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Q1. Green coating on copper is an example of:
CorrosionThe green coating on copper (basic copper carbonate, Cu₂(OH)₂CO₃) forms when copper reacts with moist air containing oxygen, carbon dioxide, and water. This gradual deterioration of the metal surface due to environmental reactions is called corrosion, specifically “patination” for copper.
Q2. Rusting of iron is called:
CorrosionRusting is the specific term for corrosion of iron and its alloys. Corrosion is the broader term for the gradual destruction of any metal by chemical or electrochemical reactions with its environment. Since rusting is a type of corrosion, that is the correct classification.
Q3. Silver articles turn black due to:
CorrosionSilver articles develop a black coating of silver sulphide (Ag₂S) when they react with hydrogen sulphide (H₂S) present in air. This is another example of corrosion, specifically called “tarnishing” for silver. The reaction is: 2Ag + H₂S + ½O₂ → Ag₂S (black) + H₂O.
Q4. Chip packets are filled with nitrogen gas because it:
Prevents oxidationNitrogen is an inert gas that does not react with food. By flushing nitrogen into chip packets, oxygen is displaced, creating an oxygen-free environment. This prevents oxidation of fats and oils in the chips, which would otherwise cause rancidity and spoilage.
Q5. Fats and oils become rancid due to:
OxidationRancidity occurs when unsaturated fats and oils undergo auto-oxidation, reacting with atmospheric oxygen. This produces volatile aldehydes, ketones, and carboxylic acids that cause unpleasant smell and taste. The process is accelerated by light, heat, and certain metals.
Q6. Rancidity affects:
Taste and smellRancidity primarily alters the organoleptic properties of food – it develops a sharp, unpleasant odour (often described as “off” or sour) and a bitter or metallic taste. While colour may change slightly in some cases, the most noticeable and undesirable effects are on taste and smell.
Q7. Substances that prevent oxidation in food are called:
AntioxidantsAntioxidants are compounds that inhibit oxidation by being preferentially oxidized themselves, thus protecting the food. Common food antioxidants include vitamin C (ascorbic acid), vitamin E (tocopherols), BHA (butylated hydroxyanisole), and BHT (butylated hydroxytoluene).
Q8. In the reaction CuSO₄ + Fe → FeSO₄ + Cu, copper is:
ReducedIn this displacement reaction, copper ions (Cu²⁺ in CuSO₄) gain 2 electrons to become copper metal (Cu). Gain of electrons is reduction. Iron simultaneously loses electrons (oxidation). So copper is reduced from +2 oxidation state to 0.
Q9. A substance is oxidised if it:
Gains oxygenIn the classical definition of oxidation, a substance is said to be oxidized when it gains oxygen. For example, 2Mg + O₂ → 2MgO (magnesium gains oxygen). Alternatively, oxidation can also be defined as loss of hydrogen or loss of electrons.
Q10. A substance is reduced if it:
Loses oxygenIn the classical definition, reduction occurs when a substance loses oxygen. For example, CuO + H₂ → Cu + H₂O (copper oxide loses oxygen to become copper). Reduction can also be defined as gain of hydrogen or gain of electrons.
Q11. In the reaction PbO + C → Pb + CO₂, carbon is:
OxidisedCarbon (C) gains oxygen to form carbon dioxide (CO₂). According to the definition, gaining oxygen is oxidation. So carbon is oxidized. Meanwhile, lead (in PbO) loses oxygen to become Pb metal, so lead is reduced. This is a redox reaction.
Q12. A reaction in which both oxidation and reduction occur is called:
Redox reaction“Redox” stands for reduction-oxidation. In any chemical reaction where one species loses electrons (oxidation), another species must gain those electrons (reduction). They occur simultaneously. Displacement reactions, combustion, and many decomposition reactions are redox reactions.
Q13. Magnesium ribbon burns to form:
Magnesium oxideWhen a magnesium ribbon is burned in air (oxygen), it forms magnesium oxide. The balanced equation is: 2Mg + O₂ → 2MgO. The reaction produces intense white light and a lot of heat, making it an exothermic combination reaction.
Q14. The white powder formed when magnesium burns is:
MgOThe white powder is magnesium oxide (MgO). It is a basic oxide that reacts with water to form magnesium hydroxide, which turns red litmus blue. Students often mistake this white powder for ash, but it is chemically MgO.
Q15. Which metal can displace iron from iron sulphate solution?
ZincZinc is more reactive than iron in the reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au). A more reactive metal can displace a less reactive metal from its salt solution. Zinc + Iron sulphate → Zinc sulphate + Iron.
Q16. Barium chloride reacts with aluminium sulphate to form:
BaSO₄ precipitate and AlCl₃The double displacement reaction is: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ (white precipitate) + 2AlCl₃ (remains in solution). Barium sulphate is insoluble, so it forms a precipitate, while aluminium chloride is soluble.
Q17. The reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of:
Displacement reactionThis is a single displacement (or thermite) reaction where aluminium (more reactive) displaces iron (less reactive) from iron(III) oxide. Aluminium is above iron in the reactivity series. The reaction is highly exothermic and used in welding railway tracks.
