How does higher salinity affect coastal water chemistry?

• A. It reduces ion concentrations and lowers ion activity.
• B. It increases water density and changes ion concentrations, affecting ion activity and reactions.
• C. It neutralizes all chemical processes in coastal waters.
• D. It has no effect on chemical reactions in coastal waters.

Answer: (B)

Higher salinity changes coastal water chemistry by increasing the amount of dissolved ions and altering the physical environment where reactions occur. As more salts dissolve, the ionic strength of the water goes up. This changes how ions behave: the activity of each ion (a_i) depends on both its concentration and its activity coefficient (a_i = gamma_i × [i]). When ionic strength rises, activity coefficients typically decrease, so the same concentration of ions has a different effective strength, shifting chemical equilibria, solubilities, and reaction rates. At the same time, more dissolved salts make the water denser, which affects stratification, mixing, and transport of species. All of this means higher salinity does not simply reduce ion counts or halt chemistry; it increases ion content while changing how strongly those ions participate in reactions.