Why do seawater’s density and viscosity increase when salinity increases?

The density and viscosity of seawater increase with higher salinity due to the presence of dissolved salts, primarily sodium chloride.1

• Density: Density is defined as mass per unit volume. When salt is added to water, the dissolved ions—such as sodium and chloride—occupy space and add significant mass to the solution without increasing the volume proportionally. Consequently, the mass per unit volume increases, resulting in a higher density.12
• Viscosity: Viscosity represents a fluid's internal resistance to flow. The addition of dissolved salts increases the interactions between the water molecules and the ions. These interactions create greater internal friction within the fluid, which manifests as an increase in viscosity compared to fresh water.1

In essence, the higher concentration of dissolved substances alters the physical properties of the water, making it both heavier and more resistant to flow as salinity rises.1