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Q1. The slow process in which iron reacts with oxygen and moisture from the air to form a reddish-brown substance is called:
Rusting is the slow chemical process in which iron reacts with oxygen and moisture (water) present in the air to form a reddish-brown flaky substance called rust (hydrated iron oxide). It is a chemical change that weakens the iron. Galvanisation is a prevention method, crystallisation is a purification process, and alloy formation is mixing metals.
Q2. The chemical formula of rust is:
Rust is chemically known as hydrated iron(III) oxide, with the formula Fe₂O₃·xH₂O. FeO is iron(II) oxide, Fe₂O₃ is anhydrous iron(III) oxide, and Fe₃O₄ is magnetite. The presence of water molecules makes rust hydrated.
Q3. Which of the following conditions is necessary for rusting to occur?
For rusting to occur, both oxygen and water (moisture) are essential. Iron reacts with oxygen and water to form hydrated iron(III) oxide. Without either one, rusting does not take place. Carbon dioxide is not directly involved in rusting.
Q4. The process of coating iron with a layer of zinc to prevent rusting is called:
Galvanisation is the process of coating iron with a layer of zinc to protect it from rusting. The zinc acts as a protective barrier and, being more reactive, corrodes first (sacrificial protection). Crystallisation, alloying, and electroplating with copper are different processes.
Q5. Stainless steel is an alloy of iron that does not rust easily because it contains:
Stainless steel contains chromium and nickel. Chromium forms a thin, invisible, self-healing oxide layer on the surface that prevents rusting. This makes stainless steel corrosion-resistant. Zinc and copper are not the main alloying elements in stainless steel.
Q6. The famous iron pillar located at the Qutub Minar complex in Delhi has not rusted significantly for over:
The Iron Pillar of Delhi is approximately 1600 years old. It has withstood corrosion remarkably well due to a thin protective layer that formed on its surface. It was built during the Gupta period (around 4th-5th century AD).
Q7. The main reason the Iron Pillar of Delhi has not rusted is:
The Delhi Iron Pillar has a thin protective layer made of iron phosphate, hydrogen phosphate, and misawite (a type of iron oxyhydroxide). This layer formed naturally due to the high phosphorus content of the iron and prevented further rusting. It was not coated with zinc or painted.
Q8. Which of the following methods can be used to prevent rusting of iron?
Painting, oiling/greasing, and galvanisation are all effective methods to prevent rusting. Painting and oiling create a barrier that prevents moisture and oxygen from reaching the iron. Galvanisation coats iron with zinc, which protects it even if scratched (sacrificial protection).
Q9. In galvanisation, the layer of zinc protects iron even if scratched because:
Zinc is more reactive than iron. When the zinc layer is scratched, zinc corrodes preferentially (acts as a sacrificial anode) and protects the iron underneath. This is called sacrificial protection. Zinc does not make iron waterproof or form a paint-like layer.
Q10. Which of the following is an example of a rusting prevention method used on bicycle chains?
Bicycle chains are often oiled or greased to prevent rusting by creating a barrier that keeps moisture and oxygen away from the metal. Painting is not practical for moving parts, galvanisation is not used for chains, and gold plating is too expensive and not used for this purpose.
Q11. The process of obtaining large, pure crystals of a substance from its solution is called:
Crystallisation is a purification technique used to obtain pure crystals from a solution. It involves dissolving the substance, filtering, and then allowing the solution to cool slowly so that pure crystals form. Rusting, galvanisation, and alloying are different processes.
Q12. Stainless steel is an example of:
Stainless steel is an alloy, which is a mixture of two or more metals (or a metal and a non-metal). It is primarily iron mixed with chromium, nickel, and small amounts of carbon. It is not a pure metal, non-metal, or a compound.
Q13. Which of the following conditions speeds up rusting the most?
Saltwater speeds up rusting because dissolved salts act as electrolytes, increasing the electrical conductivity of water and facilitating the electrochemical reactions involved in rusting. Dry air and pure water without oxygen slow down rusting, and ice-cold water slows chemical reactions.
Q14. The Iron Pillar of Delhi is made of:
The Iron Pillar of Delhi is made of wrought iron with a high phosphorus content. The phosphorus played a crucial role in forming the protective layer that prevented rusting. It is not pure iron, stainless steel, or cast iron.
Q15. Which of the following statements about rusting is correct?
Rusting is a chemical change because a new substance (rust) is formed. It is irreversible because the rusted iron cannot be converted back to iron by simple physical methods. Rusting requires both oxygen and water; without oxygen, rusting does not occur.
Q16. The method of protecting iron by coating it with a layer of another metal that is less reactive than iron is:
Tin is less reactive than iron. Tin plating protects iron by providing a barrier layer that prevents air and moisture from reaching the iron. However, if the tin layer is scratched, the iron will rust faster because tin is less reactive. Galvanisation uses zinc, which is more reactive (sacrificial protection).
Q17. What is the colour and nature of rust?
Rust is reddish-brown in colour and has a flaky, brittle nature. It is not shiny black, silvery white, or green. The flaky nature allows moisture and air to penetrate further, accelerating the rusting process.
