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Oceanography >> Seawater >> A Salty Sea
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Two of the most defining characteristics of seawater is that it is both liquid and salty! Where did the salt come from? How can we measure it? What does it do? Why can't we drink it? How can we obtain freshwater from the sea?
2.1 - A Salty Sea
Objectives:
- To know the percentage of dissolved salt in seawater
- To know the sources and approximate percentages of freshwater on Earth.
- To know the origin of the salt in the ocean.
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How does salt dissolve in water? Seems simple - add salt to water and stir. But the science of dissolving salt is not as trivial as it first looks. The water is a polar molecule - it is split into a positive and negative charge by its asymmetry. When the salt crystals are surrounded by moving water molecules the outer sodium chloride molecules are removed and broken into two. The water molecules surround the sodium and chloride ions. As they are too small to be seen, they become invisible. You can tell a solution from a suspension as solutions are transparent.
The amount of salt that a certain volume of water can dissolve depends on the type of salt (although we will stick with sodium chloride) and the temperature of water. |
Salinity
Definition: salinity is the number of grams of dissolved solids in a kilogram of seawater. As the amount of salt dissolved is relatively small, around \(35\,\text{g}\), it is customary to measure in parts per thousand, which is written in a similar manner to the percentage symbol - \( 35\,\unicode{x2030}\). The other more precise unit is the Practical Salinity Unit \( \text{(PSU)}\), which to all intents and purposes is the same thing.
So, the salinity is a measure of how many grams of salt are in each kilogram of water. For example a salinity of \( 35\,\unicode{x2030}\) means that there are \(35\,\text{g}\) of salt dissolved in \(1\,\text{kg}\) of seawater. BUT this is NOT \(1\,\text{kg}\) of freshwater.
Total mass \(=1.0\,\text{kg} = 1000\,\text{g}\)
Mass of salt \(=35\,\text{g}\)
Mass of water \( =1000 - 35 = 965\,\text{g} = 0.965\,\text{kg}\)
Percentage \(= \frac{35}{1000} \times 100\,\text{%} = 3.5 \,\text{%}\)
The common mistake is to think that \( 35\,\unicode{x2030}\) water is \(35\,\text{g}\) of salt dissolved in \(1.0\,\text{kg}\) of fresh water.
Definition: salinity is the number of grams of dissolved solids in a kilogram of seawater. As the amount of salt dissolved is relatively small, around \(35\,\text{g}\), it is customary to measure in parts per thousand, which is written in a similar manner to the percentage symbol - \( 35\,\unicode{x2030}\). The other more precise unit is the Practical Salinity Unit \( \text{(PSU)}\), which to all intents and purposes is the same thing.
So, the salinity is a measure of how many grams of salt are in each kilogram of water. For example a salinity of \( 35\,\unicode{x2030}\) means that there are \(35\,\text{g}\) of salt dissolved in \(1\,\text{kg}\) of seawater. BUT this is NOT \(1\,\text{kg}\) of freshwater.
Total mass \(=1.0\,\text{kg} = 1000\,\text{g}\)
Mass of salt \(=35\,\text{g}\)
Mass of water \( =1000 - 35 = 965\,\text{g} = 0.965\,\text{kg}\)
Percentage \(= \frac{35}{1000} \times 100\,\text{%} = 3.5 \,\text{%}\)
The common mistake is to think that \( 35\,\unicode{x2030}\) water is \(35\,\text{g}\) of salt dissolved in \(1.0\,\text{kg}\) of fresh water.
LAB 2.1 - Making Seawater
Aim: To make samples of water of different salinity for use for future experiments and gain an understanding of what salinity means.
Method: Using a plastic beaker and a balance, weigh out a specified mass of salt (e.g. \(10.0\,\text{g}\). Slowly add water until the total mass is exactly \(1000.0\,\text{g}\). Stir until all the salt has been dissolved.
Label the beaker clearly with the salinity, in this case \( 10.0\,\unicode{x2030}\) .
By the end, we should have a wide range of water of different salinities.
Method: Using a plastic beaker and a balance, weigh out a specified mass of salt (e.g. \(10.0\,\text{g}\). Slowly add water until the total mass is exactly \(1000.0\,\text{g}\). Stir until all the salt has been dissolved.
Label the beaker clearly with the salinity, in this case \( 10.0\,\unicode{x2030}\) .
By the end, we should have a wide range of water of different salinities.
This can lead us to do some interesting calculations:
How much salt is required to turn the swimming pool into seawater? Assume that the dimensions of the pool are: \(25\,\text{m} \times15\,\text{m}\times2.5\,\text{m}\) and the density of water is \(1000\,\text{kg/m}^3\).
How much salt is in the ocean assuming a salinity of \(35\,\unicode{x2030}\) and an overall mass of roughly \(50\times10^{15}\,\text{tonnes}\)?
How much salt is required to turn the swimming pool into seawater? Assume that the dimensions of the pool are: \(25\,\text{m} \times15\,\text{m}\times2.5\,\text{m}\) and the density of water is \(1000\,\text{kg/m}^3\).
How much salt is in the ocean assuming a salinity of \(35\,\unicode{x2030}\) and an overall mass of roughly \(50\times10^{15}\,\text{tonnes}\)?
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Why is the ocean salty?
Now this is a BIG question! The answer comes from the water cycle. The water vapour that evaporates from the surface of the ocean is fresh. It condenses to form clouds, which eventually deposit the fresh water as rain. Rain water absorbs carbon dioxide and become slightly acidic carbonic acid. This reacts with rocks to produce a salt and more carbon dioxide. So ultimately the salt has come from the rocks of the Earth. Based on the article above, it is estimated that the salt of the oceans would form a crust over the land up to 500 ft (166 m) depth. Certainly, the evaporated salt piles that I have seen in the Mediterranean Salinas and the Salt Lake plains in the USA are massive! |
How much of the World's Water is Fresh?
As interesting as seawater is, we cannot drink it or irrigate our crops with it. We need fresh water. Surprisingly little - some \(2.5\,\text{%}\) of the Earth's water is fresh, even if we include the Great Lakes of the US, the frozen icesheets of Greenland and Antarctica and all the other lakes and rivers of the world. Fresh water is a precious resource - one that is coming under increasing pressure due to population growth and climate change.
As interesting as seawater is, we cannot drink it or irrigate our crops with it. We need fresh water. Surprisingly little - some \(2.5\,\text{%}\) of the Earth's water is fresh, even if we include the Great Lakes of the US, the frozen icesheets of Greenland and Antarctica and all the other lakes and rivers of the world. Fresh water is a precious resource - one that is coming under increasing pressure due to population growth and climate change.
Only 2.5 % of the Earth's water is fresh - and a large amount of it is unusable. We had better be careful with it!
Activities:
- Research - how does the salinity of the oceans vary? Are the seas around Bermuda really saltier than the US and Europe as people seem to think?
| salinity.pptx |