The next time you sputter after accidentally swallowing seawater at the beach, consider this: That bitter liquid around you once washed the land. Each drop of refreshing rainwater contains an itty-bit of carbon dioxide absorbed from the air. That gas gives rain a slightly acidic bite, which washes away rocks and soil when the drops splash against the ground. This process creates salty sodium and chloride ions that follow streams and rivers into the ocean. All those ions add up; roughly 3.5 percent of the seawater’s weight is from salt.
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