CBSE Class 10 Science Chemical Reactions and Equations Worksheet Set 09

Answer: (c) 4 and 2. Balancing gives Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O.

Teacher's Note:
1. Count nitrogen and oxygen atoms on both sides after fixing two H₂O.
2. Four HNO₃ are needed because two NO₃ stay in the salt and two leave as NO₂.

Answer: (b) Oxidation. Copper gains oxygen, so it is oxidised: 2Cu + O₂ → 2CuO.

Teacher's Note:
1. Gain of oxygen by a substance is oxidation.
2. The reaction is also a combination reaction.

Answer: (d) a white substance settles at the bottom. Insoluble white barium sulphate is formed: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl.

Teacher's Note:
1. This is a double displacement (precipitation) reaction.
2. Both reactant solutions are colourless, so only the white precipitate is visible.

Answer: (a) decomposition reaction. One compound breaks into two simpler substances on heating.

Teacher's Note:
1. A single reactant giving two or more products signals decomposition.
2. It is also a redox reaction, but decomposition is the type asked here.

Answer: (a). PbO loses oxygen (reduction) while carbon gains oxygen (oxidation), so both happen together and the reaction is redox.

Teacher's Note:
1. A reaction in which one substance is oxidised and another reduced at the same time is a redox reaction.
2. Here R directly explains A.

Answer: (c). Burning of carbon releases heat, so A is true; R is false because heat is released, not absorbed.

Teacher's Note:
1. Exothermic reactions give out heat to the surroundings.
2. The reason contradicts the assertion.

Answer: (a). Moist air supplies both water and oxygen, which are the two conditions needed for rust to form.

Teacher's Note:
1. Rusting does not occur in dry air or in boiled water without air.
2. Hence R correctly explains A.

Answer: (d). The reaction is a displacement reaction, and zinc is more reactive than copper, so both statements are false.

Teacher's Note:
1. Zinc displaces copper from its salt solution because zinc is higher in the reactivity series.
2. Check the type of reaction and the reactivity order separately.

Answer: (i) X is copper; the process is corrosion.
(ii) The green coating is basic copper carbonate, CuCO₃·Cu(OH)₂.
(iii) Painting, greasing or oiling, galvanising, chrome plating or alloying (any two).

Teacher's Note:
1. Copper reacts slowly with moisture, CO₂ and oxygen of air.
2. Prevention works by keeping the metal away from air and moisture.

Answer: (i) Hydrogen at the cathode and oxygen at the anode.
(ii) 2H₂O → 2H₂ + O₂ shows that hydrogen and oxygen form in the volume ratio 2 : 1, so hydrogen is double.
(iii) It is a decomposition reaction (electrolytic decomposition).

Teacher's Note:
1. Water contains two hydrogen atoms for every oxygen atom.
2. Hence the gas ratio follows the coefficients of the balanced equation.

Answer: Heat: CaCO₃ → CaO + CO₂
Light: 2AgCl → 2Ag + Cl₂ (in sunlight)
Electricity: 2H₂O → 2H₂ + O₂

Teacher's Note:
1. Silver chloride turns grey in sunlight because silver is formed.
2. Other valid equations, such as 2AgBr or electrolysis of molten NaCl, are also acceptable.

Answer: (i) X is calcium oxide (quicklime), CaO.
(ii) CaO + H₂O → Ca(OH)₂ + heat. It is a combination reaction and it is exothermic.

Teacher's Note:
1. Slaked lime, Ca(OH)₂, is the whitewash solution; it slowly forms CaCO₃ on walls.
2. Much heat is released when quicklime is added to water.

Answer: (i) KCl + AgNO₃ → AgCl↓ + KNO₃
(ii) Double displacement reaction and precipitation reaction.

Teacher's Note:
1. The insoluble substance is white silver chloride.
2. Ions of the two salts exchange partners in this reaction.

Answer: Yes. Rusting of iron is destructive: rust is flaky, peels off and exposes fresh iron, so the article is eventually destroyed.
Aluminium forms a thin, tough layer of Al₂O₃ in air that does not peel and protects the metal beneath, so this corrosion is protective.

Teacher's Note:
1. Whether corrosion harms depends on whether the oxide layer sticks to the metal.
2. Anodising aluminium makes this protective layer thicker.

Answer: (i) Respiration (oxidation of glucose) and combustion of fuels such as LPG or wood.
(ii) Both need oxygen; both form oxides; both release heat; both are chemical changes (oxidation reactions).

Teacher's Note:
1. In rusting the heat released is too small to notice because the process is slow.
2. Redox means oxidation and reduction occur together.

Answer: (i) 3H₂ + N₂ → 2NH₃
(ii) 2H₂S + 3O₂ → 2H₂O + 2SO₂
(iii) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄↓
(iv) 2K + 2H₂O → 2KOH + H₂

Teacher's Note:
1. Balance each equation so that every element has equal atoms on both sides.
2. State symbols are optional unless asked.

Answer: (i) Silver lies below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.
(ii) Sodium is very reactive and the reaction is highly exothermic, igniting the hydrogen released.
(iii) 2Al + 6HCl → 2AlCl₃ + 3H₂ is exothermic, so the mixture becomes warm.
(iv) Lead is only slightly reactive, so it releases hydrogen slowly: Pb + 2HCl → PbCl₂ + H₂.

Teacher's Note:
1. Metals above hydrogen in the reactivity series displace hydrogen from dilute acids.
2. The more reactive the metal, the faster the bubbling.