Q18. The presence of which element in stainless steel helps it resist rusting by forming a self-healing oxide layer?
Chromium in stainless steel forms a thin, invisible, self-healing oxide layer (chromium oxide) on the surface. This layer protects the underlying iron from rusting. Nickel adds strength and corrosion resistance, but chromium is the key element for rust resistance.
Q19. Which of the following is NOT a method to prevent rusting?
Wrapping iron in a wet cloth introduces moisture, which promotes rusting. All the other options (painting, keeping dry, and alloying) are methods to prevent rusting. Keeping iron dry prevents moisture from reaching it, and paint/alloying provide barriers.
Q20. Crystallisation is used to purify:
Crystallisation is used to purify common salt (sodium chloride) from seawater. The salt is obtained by evaporating water and then recrystallising to get pure salt. Iron, rust, and stainless steel are not purified by crystallisation.
Q21. The Iron Pillar of Delhi was made during the reign of which dynasty?
The Iron Pillar of Delhi is believed to have been built during the Gupta dynasty (around 4th-5th century AD). It is a testament to the advanced metallurgical skills of ancient Indian artisans. It was not made during the Maurya, Mughal, or Chola periods.
Q22. What happens to the mass of an iron object when it rusts?
When iron rusts, its mass increases because it combines with oxygen and water from the air. The rust contains additional elements (oxygen and hydrogen) that were not present in the pure iron. However, rust flakes off, so the visible object may lose mass over time.
Q23. Which of the following is NOT an alloy?
Galvanised iron is not an alloy; it is iron coated with a layer of zinc. Brass is an alloy of copper and zinc, bronze is an alloy of copper and tin, and stainless steel is an alloy of iron, chromium, and nickel. Galvanised iron is a coated metal, not a homogeneous mixture of metals.
Q24. In the crystallisation process, the substance that is dissolved in water to form a solution is called the:
In crystallisation, the substance that is dissolved in the solvent (usually water) is called the solute. The solvent dissolves the solute to form a solution. The filtrate is the liquid that passes through the filter paper, and the residue is the solid left behind on the filter paper.
Q25. Why do iron ships rust faster in seawater than in river water?
Seawater contains dissolved salts (like sodium chloride) that act as electrolytes, increasing the electrical conductivity of water. This speeds up the electrochemical reactions involved in rusting. River water has fewer dissolved salts, so rusting is slower. Seawater is not colder, and it does not have less oxygen.
Q26. The protective layer formed on the Iron Pillar of Delhi contains a compound called misawite, which is a type of:
Misawite is a type of iron oxyhydroxide (δ-FeOOH) that formed as part of the protective layer on the Iron Pillar. This layer, along with iron phosphate and hydrogen phosphate compounds, helped prevent rusting. Iron oxide and hydroxide are other compounds, but misawite is specifically an oxyhydroxide.
Q27. Which of the following statements about galvanisation is FALSE?
Galvanised iron can still rust if the zinc coating is completely damaged or worn away. The zinc layer provides protection, but it is not permanent under all conditions. The other statements are true: galvanisation involves zinc coating, zinc is more reactive, and it can be applied by hot-dipping.
Q28. What is the common name for the process of forming pure copper sulphate crystals from an impure sample?
Crystallisation is the process used to obtain pure copper sulphate crystals from an impure sample. The impure copper sulphate is dissolved in hot water, filtered, and then allowed to cool slowly to form pure crystals. This is a purification technique.
Q29. The iron pillar at Dhar (Madhya Pradesh) is another ancient iron pillar, but it has rusted more than the Delhi pillar because:
The Dhar iron pillar lacks the high phosphorus content that helped form the protective layer on the Delhi Iron Pillar. This allowed rusting to occur more readily. The Delhi pillar’s high phosphorus content was key to its corrosion resistance. The Dhar pillar is not made of a different metal and is not necessarily older.
Q30. In the rusting of iron, which gas from the air is essential?
Oxygen is essential for rusting. Iron reacts with oxygen and moisture to form rust. Nitrogen, carbon dioxide, and hydrogen are not directly involved in the rusting process. Oxygen is the key gas required for oxidation of iron.
Q31. Stainless steel is used to make surgical instruments because:
Stainless steel is used for surgical instruments because it is corrosion-resistant (does not rust) and can be easily sterilised by boiling or autoclaving. It is not cheap, not soft, and does not react with blood in a harmful way.
Q32. Which of the following will rust the fastest when left outdoors?
An unpainted iron nail has no protective coating, so it will rust the fastest. Galvanised iron, painted iron, and stainless steel all have protective layers that slow or prevent rusting. The iron nail is directly exposed to moisture and oxygen.
Q33. The process of crystallisation is used to obtain:
Crystallisation is used to obtain pure crystals of many substances, including alum, sugar, and copper sulphate. It is a widely used purification technique for solid substances. All the options are correct.
Q34. What is the role of moisture (water) in rusting?
