As the salinity of water increases, how does its density change?

• A. The density of water first increases, then decreases.
• B. The density of water remains unchanged.
• C. The density of water increases.
• D. The density of water decreases.

Answer: (C)

Verified step by step guidance

1. Understand that salinity refers to the concentration of dissolved salts in water, primarily sodium chloride (NaCl).
2. Recall that density is defined as mass per unit volume, given by the formula \(\rho = \frac{m}{V}\), where \(\rho\) is density, \(m\) is mass, and \(V\) is volume.
3. When salt dissolves in water, it increases the mass of the solution without significantly increasing the volume, because the salt ions fit into the spaces between water molecules.
4. Since the mass increases while the volume changes only slightly, the overall density of the water increases as salinity increases.
5. Therefore, the density of water increases with increasing salinity, which explains why seawater is denser than pure freshwater.