CBSE Class 10 Science – Chapter Wise Study Materials

Case 1

Scenario: A student places an iron nail in a copper(II) sulfate solution. Over time the blue colour fades and a reddish-brown deposit forms on the nail.

(a) Write the chemical equation for the observation.
(b) Explain why this happens using the reactivity series.

Answer:

(a) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
(b) Iron is more reactive than copper (higher in reactivity series) so Fe displaces Cu²⁺ from solution. Fe is oxidised to Fe²⁺ while Cu²⁺ is reduced to metallic copper (reddish-brown deposit). This is a single displacement redox reaction.

Case 2

Scenario: On heating calcium carbonate in the laboratory a white solid remains and a gas evolves that turns limewater milky.

(a) Give the balanced equation for the reaction.
(b) Name the gas and explain the limewater test.

Answer:

(a) CaCO₃(s) → CaO(s) + CO₂(g).
(b) Gas: carbon dioxide (CO₂). Limewater (Ca(OH)₂) turns milky due to formation of insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O. This is the standard test for CO₂.

Case 3

Scenario: A teacher demonstrates the decomposition of hydrogen peroxide. With MnO₂ added, oxygen bubbles appear rapidly.

(a) Write the decomposition equation for H₂O₂.
(b) Explain the role of MnO₂.

Answer:

(a) 2 H₂O₂(aq) → 2 H₂O(l) + O₂(g).
(b) MnO₂ acts as a catalyst — it provides an alternate pathway with lower activation energy, increasing the rate without being consumed. It speeds up O₂ evolution but is unchanged chemically at the end.

Case 4

Scenario: Mixing aqueous solutions of silver nitrate and potassium chloride gives a white solid that sediments on standing.

(a) Give the molecular and net-ionic equations.
(b) Identify the precipitate and its colour.

Answer:

(a) Molecular: AgNO₃(aq) + KCl(aq) → AgCl(s)↓ + KNO₃(aq).
Net ionic: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)↓.
(b) Precipitate: silver chloride (AgCl), white in colour (may darken on sunlight due to photoreduction to Ag).

Case 5

Scenario: Molten sodium chloride is electrolysed in class demonstration producing a gas at the anode and a liquid metal at the cathode.

(a) Identify the products and write half-equations.
(b) Why is molten NaCl used instead of solid NaCl?

Answer:

(a) Cathode (reduction): Na⁺ + e⁻ → Na(l). Anode (oxidation): 2 Cl⁻ → Cl₂(g) + 2 e⁻. Overall: 2 NaCl(l) → 2 Na(l) + Cl₂(g).
(b) In solid NaCl ions are locked in a lattice and cannot move; molten NaCl provides mobile ions necessary for conduction and electrolysis.

Case 6

Scenario: A metal sample when dropped into dilute HCl produces effervescence and the gas gives a 'pop' with a burning splint.

(a) Identify the gas and the type of reaction.
(b) Give a likely balanced equation if the metal is magnesium.

Answer:

(a) Gas: hydrogen (H₂). Reaction type: metal + acid displacement producing salt and hydrogen (single displacement).
(b) Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g).

Case 7

Scenario: A copper cooking vessel becomes dull over time. Home remedy uses lemon (citric acid) and salt to clean it.

(a) Suggest the chemical reason behind the dullness (oxide formation).
(b) Explain how lemon + salt cleans the copper surface.

Answer:

(a) Copper reacts slowly with oxygen and moisture to form oxides (and sometimes basic copper carbonate) which appear dull. Surface oxidation and dirt cause loss of metallic lustre.
(b) Acid (citric acid) dissolves oxides and salt (NaCl) provides mild abrasive action and increases ionic strength helping removal. Reaction example (simplified): CuO + 2 HCl → CuCl₂ + H₂O, soluble copper salts are washed away restoring shine.

Case 8

Scenario: A water sample causes heavy scaling in a kettle; adding washing soda (Na₂CO₃) removes the white deposits.

(a) Explain the chemistry behind the scaling.
(b) Show how Na₂CO₃ removes hardness (equation).

Answer:

(a) Scale is mainly calcium carbonate (CaCO₃) formed from temporary hardness (bicarbonates) on heating; Ca(HCO₃)₂ decomposes to CaCO₃ and CO₂.
(b) Na₂CO₃ precipitates Ca²⁺ as CaCO₃ which can be filtered away: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)↓. This removes hardness-causing ions from water.

