What is the term for the reaction that is the opposite of oxidation?
ReductionReduction is the opposite of oxidation. While oxidation involves gain of oxygen/loss of hydrogen/loss of electrons, reduction involves loss of oxygen/gain of hydrogen/gain of electrons. In any redox reaction, oxidation and reduction occur together as opposite processes.
Q31. Which of the following processes is an example of oxidation in everyday life?
Cutting an apple and it turns brownWhen an apple is cut, enzymes in the fruit catalyze the oxidation of phenolic compounds by atmospheric oxygen, producing brown-colored melanins. This is called enzymatic browning and is a classic example of oxidation in everyday life.
Q32. What is the visual sign that a redox reaction has occurred in the copper heating experiment?
Change in colour of the metalIn the copper heating experiment, reddish-brown copper turns black when it forms copper(II) oxide (oxidation). When hydrogen is passed over this black copper oxide, it turns back to reddish-brown copper (reduction). The colour change is the key observable evidence.
Q33. HCl is oxidised to Cl₂ because it:
Loses hydrogen.In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, HCl (hydrogen chloride) loses hydrogen to form Cl₂. According to the classical definition, loss of hydrogen is oxidation. Therefore, HCl is oxidized to Cl₂. This is an alternative way oxidation can occur without oxygen involvement.
Q34. What is the primary cause of the rancidity of fatty foods?
Oxidation reactionRancidity is primarily caused by the auto-oxidation of unsaturated fats and oils. Atmospheric oxygen reacts with carbon-carbon double bonds in fatty acids, forming peroxides that break down into volatile aldehydes, ketones, and acids — responsible for the unpleasant smell and taste.
Q35. Why is corrosion considered a redox process?
Because it involves both oxidation of the metal and reduction of oxygen.In corrosion, the metal loses electrons (oxidation) to form metal ions, while oxygen (or other species) gains those electrons (reduction). For example, in rusting: Fe → Fe²⁺ + 2e⁻ (oxidation) and O₂ + 4e⁻ + 2H₂O → 4OH⁻ (reduction).
Q36. In the reaction CuO + H₂ → Cu + H₂O, hydrogen:
Is oxidised to water.Hydrogen (H₂) gains oxygen to form water (H₂O). Gaining oxygen is oxidation. Therefore, hydrogen is oxidized. Since it causes the reduction of CuO, hydrogen acts as a reducing agent, not an oxidising agent.
Q37. What is the common element involved in both corrosion and rancidity?
OxygenOxygen is the common element in both processes. Corrosion of metals (like rusting) involves reaction of metals with oxygen. Rancidity involves reaction of fats and oils with atmospheric oxygen. Both are oxidation reactions where oxygen is the oxidizing agent.
Q38. What is the main purpose of the reaction where hydrogen gas is passed over heated copper oxide?
To demonstrate reduction of a metal oxide.This classic experiment demonstrates that hydrogen can reduce a metal oxide to the metal. The black copper(II) oxide is reduced to reddish-brown copper metal. The reaction CuO + H₂ → Cu + H₂O shows that hydrogen is a good reducing agent.
Q39. Which of the following is a consequence of rancidity?
Change in smell and taste of foodRancidity results in an unpleasant “off” smell (sharp, sour, or metallic) and a bitter taste. The food becomes unpalatable and may be harmful to consume. Nutritional value decreases, and certain vitamins (like vitamin E) are destroyed in the process.
Q40. What is the primary reason that stainless steel resists rusting better than ordinary iron?
Stainless steel contains chromium which forms a protective oxide layer.Stainless steel contains at least 10.5% chromium. Chromium reacts with oxygen to form a thin, stable, transparent layer of chromium(III) oxide (Cr₂O₃) on the surface. This layer is self-healing and prevents further oxidation of the underlying iron.
Q41. The green coating on copper is a result of:
CorrosionThe green coating on copper is basic copper carbonate (Cu₂(OH)₂CO₃), formed by the corrosion of copper when exposed to moist air containing carbon dioxide and oxygen. This is a type of corrosion (specifically called patination), not rancidity or displacement.
Q42. In a redox reaction, the total amount of oxygen lost by one substance is:
Equal to the oxygen gained by another.In a redox reaction, oxygen is transferred from one substance to another. The oxygen lost by the substance that is reduced equals the oxygen gained by the substance that is oxidized. This satisfies the conservation of mass.
Q43. What common household practice can slow down food spoilage due to rancidity?
RefrigerationBoth refrigeration and storing in airtight containers slow down rancidity. Refrigeration lowers the temperature, reducing the rate of chemical reactions including oxidation. Airtight containers exclude oxygen. The question asks for a “common household practice” — both are common, but refrigeration is a very widespread practice.
Q44. What is the chemical nature of the “black substance” formed on heating copper?
Copper oxideWhen copper is heated in air, it reacts with oxygen to form copper(II) oxide (CuO), which is black. The equation is 2Cu + O₂ → 2CuO. Copper carbonate is green (basic copper carbonate), not formed by simply heating copper in air.
Q45. The conversion of ZnO to Zn in the reaction with carbon is an example of:
Reduction of zinc oxideZnO + C → Zn + CO. Zinc oxide (ZnO) loses oxygen to become zinc metal (Zn). Losing oxygen is reduction. So, zinc oxide is reduced to zinc. Carbon is oxidized to carbon monoxide. This reaction is used in the extraction of zinc from its ore.
Q46. Which of the following is a problem caused by oxidation reactions?
Corrosion of metalsOxidation causes many problems, most notably corrosion of metals (rusting of iron, tarnishing of silver, etc.) and rancidity of food. While oxidation is also used beneficially (e.g., in batteries, combustion for energy), the question asks for a problem caused by oxidation.
Q47. In the context of food preservation, flushing with nitrogen is effective because nitrogen:
Is chemically unreactive (inert).Nitrogen is an inert gas — it does not react with food at room temperature. By displacing oxygen, it removes the reactant needed for oxidation (rancidity). If nitrogen were reactive, it would defeat the purpose. Its inertness is the key property.
Q48. The reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ demonstrates that oxidation can involve:
Only loss of hydrogen.In this reaction, no oxygen is transferred. HCl is oxidized to Cl₂ by losing hydrogen. This demonstrates that the classical definition of oxidation includes “loss of hydrogen” as an alternative to “gain of oxygen.” Manganese dioxide acts as the oxidizing agent.
Q49. What is the scientific term for the “reverse reaction” mentioned when hydrogen is passed over CuO?
ReductionWhen copper(II) oxide (CuO) is heated with hydrogen, it is reduced to copper metal (Cu). If the previous reaction (copper heating in air) was oxidation (Cu → CuO), then passing hydrogen over CuO reverses that — it is reduction (CuO → Cu). So reduction is the reverse of oxidation.
Q50. The fundamental idea behind the concept of redox reactions is the:
Transfer of oxygen or hydrogen between substances.Classically, redox reactions involve the transfer of oxygen or hydrogen between substances. Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. The modern concept is transfer of electrons, but the fundamental idea remains the complementary nature of the two processes.