Although fresh water runs into the sea, the sea is salty. Its strange really, and so in this article I’ll explain why the sea is salty.
The ground is not just inert dirt and rocks – it contains an abundance of chemicals and minerals. Amongst all these minerals is sodium, which is the the 6th most abundant mineral found in the crust. Sodium is most commonly found bonded to less abundant (but still relatively plentiful) chlorine, which forms sodium chloride – what we call table salt. Salt is water soluble, and so when it rains, the salt in the ground dissolves in the rain water and runs into the rivers, and ultimately into the seas.
When the water in the sea heats up from the sun it evaporates and forms clouds, but the salt is far to heavy to evaporate with the water, and so stays in the seas.
This process continues over many many years, and the weather/ rain will break rocks up/ wear rocks down, which exposes more mineral rich rocks/ soil, which rain water will also dissolve and wash into the sea.
This process concentrates salts and other minerals in the sea, because the salt cannot escape, but the water can through evaporate. Gradually this builds up the salinity of the seas to the point where the seas are significantly saltier than water in the rivers which feeds them.
When underwater volcanoes erupt they release a number of chemicals into the seas and oceans which were previously trapped in the crust below the ocean floor. Amongst these chemicals will be sodium chloride (salt), which will further contribute to the salinity of the sea, but to a much lesser extent than the rivers do.
With sodium only being the 6th most abundant mineral in the crust, and chlorine being even less abundant, you might wonder why the sea is salty and not another kind of flavour (minerally?). Indeed aluminium, calcium and iron are far more abundant that chlorine and sodium, so you might expect the sea to be more abundant in these minerals than sodium and chlorine.
Iron – Iron is nearly 5% of the crust, which makes it almost twice as abundant as sodium, and it is in fact the most abundant element on the planet. Very little iron reaches the sea e sea because when water comes into contact with iron it readily forms a compound called iron oxide (which we know as rust). Iron oxides are not soluble, and so don’t easily run off into the sea
Aluminium – Aluminium is nearly three times as abundant as sodium is, but very little is found in the sea for the same reason as iron. Aluminium reacts with oxygen far too quickly, and becomes insoluble too.
Calcium – Calcium is usually found in the crust in the form of calcium carbonate, which is very stable and so doesn’t dissolve very well in the rain water. Calcium compounds do dissolve in water to some extent though and do make it to the sea, which makes calcium one of the more abundant minerals in the sea.
Other minerals are present in the water, the most notable of which is potassium and magnesium, which are moderately abundant in the crust, but due to solubility, are found in higher concentrations in the sea.
Interestingly, it is the presence of these additional minerals which makes using sea salt to season food a healthier option than regular table salt.
The sea is salty because rain water continuously washes salt from the rocks/ soil on land into the sea. The salt cannot escape because the salt atoms are too heavy to evaporate with the water the dissolved in.
Despite not being the most abundant minerals in the sea both sodium and chloride ions are water soluble, and so will readily wash into the sea where other more abundant elements won’t. This is why sodium and chloride concentrations are higher in the sea than in the crust.
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Over millions of years salt from the land has been dissolved and washed into the sea by rain water. As salt it heavy, it cannot evaporate, and so stary in the sea.