Why sea water weighs more than pure water The specific weight (weight per unit volume) of sea water exceeds that of pure water primarily because of what factor?

Introduction / Context:
Specific weight (γ = ρ g) reflects the density of a fluid. Comparing sea water to pure water highlights how composition affects density and, consequently, buoyancy and hydrostatics. Understanding the main contributor is essential for coastal and naval engineering calculations.

Given Data / Assumptions:

• Sea water contains dissolved salts (primarily NaCl, MgSO4, etc.).
• Presence of some dissolved gases and fine suspended matter.
• Temperatures near ambient and moderate pressures.

Concept / Approach:

The dominant reason sea water is denser than pure water is its salinity: dissolved ions increase mass per unit volume much more than trace dissolved air or typical suspended particulates in clear sea water. Thus ρ_sea > ρ_fresh and γ_sea > γ_fresh, which affects buoyancy, stratification, and wave dynamics.

Step-by-Step Solution:

Verification / Alternative check:

Oceanographic data show typical sea water density near 1025 kg/m^3 at 15 °C and standard salinity, compared to 999–1000 kg/m^3 for fresh water at similar conditions.

Why Other Options Are Wrong:

(a) Dissolved air has a negligible effect and often decreases density slightly; (c) suspended matter is episodic and small for clear water; (d) is too broad—salts are the principal cause.

Common Pitfalls:

Confusing salinity effects with temperature; assuming turbidity is always significant—open-ocean suspended loads are usually low.

Final Answer:

dissolved salts