CBSE Class 10 Science Chemistry and Physics Questions Assignment Set 06

Answer: Iron is more reactive than copper, so it displaces copper from the solution: Fe + CuSO₄ → FeSO₄ + Cu. The blue colour of CuSO₄ fades to pale green as FeSO₄ forms, and a reddish-brown layer of copper deposits on the nail.

Teacher's Note:
1. This is a displacement reaction based on the reactivity series.
2. Mention both the colour change and the deposit.

Answer: (i) HCl is oxidised (it loses hydrogen to form Cl₂) and MnO₂ is reduced (it loses oxygen to form MnCl₂).
(ii) Al is oxidised (it gains oxygen) and Fe₂O₃ is reduced (it loses oxygen).

Teacher's Note:
1. Oxidation is gain of oxygen or loss of hydrogen, and reduction is the reverse.
2. In each reaction one reactant is oxidised while the other is reduced.

Answer: In a displacement reaction a more reactive element displaces a less reactive element from its compound: Fe + CuSO₄ → FeSO₄ + Cu. In a double displacement reaction two compounds exchange ions to form two new compounds: Na₂SO₄ + BaCl₂ → BaSO₄(s) + 2NaCl.

Teacher's Note:
1. Displacement involves one element and one compound, while double displacement involves two compounds.
2. Write one balanced equation for each.

Answer: Dry HCl gas does not ionise because there is no water, so it gives no H⁺ ions. Acidic behaviour needs H⁺ ions, so the litmus stays unchanged.

Teacher's Note:
1. Acids show their properties only in aqueous solution.
2. Without water there are no free hydrogen ions.

Answer: Mixing acid and water releases a large amount of heat. Adding acid slowly to water spreads the heat in the large volume of water, but adding water to concentrated acid can make the mixture spurt out and cause burns or break the glass.

Teacher's Note:
1. The process is highly exothermic.
2. Safety is the reason behind the rule.

Answer: Washing soda, sodium carbonate (Na₂CO₃·10H₂O).

Teacher's Note:
1. It removes calcium and magnesium ions as insoluble carbonates.
2. Write the formula as well as the name.

Answer: On heating, sodium hydrogen carbonate decomposes into sodium carbonate, water and carbon dioxide: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.

Teacher's Note:
1. This reaction is the basis of baking powder.
2. Balance the equation by writing 2 before NaHCO₃.

Answer: Zinc is more reactive than iron, so it displaces iron: Zn + FeSO₄ → ZnSO₄ + Fe. The pale green colour of the solution fades and iron is deposited on the zinc.

Teacher's Note:
1. Compare the positions of zinc and iron in the reactivity series.
2. The more reactive metal displaces the other.

Answer: Reduction is used. The metal oxide is heated with a suitable reducing agent such as carbon, or reduced electrolytically for highly reactive metals.

Teacher's Note:
1. Sulphide or carbonate ores are first converted to oxides by roasting or calcination.
2. The oxide is then reduced to the metal.

Answer: Rusting can be prevented by (1) painting or oiling the surface, (2) galvanisation, which is coating iron with a layer of zinc, and (3) making alloys such as stainless steel. Greasing and electroplating also work.

Teacher's Note:
1. Each method keeps air and moisture away from iron or protects it chemically.
2. Give any three.

Answer: Amphoteric oxides are metal oxides that react with both acids and bases to give salt and water. Examples are aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

Teacher's Note:
1. The word amphoteric means showing both acidic and basic behaviour.
2. Always give the formulae of the examples.

Answer: A convex mirror always forms an erect, virtual and diminished image. It also gives a wider field of view, so the driver can see a large area of traffic behind.

Teacher's Note:
1. Both the nature of the image and the field of view are required.
2. Mention both for full marks.

Answer: Refractive index n = c/v, so v = c/n = (3 × 10⁸)/1.50 = 2 × 10⁸ m/s.

Teacher's Note:
1. Rearrange n = c/v to get the speed in the medium.
2. The speed in glass is two-thirds of that in vacuum.

Answer: For a concave lens f = −2 m. Power P = 1/f = 1/(−2) = −0.5 D.

Teacher's Note:
1. Focal length of a concave lens is negative.
2. Power is measured in dioptres.

Answer: u = −10 cm, f = +15 cm. From 1/v + 1/u = 1/f: 1/v = 1/15 + 1/10 = 5/30, so v = +6 cm. The image is formed 6 cm behind the mirror and is virtual, erect and diminished (m = +0.6).

Teacher's Note:
1. Use the mirror formula with sign convention.
2. A positive v means the image is behind the convex mirror.

Answer: I = 2.5 mA = 2.5 × 10⁻³ A. R = V/I = 12/(2.5 × 10⁻³) = 4800 Ω.

Teacher's Note:
1. Convert milliamperes to amperes before using Ohm's law.
2. The resistance is 4.8 kΩ.

Answer: Separately: I = 220/24 ≈ 9.17 A.
In series: R = 48 Ω, so I = 220/48 ≈ 4.58 A.
In parallel: R = 12 Ω, so I = 220/12 ≈ 18.33 A.

Teacher's Note:
1. Find the effective resistance in each arrangement and apply I = V/R.
2. Parallel gives the least resistance and the largest current.

Answer: H = I²Rt = 5² × 20 × 30 = 25 × 20 × 30 = 15000 J.

Teacher's Note:
1. Joule's law of heating gives the heat produced.
2. The answer is 15 kJ.

Answer: Alloys have higher resistivity than pure metals, so they produce more heat. They also do not oxidise or burn easily at high temperature, so they last longer.

Teacher's Note:
1. Two reasons are expected: high resistivity and resistance to oxidation.
2. Pure metals fail on both counts.

Answer: The earth wire connects the metal body of an appliance to the ground. If a live wire touches the body, the current flows to the earth and the user does not get a shock.

Teacher's Note:
1. Earthing provides a low-resistance path for leakage current.
2. It protects people from electric shock.