What forms salt and water

Dissolving salts

Water molecules and ions - interaction

When salt crystals come into contact with water, the water molecules are deposited as dipoles on their surfaces and edges. The positively charged ions of the salt align themselves with the negative pole of the water molecule - and vice versa - the negatively charged ions with the positive pole of the water molecule.

This interaction is called ion-dipole interaction, in which the ions at the edge of the ion lattice are released from the ion lattice by water molecules and enveloped by them (hydration shell). Now the ions can diffuse into the water. This process is called hydration, during which energy is released, the hydration energy. It serves as a measure of the tendency of the ions to combine with water molecules

With the formation of hydrogen bridges to the first hydrate shell, further water molecules can accumulate and thus form another hydrate shell. In the hydration shell, the mutual attraction of the ions is greatly reduced, and in water two oppositely charged ions only attract each other with $ \ tfrac {1} {80} $ the force that would hold them together in air. As a result of hydration and the decrease in ion attraction, the salt crystal begins to dissolve.



Energy balance in the solution process

If one dissolves an ion crystal in water, an energy conversion takes place, which is characterized by two characteristics: energy is used to overcome the lattice energy, on the other hand energy is gained by hydrating the ions.

If the hydration energy is equal to or greater than its lattice energy, a salt is said to be readily soluble. In the case of salts, in which the hydration energy is significantly greater than the lattice energy, the salt-water mixture is heated during dissolution (heat of solution).


When calcium chloride hexahydrate is dissolved, however, the solution cools down. This cooling is based on the fact that the lattice energy required to dissolve the salt is not completely covered by the hydration energy and that the energy still required is taken from the thermal energy of the water. If the lattice energy is significantly greater than the hydration energy, the dissolution process does not take place: the substance in question is insoluble in water.


$ \ mathrm {hydration energy> lattice energy = warming} $

$ \ mathrm {hydration energy



The process of dissolving salts in water - formulation

After a salt crystal has dissolved, its ions are now in the salt solution. There they are however - in contrast to the solid state - surrounded by water molecules, which can be represented in simplified form with the following formula:


$ \ mathrm {NaCl \; \ xrightarrow {H_ {2} O} \; Na _ {(aq)} ^ {+} \; + \; Cl _ {(aq)} ^ {-}} $


The formula H2O above the reaction arrow indicates that the salt is dissolved in water (aqua). An "aq" on the ion indicates that the ion is hydrated, i.e. of an indefinite number of H2O molecules is enveloped.


$ \ mathrm {KBr \ quad \ xrightarrow {H_ {2} O} \ quad K _ {(aq)} ^ {+} \; + \; Br _ {(aq)} ^ {-}} $
Potassium bromide $ \ longrightarrow $ Potassium ion + Bromide ion


The sum of the positive and negative charges must be the same to the left and right of the reaction arrow, as the following example shows:


$ \ mathrm {MgCl_ {2} \ quad \ xrightarrow {H_ {2} O} \ quad Mg _ {(aq)} ^ {2+} \; + \; 2 \ Cl _ {(aq)} ^ {-}} $
Magnesium chloride $ \ longrightarrow $ Magnesium ion + Chloride ion