Final answer:
The main mineral that distinguishes sea water from fresh water is sodium chloride, or salt. Sea water's high salt content impacts marine life, influencing osmoregulation and ecosystem dynamics.
Step-by-step explanation:
The key mineral that makes sea water different from fresh water is sodium chloride (NaCl), commonly known as salt. While fresh water contains low levels of dissolved minerals, sea water contains a high concentration of various salts, with sodium and chloride ions being the most prevalent. These minerals dissolve from rocks and soil into rivers that flow into the oceans, where they accumulate over time. As sea water evaporates, it leaves these minerals behind, increasing the ocean's salinity.
Other minerals present in sea water include magnesium, calcium, and potassium, but sodium chloride is by far the most abundant. This abundance of salt affects marine life: while many marine invertebrates are isotonic with their environment, saltwater fish must exert considerable energy maintaining osmotic balance, secreting excess salt through their gills and producing highly concentrated urine.