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Why is the sea salty and rivers and lakes aren’t?

July 5, 2010

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Why is the sea salty and rivers and lakes aren’t? Also, what’s the difference between sea salt and regular table salt?

The earth’s water goes through a continuous cycle of being evaporated and rained back down. Whenever evaporation happens, water goes up as vapor with essentially no salt in it.

Water falls all over the planet, but when it passes through soil and rocks it slowly dissolves bits of minerals, including sodium chloride (salt). This means rivers and lakes have tiny fragments of salt in them, which — little by little — are carried into the sea.

Rivers and lakes are replenished with fresh rainwater, but oceans are a sort of dumping ground where water with accumulated salt keeps adding to the salinity. There are also vents and volcanoes under the ocean that increase the amount of minerals, especially salt.

A detailed account of why some elements and compounds are more abundant is a long story, but take it as fact that of all minerals, sodium chloride is the most common highly soluble one.

This might lead you to worry that the oceans will keep getting saltier with time, but other processes, such as the formation of minerals at the bottom of the ocean, take some salt out.

Sea salt is obtained by evaporating sea water, so it tends to include tiny pieces of whatever else is in the sea, affecting taste and color.

Table salt is mined from underground deposits, but is usually processed to remove impurities and is nearly 98 percent sodium chloride. Sea salt is about 85 percent pure (with 15 percent mainly other minerals).

Iodine is usually added to table salt to help avoid dietary deficiencies, and chemicals like calcium silicate are mixed in to absorb moisture to stop the salt from clumping.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.