UPSC CSE Prelims
Chemistry and Material Science Previous Year Questions (PYQs)

Detailed Explanation:

Rare Earth Elements (REEs) such as Europium, Terbium, and Yttrium are widely used in modern display technologies.

• Statement I is Correct: Some rare earth elements are used in flat TV screens and computer monitors.

• Statement II is Correct: These elements possess phosphorescent properties, meaning they can absorb energy and emit light.

• Statement II explains Statement I because these phosphorescent properties help produce bright, vibrant, and accurate colors in display screens.

• Therefore, Option A is the correct answer.

Statement-wise Analysis

Rare Earth Elements (REEs)

What are Rare Earth Elements?

Rare Earth Elements are a group of 17 metallic elements consisting of:

• 15 Lanthanides

• Scandium

• Yttrium

Important Applications

• Flat-screen TVs and monitors

• Smartphones

• Electric Vehicles (EVs)

• Wind turbines

• Defence equipment

• Medical imaging devices

Phosphorescence

Phosphorescence is the ability of a material to:

• Absorb energy

• Store it temporarily

• Emit light gradually

This property helps create:

• Bright displays

• High color accuracy

• Better screen efficiency

Important REEs Used in Displays

Detailed Explanation:

Answer: Option 1 — it is dipolar in nature

Water is known as the "universal solvent" because of its dipolar (polar) nature. Water molecules have a bent structure with partial positive charges on hydrogen atoms and a partial negative charge on the oxygen atom, creating a dipole. This polarity enables water molecules to surround and interact with ions and other polar molecules through electrostatic attraction, effectively separating and dissolving them. The dipolar nature is the fundamental reason why water can dissolve more substances than any other liquid.

📝 Short Notes: Properties of Water as a Solvent

• Dipolar/Polar Nature: Water has a bent molecular structure (H-O-H angle ~104.5°) with oxygen being more electronegative, creating partial charges (δ+ on H, δ- on O).
• Universal Solvent: Water dissolves ionic compounds (like salts) by surrounding ions with water molecules (hydration) and polar covalent compounds through hydrogen bonding.
• Hydrogen Bonding: The polar nature allows water molecules to form hydrogen bonds with each other and with solute particles, facilitating dissolution.
• High Dielectric Constant: Water has a high dielectric constant (~80), which reduces the electrostatic attraction between dissolved ions, keeping them separated in solution.
• Not Related to Solvent Properties: Good heat conductor, high specific heat, and being an oxide of hydrogen are properties of water but not the primary reasons for its exceptional solvent capabilities.

Detailed Explanation:

Answer: Option 3 — Lemongrass

Lemongrass (also known as Citronella grass) is widely used in preparing natural mosquito repellents due to its strong citrus-like odor. The essential oil extracted from lemongrass contains citronella oil, which is highly effective in repelling mosquitoes. Studies have shown that lemongrass oil can repel or kill up to 95% of certain mosquito species, with effects lasting approximately 2-3 hours.

📝 Short Notes: Natural Mosquito Repellents

• Lemongrass (Cymbopogon): Contains citronella oil, the most commonly used natural mosquito repellent; effective against multiple mosquito species
• Mechanism: The strong citrus odor masks human scent and carbon dioxide, making it difficult for mosquitoes to locate their targets
• Citronella Oil: Extracted from lemongrass leaves; used in candles, sprays, lotions, and incense sticks
• Duration of Effect: Typically lasts 2-3 hours; requires reapplication for extended protection
• Other Natural Repellents: Neem oil, eucalyptus oil, lavender, basil, and marigold also possess mosquito-repelling properties
• Safety: Natural repellents are generally safer than chemical alternatives like DEET, with fewer side effects

Detailed Explanation:

✅ Statement 1 – Incorrect: Crystallization of sodium chloride is a physical change; NaCl changes from dissolved to solid state without altering its chemical composition.
✅ Statement 2 – Incorrect: Melting of ice is a physical change; H₂O molecules transition from solid to liquid state while maintaining the same chemical structure.
✅ Statement 3 – Correct: Souring of milk is a chemical change; bacteria convert lactose into lactic acid, creating new substances and irreversibly altering the chemical composition.

Detailed Explanation:

✅ Statement 1 – Correct: Graphene is a two-dimensional material consisting of a single layer of carbon atoms arranged in a hexagonal lattice, and it exhibits excellent electrical conductivity due to the mobility of electrons in its structure.

✅ Statement 2 – Correct: Graphene is one of the thinnest materials (only one atom thick) yet possesses extraordinary tensile strength — approximately 200 times stronger than steel — making it one of the strongest materials tested.

❌ Statement 3 – Incorrect: Graphene is composed entirely of carbon atoms, not silicon. However, it does possess high optical transparency (absorbs only ~2.3% of visible light).

✅ Statement 4 – Correct: Due to its combination of electrical conductivity, optical transparency, and flexibility, graphene is highly suitable for use as conducting electrodes in touch screens, LCDs, and organic LEDs.