seawater chemistry


Where does the salt in seawater come from? As water moves through rock and soil on land it picks up ions. This is the flip side of weathering. Salts comprise about 3.5% of the mass of ocean water, but the salt content, or salinity, is different in different locations. What would the salinity be like in an estuary? Where seawater mixes with fresh water, salinity is lower than average. What would the salinity be like where there is lots of evaporation? Where there is lots of evaporation but little circulation of water, salinity can be much higher. The Dead Sea has 30% salinity nearly nine times the average salinity of ocean water (Figure 1.2). Why do you think this water body is called the Dead Sea? In some areas, dense saltwater and less dense freshwater mix, and they form an immiscible layer, just like oil and water. One such place is a "cenote", or underground cave, very common in certain parts of Central America. Ocean water is composed of many sub- stances, many of them salts such as sodium, magnesium, and calcium chlo- ride. Because of the increased salinity, the wa- ter in the Dead Sea is very dense, it has such high salinity that people can easily float in it!



composition of ocean water

Remember that H2 O is a polar molecule, so it can dissolve many substances (Figure 1.1). Salts, sugars, acids, bases, and organic molecules can all dissolve in water.


With so many dissolved substances mixed in seawater, what is the density (mass per volume) of seawater relative to fresh water? Water density increases as: salinity increases temperature decreases pressure increases Differences in water density are responsible for deep ocean currents, as will be discussed in the "Deep Ocean Currents" concept. Click image to the left or use the URL below. URL:

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water is good at dissolving salts and other substances because

-->  a) it is a polar molecule.

b) it has more ionic charge than other substances.

c) it is present on earth as a gas, liquid and solid.

d) it is wet.

in an estuary,

a) there is a lot of evaporation so the water is very saline.

b) calcium chloride is more abundant than sodium chloride due to river runoff.

-->  c) seawater mixes with freshwater so the water has intermediate salinity.

d) the salinity is constant.

salts in seawater are made with

a) chlorine as the anion.

b) sodium as the most abundant cation.

c) magnesium or calcium as cations.

-->  d) all of the above.

all salt is dangerous for humans and we should eat only a tiny amount each day.

a) true

-->  b) false

the salts in seawater come from

a) rain

b) evaporation

-->  c) weathering of rock and soil

d) all of the above

the salinity is high in water in the great salt lake in utah because

a) evaporation rates are high.

b) the input of fresh water is low.

c) the lake has no outlet to the sea.

-->  d) all of the above

chlorine is the most abundant cation in seawater.

a) true

-->  b) false

why might seawater and freshwater not mix?

-->  a) saltwater is denser so it sinks.

b) freshwater is denser so it sinks.

c) saltwater is colder so it sinks.

d) freshwater is colder so it sinks.

water density increases when

a) salinity decreases.

-->  b) temperature decreases.

c) pressure decreases.

d) all of the above.

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