Case 9

Scenario: During extraction of iron in a blast furnace, coke, limestone and iron ore are charged. Molten iron and slag are collected separately.

(a) Write a key reduction equation showing how Fe₂O₃ is reduced.
(b) Explain the role of limestone.

Answer:

(a) Reduction: Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂ (carbon monoxide acts as reducing agent).
(b) Limestone (CaCO₃) decomposes to CaO and CO₂. CaO reacts with silica impurities to form slag (CaSiO₃), which is less dense and floats on molten iron, allowing removal: CaO + SiO₂ → CaSiO₃.

Case 10

Scenario: A silver nitrate solution stored in sunlight gradually darkens and metallic silver is detected on glass surface.

(a) Suggest the photochemical change occurring (equation).
(b) State a historical application of this property.

Answer:

(a) Photoreduction: 2 AgCl(s) → 2 Ag(s) + Cl₂(g) (under sunlight; actual mechanism involves photoelectrons converting Ag⁺ to Ag).
(b) Historical application: Light sensitivity of silver halides was the basis of traditional photography—silver-halide coated films form images on exposure to light which are developed chemically.

Case 11

Scenario: A farmer notices iron gates near the sea corrode faster than those inland. Salt-sprayed water appears to accelerate rusting.

(a) Explain why corrosion is faster in the presence of salt.
(b) Suggest two suitable protection methods for marine environments.

Answer:

(a) Salt (NaCl) increases water conductivity and provides chloride ions which disrupt protective oxide layers, enhancing electrochemical corrosion (rusting) rates by facilitating electron/ion transfer.
(b) Prevention: (1) galvanisation (zinc coating) for sacrificial protection; (2) protective marine-grade paints/coatings and regular maintenance; cathodic protection (sacrificial anodes) is also used for ships/pipelines.

Case 12

Scenario: A student tests unknown salts: one gives white precipitate with AgNO₃, another gives white precipitate with BaCl₂. Teacher asks to identify the anions.

(a) Which anion is confirmed by AgNO₃ test? Give equation.
(b) Which anion is confirmed by BaCl₂ test? Give net ionic equation.

Answer:

(a) Chloride ion (Cl⁻) gives white AgCl precipitate: AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq).
(b) Sulfate ion (SO₄²⁻) gives white BaSO₄ precipitate: net ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)↓.

Case 13

Scenario: A piece of aluminium remains shiny despite being exposed to air; another piece of iron forms rust quickly. Students are asked to explain the difference.

(a) Why does aluminium not show progressive corrosion?
(b) What property of aluminium oxide helps protect the metal?

Answer:

(a) Aluminium forms a thin, adherent oxide layer (Al₂O₃) almost instantly on exposure to air; this layer is tightly bound and prevents further oxidation of underlying metal.
(b) The oxide layer is impermeable, strongly adherent and non-porous, which stops oxygen and moisture reaching the metal — giving passive protection against further corrosion.

Case 14

Scenario: An experiment uses MnO₂ as a catalyst to speed one reaction; in another, a sacrificial zinc block protects an iron pipeline. Students compare these two uses.

(a) How is the role of MnO₂ different from the role of sacrificial zinc?
(b) Categorise each as catalysis or electrochemical protection.

Answer:

(a) MnO₂ acts as a catalyst — it increases reaction rate and is chemically unchanged. Sacrificial zinc corrodes (is consumed) to protect iron; it is not catalytic.
(b) MnO₂: catalysis (lowers activation energy). Zinc block: electrochemical sacrificial protection (anode corrodes, cathode protected).

Case 15

Scenario: A student tries to balance the combustion of carbon by changing subscripts (writing CO₂ as CO to force balance). Teacher corrects the method.

(a) Explain why changing subscripts is incorrect.
(b) Provide the correct balanced equation(s) for carbon combustion in excess and limited oxygen.

Answer:

(a) Subscripts define chemical identity; changing them creates a different compound (e.g., CO vs CO₂). Balancing must use coefficients which change amounts (moles) without altering formulas.
(b) Correct balanced equations:
- Complete combustion (excess O₂): C + O₂ → CO₂.
- Incomplete/limited oxygen: 2 C + O₂ → 2 CO (carbon monoxide forms under limited oxygen).

Metals and Non-metals – Case-based Questions with Answers

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