Q18. A balanced chemical equation follows the law of:
Conservation of massThe Law of Conservation of Mass, proposed by Antoine Lavoisier, states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total mass of reactants equals the total mass of products, requiring the same number of atoms of each element on both sides of the equation.
Q19. In a chemical equation, the symbols (s), (l), (g), (aq) represent:
Solid, liquid, gas, aqueous solution(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution (substance dissolved in water). These state symbols provide important information about the physical states of reactants and products.
Q20. Sodium hydroxide reacts with sulphuric acid to form:
Na₂SO₄ + H₂OThis is a neutralization reaction between a base (NaOH) and an acid (H₂SO₄). The balanced equation is: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. The products are sodium sulphate (salt) and water.
Q21. Silver nitrate reacts with sodium chloride to form:
AgCl precipitate + NaNO₃This is a double displacement/precipitation reaction: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Silver chloride (AgCl) is insoluble in water and forms a curdy white precipitate, while sodium nitrate remains dissolved.
Q22. Which of the following is an example of a decomposition reaction?
2KClO₃ → 2KCl + 3O₂Decomposition reactions have a single reactant breaking down into multiple products. Here, potassium chlorate (KClO₃) decomposes on heating into potassium chloride (KCl) and oxygen gas (O₂). Option A is combination, C is displacement, D is double displacement.
Q23. Photolytic decomposition of silver chloride in sunlight produces:
Silver metal and chlorine gas2AgCl (white) —sunlight→ 2Ag (grey) + Cl₂ (gas). This photolytic decomposition reaction is the basis for black and white photography. The silver metal formed gives a grey/black appearance.
Q24. Which of the following is an exothermic reaction?
RespirationRespiration is an exothermic reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (heat). Photosynthesis is endothermic (absorbs light energy). Decomposition of CaCO₃ is endothermic (requires heat). Electrolysis requires electrical energy (endothermic overall).
Q25. Potassium reacts with water to give:
KOH + H₂Potassium is an extremely reactive alkali metal. The reaction with water is: 2K + 2H₂O → 2KOH + H₂ (hydrogen gas). The reaction is highly exothermic and can ignite the hydrogen produced. The solution becomes alkaline due to KOH formation.
Q26. Iron reacts with hydrochloric acid to produce:
Hydrogen gas + iron chlorideFe + 2HCl → FeCl₂ (iron(II) chloride) + H₂ (hydrogen gas). This is a single displacement reaction where iron displaces hydrogen from hydrochloric acid. Hydrogen gas is released as effervescence (bubbles).
Q27. The colour of rust is:
Reddish-brownRust is hydrated iron(III) oxide with the approximate formula Fe₂O₃·xH₂O. It has a characteristic reddish-brown appearance. The exact shade can vary depending on the degree of hydration and presence of other iron compounds.
Q28. Corrosion causes damage to:
All of theseCorrosion affects all iron and steel structures exposed to moisture and oxygen. Car bodies develop rust spots, iron railings corrode, and bridges suffer structural weakening, leading to enormous economic costs for prevention, maintenance, and replacement.
Q29. Which of the following is an endothermic reaction?
Decomposition of calcium carbonateDecomposition of calcium carbonate (CaCO₃ → CaO + CO₂) requires continuous heat input to proceed, making it endothermic. Burning magnesium, respiration, and neutralization all release heat (exothermic).
Q30. Barium chloride reacts with sulphuric acid to produce:
BaSO₄ + HClBaCl₂ + H₂SO₄ → BaSO₄ (white precipitate) + 2HCl. This is a double displacement reaction. Barium sulphate is insoluble and forms a precipitate, confirming the presence of sulphate ions in qualitative analysis.
Q31. Hydrogen gas reacts with nitrogen gas to form:
NH₃N₂ + 3H₂ ⇌ 2NH₃ (ammonia). This is the Haber process, used industrially to produce ammonia. The reaction is reversible and requires high pressure (200 atm), high temperature (450°C), and an iron catalyst.
Q32. Burning of hydrogen sulphide (H₂S) forms:
H₂O + SO₂2H₂S + 3O₂ → 2SO₂ + 2H₂O. When hydrogen sulphide (rotten egg smell) burns in sufficient oxygen, it produces sulphur dioxide (SO₂) and water vapour. The reaction is a combustion reaction.
Q33. More reactive metal displaces a less reactive metal from its salt solution. This is called:
Single displacement reactionIn a single displacement reaction, one element displaces another from its compound, following A + BC → AC + B. This occurs based on the reactivity series – a more reactive element can displace a less reactive one.
Q34. Which metal does not corrode easily?
GoldGold is a noble metal that is extremely unreactive. It does not react with oxygen, moisture, or most acids. This is why gold artifacts from ancient civilizations remain intact for thousands of years. Iron rusts readily, copper turns green, silver tarnishes black.
Q35. Rancidity can be prevented by:
Adding antioxidantsAntioxidants prevent oxidation of fats and oils. Other prevention methods include: storing food in airtight containers to exclude oxygen, refrigeration to slow reaction rates, flushing with inert gases like nitrogen, and avoiding light and heat exposure.