|What percent of the ocean's water is salt?
|Question Date: 2004-05-06|
The ocean is about 3% salt. This varies a bit
though, depending on where you measure it. Near
where rivers reach the ocean, the salt content
(salinity) will be lower. In fact, because salt
water is more dense than fresh water, the river's
water will actually "float" on top of the ocean
water until it is mixed together. So if you took
a reading near the surface of the ocean where a
river flowed in, the salinity would be lower than
that near the bottom.
Evaporation takes away
water, but leaves behind salt, so in places where
there is a lot of evaporation, the surface water
may be saltier. Where do you think the saltiest
water might be? You might want to look in the
Scienceline archives for my answer to a question
about salinity in tidepools.
What a great question!
Here's how you'd find
your answer: filter some seawater to remove all
the tiny animals and bacteria swimming around
init, fill a plastic container with seawater,
weigh the container and seawater, wait for the
water to evaporate and then re-weigh the container
and the salt left behind.
You can then
calculate the percentage of seawater that is salt
by weight. You'd need a precise scientific
scale(your bathroom scale won't work) because only
a small fraction of the total weight of seawater
is salt -- about 3.47% on
Oceanographers call this percentage
"salinity", as in: "Seawater has a salinity of
3.47% on average." Because it's so small,
oceanographers actually measure salinity in parts
per thousand instead of parts per hundred ("per
mil" instead of "per cent"), so the average
salinity of the ocean is 34.7 parts per mil.
Some oceanographers dedicate their life to
measuring very small changes in salinity, since
this can affect large-scale ocean circulation
patterns and can also give valuable information
about changes in rainfall and storm patterns. In
fact, small changes in salinity are what first
alerted scientists that global warming has already
caused large-scale changes in the Pacific and
Atlantic Oceans. (Read about it at salt
Of these three bodies of water, how
do you think they rank in terms of salinity, from
most to least? What do you think makes these
a) Atlantic Ocean
c) Pacific Ocean
b) The salinity of the Mediterranean is about
38 parts per mil.
a) The Atlantic Ocean is
about 34.90 parts per mil on average.
Pacific Ocean is about 34.62 parts per mil on
You mean the ocean's salinity? A quick Google
search came up with the salinity of the ocean
being about 35 parts per thousand, or about 3.5%.
In any case, I can suggest an easy way to find
out. Get a cup or a bowl or something and weigh
it. Then go down to the beach and fill it with
water. Bring it back and weigh it again. The
difference in weights is the weight of the sea
Now, put your bowl out in the sun and
let it dry. When all of the water has dried up,
weigh it again. The difference between the bowl's
original weight and its weight now is the salt.
You now have the weight of the salt and the weight
of the water, so dividing the amount of salt by
the amount of water will give you the weight ratio
of salt to water in the ocean.
Seawater varies in its salt content from place to
place. For example, there is a very low salt
concentration in areas where a large river flows
in to the ocean, due to high mixing with
freshwater. However, in the open ocean away from
land the salt content (or salinity) of the
seawater tends to range between 30 and 35 ppt
(parts per thousand). This is equivalent to 3.0
to 3.5 % (or parts per hundred). In hot areas
where there is a lot of evaporation the number
tends to be higher, whereas in areas that receive
a lot of water input (such as from rain) the
number will be lower.
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