Moisture (water) provides an electrolyte medium that allows ions to move, facilitating the electrochemical reactions involved in rusting. Iron atoms lose electrons at the anode, and oxygen gains electrons at the cathode. Water is essential for this process. It does not directly combine with iron to form rust; it facilitates the reaction.
Q35. Which ancient Indian text mentions advanced knowledge of metals and alloys?
The Arthashastra, written by Kautilya (Chanakya), contains references to metals, alloys, and metallurgical practices in ancient India. It mentions various types of metals and their uses. The Ramayana, Rigveda, and Manusmriti are not primarily texts on metallurgy.
Q36. When iron is coated with tin (tin plating), the main purpose is:
Tin plating is used to create a barrier layer that prevents air and moisture from reaching the iron. Tin is less reactive than iron, so it does not act as a sacrificial metal (like zinc). The main purpose is to physically separate iron from the environment. It is used in food cans (tin cans).
Q37. The Iron Pillar of Delhi has a height of approximately:
The Iron Pillar of Delhi is approximately 7 metres (23 feet) tall. It is a famous historical monument located in the Qutub Minar complex. It weighs about 6 tonnes.
Q38. Which of the following is a disadvantage of rusting?
Rusting is a destructive process. It weakens iron objects, making them brittle and causing them to fall apart over time. Rust is flaky and does not adhere strongly, so it exposes fresh iron to further corrosion. Rust cannot be easily converted back to iron.
Q39. What is the main difference between rusting and crystallisation?
Rusting is a chemical change because a new substance (rust) is formed. Crystallisation is a physical change because no new substance is formed; the substance remains chemically the same, only its physical state (crystal form) changes. The other options are incorrect.
Q40. Which metal is used as a sacrificial anode to protect iron ships from rusting?
Zinc is used as a sacrificial anode to protect iron ships from rusting. Zinc is more reactive than iron, so it corrodes first (sacrifices itself) while protecting the iron hull. This is cathodic protection. Copper, tin, and lead are less reactive than zinc.
Q41. The presence of which impurity in the Iron Pillar of Delhi helped in forming a protective layer?
The Iron Pillar of Delhi has a high phosphorus content (about 0.1%). The phosphorus reacted with iron to form a protective layer of iron phosphate and hydrogen phosphate compounds, which prevented rusting. Sulfur, carbon, and silicon did not play this role.
Q42. Which of the following will NOT rust even if left in moist air for a long time?
A stainless steel spoon will not rust because stainless steel contains chromium, which forms a protective oxide layer that prevents rusting. Iron nails, steel wool, and cast iron pans are all made of iron or plain steel and will rust in moist air.
Q43. In the crystallisation of copper sulphate, why is the solution heated before filtering?
Heating the solution increases the solubility of copper sulphate, allowing more of it to dissolve. This ensures that a saturated solution is formed, from which pure crystals will separate upon cooling. It does not destroy impurities or convert the compound.
Q44. Why is the Iron Pillar of Delhi considered a marvel of ancient metallurgy?
The Iron Pillar of Delhi is a marvel because it has withstood corrosion for about 1600 years without any modern protective coating. This demonstrates the advanced metallurgical knowledge of ancient Indian craftsmen. It is not made of gold, not the tallest pillar, and not made by one person.
Q45. Which of the following conditions would completely prevent rusting?
Rusting requires both oxygen and moisture. If iron is kept in dry air with no moisture, rusting will be completely prevented. Damp rooms, rain, and wet cloth all provide moisture, which promotes rusting.
Q46. What happens when a crystal of alum is added to a saturated solution of alum?
In a saturated solution, no more solute can dissolve. When a small crystal of alum is added, the excess alum in the solution deposits on the crystal, causing it to grow larger. This is the principle behind crystallisation. The crystal does not dissolve, and the solution does not change colour.
Q47. Which of the following is NOT a way to prevent rusting of iron tools on a farm?
Leaving iron tools outside in the rain exposes them to moisture, which promotes rusting. Keeping them dry, applying grease/oil, and painting are all effective ways to prevent rusting. The farm tools should be stored properly to prevent rust.
Q48. The reddish-brown colour of rust is due to:
The reddish-brown colour of rust is due to the presence of iron(III) oxide (Fe₂O₃). Iron(II) oxide is black, iron(II,III) oxide (magnetite) is black, and iron metal is silvery-grey. The hydrated form of iron(III) oxide gives rust its characteristic colour.
Q49. In the crystallisation method, what is the purpose of filtering the hot solution?
The hot solution is filtered to remove undissolved solid impurities (such as dust, dirt, or other insoluble materials). Dissolved impurities are removed by the crystallisation process itself, as they remain in the solution when the pure crystals form. Filtration does not cool the solution or add more solute.
Q50. The fact that the Iron Pillar of Delhi has not rusted proves that:
The Iron Pillar of Delhi is evidence that ancient Indian metalworkers possessed advanced knowledge of metallurgy and corrosion prevention. They used high-phosphorus iron, which formed a protective layer naturally. This does not diminish modern science; it highlights the skills of ancient craftsmen. The pillar was not made of plastic, and rusting does occur in Delhi.