Why are boats able to float on seawater compared to freshwater?

• A. Seawater is denser than freshwater, providing greater buoyant force that helps objects float.
• B. Freshwater is denser than seawater.
• C. Salt makes boats heavier and sinks them.
• D. Buoyancy is independent of water density.

Answer: (A)

The main idea is that buoyant force depends on the density of the fluid. When an object is in a fluid, the fluid pushes upward with a force equal to the weight of the fluid that would occupy the space the object displaces. Seawater is denser than freshwater because of the dissolved salts, so for the same submerged volume a boat displaces more mass of seawater. That larger buoyant force counteracts the boat’s weight more, allowing it to float (often with less of it submerged) in seawater than in freshwater. In freshwater, the buoyant force is smaller due to lower density, so the boat must sink deeper to balance its weight. The other ideas—that freshwater is denser, that salt makes boats heavier and sinks them, or that buoyancy doesn’t depend on density—don’t fit because buoyancy clearly depends on the density of the surrounding water.