Basic Property Identification – Hydrogen Compared to Other Substances Among chemical elements and common substances, hydrogen is the lightest in terms of density at standard conditions.

Introduction / Context:
Recognizing basic physical properties such as relative density helps in safety, storage, and application decisions. Hydrogen's low density influences buoyancy, diffusion rates, and its use as a lifting gas and potential energy carrier.

Given Data / Assumptions:

• Standard ambient conditions (near 1 atm, room temperature).
• Comparison across common elements and gases.
• Density basis by mass per unit volume.

Concept / Approach:

Hydrogen (H2) has a very low molecular mass (approximately 2 g/mol), leading to extremely low density compared with air (≈ 29 g/mol average) or other gases. Consequently, a hydrogen-filled balloon experiences strong buoyant force in air and rises rapidly. This property also impacts storage design (e.g., high-pressure tanks) and safety (rapid dispersion).

Step-by-Step Solution:

Verification / Alternative check:

Empirical density at 1 atm, 0°C: hydrogen ≈ 0.0899 kg/m^3; air ≈ 1.29 kg/m^3; helium ≈ 0.1785 kg/m^3, confirming hydrogen's status as lightest among these.

Why Other Options Are Wrong:

Heaviest/most dense contradicts data. Viscosity and volatility are different properties and do not define “lightest”.

Common Pitfalls:

Confusing helium with hydrogen in lifting applications; overlooking safety concerns like flammability despite the low density benefit.

Final Answer:

lightest