📌 SECTION E
Questions 37 to 39 · Source-based · Internal choice in each
🧂 Common Salt & Electrolysis
Seawater contains many salts dissolved in it. Common salt is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt and is mined like coal. The common salt is an important raw material for chemicals of daily use.
(a) Write balanced chemical equations to show the products formed during electrolysis of brine. (1 mark)
(b) List two uses of any one product obtained during electrolysis of brine. (1 mark)
(c) (i) A mild non-corrosive basic salt ‘A’, used for faster cooking, is strongly heated to produce a compound ‘B’, that is used for removing permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated. (2 marks)
🔍 VIEW DETAILED ANSWER (4 MARKS)
(a) Electrolysis of brine (NaCl solution):
At cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
At anode: 2Cl⁻ → Cl₂ + 2e⁻
Overall: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
(b) Uses of products: (Any one product with two uses)
Sodium hydroxide (NaOH): (1) Manufacture of soap, (2) Manufacture of paper.
Chlorine gas (Cl₂): (1) Water purification, (2) Manufacture of PVC, bleaching powder.
Hydrogen gas (H₂): (1) As fuel, (2) Manufacture of ammonia (Haber process).
(c)(i) Identification:
A = Sodium hydrogen carbonate (NaHCO₃, baking soda) – mild non-corrosive basic salt, used for faster cooking.
B = Sodium carbonate (Na₂CO₃, washing soda) – used to remove permanent hardness of water.
Reaction on heating: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
━━━━ OR (c)(ii) ━━━━
(c)(ii) Define water of crystallisation. Give two examples of salts that have water of crystallisation. (2 marks)
🔍 VIEW DETAILED ANSWER (OR)
Water of crystallisation: Fixed number of water molecules present in one formula unit of a salt, responsible for its crystal shape.
Examples:
1. Copper sulphate pentahydrate (CuSO₄·5H₂O) – blue crystals.
2. Washing soda (Na₂CO₃·10H₂O) – sodium carbonate decahydrate.
🌿 Photosynthesis
The maintenance functions of all living organisms must go on even when they are not doing anything particular. Even when we are just sitting in a class or even asleep, this maintenance job has to go on. These maintenance processes require energy to prevent damage and break-down of cells and tissues, which is obtained by the individual organism from the food prepared by the autotrophs, called producers.
(a) Name and define the process by which green plants prepare food. (1 mark)
(b) Write chemical equation involved in the above process. (1 mark)
(c) (i) Mention in correct sequence the events occurring during food synthesis by desert plants. (2 marks)
🔍 VIEW DETAILED ANSWER (4 MARKS)
(a) Process: Photosynthesis – the process by which green plants synthesize organic food (glucose) from carbon dioxide and water using sunlight energy in the presence of chlorophyll.
(b) Chemical equation:
6CO₂ + 6H₂O \(\xrightarrow{sunlight, chlorophyll}\) C₆H₁₂O₆ + 6O₂
(c)(i) Events in desert plants (xerophytes):
1. Stomata open at night to absorb CO₂ (to reduce water loss).
2. CO₂ is fixed into organic acids (CAM pathway).
3. During day, stomata close; organic acids are broken down to release CO₂.
4. CO₂ is used in Calvin cycle to synthesize glucose.
━━━━ OR (c)(ii) ━━━━
(c)(ii) Explain with reasons what happens to the rate of food preparation by green plants (I) during cloudy weather, (II) when stomata get blocked due to dust. (2 marks)
🔍 VIEW DETAILED ANSWER (OR)
(I) Cloudy weather: Rate of photosynthesis decreases because sunlight intensity is reduced. Light is essential for light-dependent reactions.
(II) Stomata blocked by dust: Rate of photosynthesis decreases because CO₂ absorption is reduced. Dust blocks stomatal pores, limiting gas exchange.
⚡ Domestic Electricity
In our homes, we receive the supply of electric power through a main supply also called mains, either supported through overhead electric poles or by underground cables. In our country the potential difference between the two wires (live wire and neutral wire) of this supply is 220 V.
(a) Write the colours of the insulation covers of the line wires through which supply comes to our homes. (1 mark)
(b) What should be the current rating of the electric circuit (220 V) so that an electric iron of 1 kW power rating can be operated? (1 mark)
(c) (i) What is the function of the earth wire? State the advantage of the earth wire in domestic electric appliances such as electric iron. (2 marks)
🔍 VIEW DETAILED ANSWER (4 MARKS)
(a) Wire colours:
– Live wire (Phase): Brown or Red
– Neutral wire: Black or Light Blue
– Earth wire: Green or Yellow/Green striped
(b) Current rating calculation:
P = V × I ⇒ I = P/V = 1000 W / 220 V = 4.55 A.
So, circuit should have current rating at least 5 A (standard 5 A or 15 A).
(c)(i) Function of earth wire:
Earth wire provides a low-resistance path for leakage current to flow to the ground, preventing electric shock.
Advantage in electric iron: If the live wire touches the metal body due to insulation failure, current flows to earth instead of through the user, protecting from fatal shock.
━━━━ OR (c)(ii) ━━━━
(c)(ii) List two precautions to be taken to avoid electrical accidents. State how these precautions prevent possible damage to the circuit/appliance. (2 marks)
🔍 VIEW DETAILED ANSWER (OR)
Precaution 1: Use of proper fuses/MCBs
– Prevents overloading; if current exceeds safe limit, fuse melts / MCB trips, cutting off supply.
Precaution 2: Proper earthing of appliances
– Provides safe path for leakage current; prevents electric shock and damage to appliance.
Precaution 3: Insulated handles on tools/plugs
– Prevents direct contact with live wire, reducing shock risk.
✅ CBSE 2025 Science · Section E · Detailed Case Study Solutions
📘 Chemistry · Biology · Physics – All internal choices covered