Solubility

​​In a nutshell 

How soluble a substance is will depend on the type of bonding in the substance. Substances are dissolved in two types of solvents: polar and non-polar solvents. 

Solubility 

​​Definition 

Solubility is how easily a substance is dissolved in another substance (solvent). 

In order for a substance to be soluble the bonds of the substance and solvent need to break. New bonds must form between the substance and the solvent. Dissolubility is favoured by a substance if the strength of the new bonds are the same or greater than the original bonds.

Solvents

There are two types of solvents: polar and non-polar solvents.

Polar solvents 

Polar solvents are made from polar molecules. These molecules will experience London forces and permanent dipole-permanent dipole bonds. 

Example 

Water is made from polar molecules bonded together via hydrogen bonding between the hydrogen and oxygen atoms. Therefore, water is a polar solvent. 

Note: Not all polar solvents are made from molecules that contain hydrogen bonding. Remember, hydrogen bonding occurs between hydrogen atoms and either fluorine, nitrogen and oxygen atoms. 

Non-polar solvents 

Non-polar solvents are made from non-polar molecules. These molecules will experience London forces only. 

Example 

Pentane is made from non-polar molecules. There are no polar bonds between the carbon and hydrogen atoms as they each have similar electronegativity values. Therefore, pentane is a non-polar solvent. 

Polar solvents 

Several substances favour dissolution in polar solvents.

Ionic substances 

Ionic substances dissolve in polar solvents, such as water. The ions in an ionic bond are attracted to the oppositely charged ends of a water molecule and the lattice structure breaks down. A hydration process is occurring.  

Note: Not all ionic substances dissolve in polar solvents. Ionic compounds with very strong bonds will not dissolve, as the bonds formed with the solvent are weaker compared to the original ionic bonds are weaker. Ions with higher charges have a higher charge density and form strong ionic compounds.  

Alcohols

Alcohols dissolve in polar solvents, such as water. Although alcohols are not ionic compounds, they will still dissolve in water as hydrogen bonds will form between the lone pairs of electrons on the oxygen atoms and hydrogen atoms in both the alcohol and water.  

The longer the carbon chain, the less soluble the alcohol will be due to the carbon chains not forming hydrogen bonds with the water molecules. 

Not all molecules that contain polar bonds will dissolve in water. Some polar molecules do not contain strong enough dipoles to form hydrogen bonds with water and will not dissolve. However, they will dissolve in solvents forming permanent dipole-permanent dipole bonds without hydrogen bonds. 

Example

Halogenoalkanes are polar, but will not dissolve in water. The hydrogen bonding in water are strong bonds compared to the bonds formed with halogenoalkanes. Halogenoalkanes form permanent dipole-permanent dipole bonds and will dissolve in a solvent with permanent dipole-permanent dipole bonds. 

Non-polar solvents 

Non-polar substances will dissolve in non-polar solvents, such as pentane. Both have London forces with similar bonds so will dissolve. Water will not dissolve non-polar substances as the hydrogen bonds are stronger than the bonds formed with the non-polar substance.