Water is made up of two molecules of oxygen and one of hydrogen (H2O). Hydrogen has a positive polarity while oxygen has a negative polarity. It is thanks to this that an electrostatic force holds the atoms together to create the water molecules.
Salt, on the other hand, is also made up of two ions. Chloride (Cl-) and sodium (Na+), hence the name sodium chloride.
In short, salt has the ability to dissolve in water due to electrical charges and the fact that water and salt compounds are polarized. Thanks to positive and negative charges on opposite sides of the molecules, a bond is created. The attraction between these molecules is said to be ionic, because it is possible thanks to the opposite electrical charges of the molecules.
Moreover, the water molecule itself has ionic characteristics since the positive charges of hydrogen allow oxygen to adhere to it. This is called a covalent bond.
When salt is added to water, it dissolves because the covalent bonds in water are stronger than the ionic bonds that hold the salt molecules together.
As the salt dissolves in the water, the sodium and chloride ions, the ones needed for the salt to be present, get separated by the water molecules. Once separated, the sodium and chloride atoms are surrounded by water molecules and form a homogeneous solution.
The amount of salt in water is measured in grams/liter and is called the salinity of water. There are 3 categories of water.
- Fresh water: less than 1g/L
- Brackish water: [1g/l to 10g/l].
- Salt water: more than 10g/l
As an indication, it is estimated that the average salinity of sea water on earth is about 35g/l.
Finally, salt has an important impact on water. Starting with its density. Salt water is indeed denser than fresh water. As a result, the freezing temperature of salt water is -1.9°C instead of 0°C and its boiling point is 1.5°C higher.
Moreover, salt water increases its conductivity since electricity can travel between the salt ions.