What NEET Asks
- Direct conceptual questions on physical state, color, odor, and density.
- Statement-based MCQs testing understanding of its unique lightness.
- Typically easy to medium difficulty, often part of a multiple-correct statement question.
Key Points
- Colorless, Odorless, Tasteless: Hydrogen (H₂) is an invisible gas with no smell or taste.
- Lightest Gas: It has the lowest molecular mass (approx. 2 g/mol), making it the least dense gas among all elements.
- Highly Flammable: Burns with a pale blue flame in air, producing water (2H₂(g) + O₂(g) → 2H₂O(l)).
- Non-toxic: Not poisonous, but can cause asphyxiation if it displaces oxygen in confined spaces.
- Insoluble in water: Very low solubility in water.
- Low Melting & Boiling Points: Possesses very low melting (-259.16 °C) and boiling (-252.87 °C) points.
Must-Know Formula / Reaction
- Density (ρ) ∝ Molar Mass (M): For ideal gases at constant temperature and pressure, density is directly proportional to molar mass (ρ = PM/RT). This explains why H₂ (M≈2 g/mol) is the lightest compared to air (M≈29 g/mol).
- Combustion Reaction: 2H₂(g) + O₂(g) → 2H₂O(l) (Demonstrates its flammability).
Common Mistakes
- Students often confuse hydrogen's non-toxic nature with being completely harmless (e.g., safe to inhale in large quantities, which is false due to oxygen displacement).
- Don't confuse "lightest" with "inert." Hydrogen is the lightest, but highly reactive and flammable.
- Mistaking the pale blue flame produced during combustion for non-flammability or inertness.
Rapid Revision
Hydrogen is a colorless, odorless, tasteless, and the lightest gas. Its low density is due to its smallest molecular mass. It is highly flammable, burning with a pale blue flame to form water, making it hazardous despite being non-toxic.