In an experiment with freshwater and saltwater, the same object sinks in freshwater but floats in saltwater. What explains this observation?

• A. The saltwater is denser than the freshwater
• B. The freshwater is denser than the saltwater
• C. The object expands in saltwater
• D. Saltwater has lower viscosity

Answer: (A)

Buoyancy depends on how dense the surrounding fluid is. Saltwater is denser than freshwater because dissolved salts add mass without a proportional increase in volume. A denser fluid provides a larger buoyant force on the same object (Archimedes’ principle), so the object can be supported and float in saltwater even though it sinks in the less dense freshwater. The key idea is the density difference: the object’s density relative to the fluids determines whether it sinks or floats, not the viscosity.