Which of the following occurs when the ocean's salinity increases?

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

Answer: (D)

Verified step by step guidance

1. Understand the relationship between salinity and seawater density: Salinity refers to the concentration of dissolved salts in water. Higher salinity means more dissolved salts, which increases the mass of the water without significantly increasing its volume, leading to higher density.
2. Recall the formula for density: Density is defined as mass divided by volume ($\frac{\mathrm{mass}}{\mathrm{volume}}$). When salinity increases, the mass of seawater increases while the volume remains relatively constant, resulting in an increase in density.
3. Consider the physical properties of seawater: As salinity increases, the dissolved salts contribute to the overall mass of the water, making it denser. This is a direct relationship and does not involve intermediate steps like first increasing and then decreasing.
4. Eliminate incorrect options: The density of seawater does not remain unchanged because salinity directly affects density. It does not decrease because higher salinity adds mass. The option stating that density first increases and then decreases is not supported by scientific evidence.
5. Conclude that the correct answer is: 'The density of seawater increases,' based on the direct relationship between salinity and density as explained above.