Which statement best describes the density and viscosity of water relative to air?

• A. Water is much denser and more viscous than air.
• B. Water is less dense than air.
• C. Water has the same density as air.
• D. Water is denser but less viscous than air.

Answer: (A)

The idea being tested is how density and viscosity compare between water and air. Water is about 1000 kg per cubic meter, while air at sea level is roughly 1.2 kg per cubic meter, so water is around 800 times denser. In terms of viscosity, water is roughly 1 mPa·s, whereas air is about 0.018 mPa·s, meaning water is about 50 times more viscous. So water is both much denser and more viscous than air. This combination explains why moving through water offers far more resistance and buoyancy effects than moving through air: the higher density increases immovable mass per volume and buoyant forces, while the higher viscosity increases resistance to flow and shear.