Model Answer –
AS 2.4
Solubility
 Question: Discuss the solubility of
iodine, PCl3 and sodium chloride in water
and hexane.
1. Overview – types of bonding
 NaCl (metal and non-metal) is an IONIC
SOLID
 Iodine (identical non-metals) is a NONPOLAR COVALENT MOLECULAR SOLID
 PCl3 (non-identical non-metals) is a POLAR
COVALENT MOLECULAR SOLID*
 Water is a POLAR SOLVENT
 Hexane is a NON-POLAR SOLVENT
*PCl3 – why is it POLAR?
 3 POLAR BONDS as Cl is more
electronegative then P
 Trigonal pyramidal shape – as 4 areas of
electron density, 3 in bonds and 1 nonbonding and areas of electron density
REPEL
 Shape is ASSYMETRICAL, therefore polar
bonds (dipoles) DO NOT CANCEL,
therefore POLAR MOLECULE
2. Decide the solubility
 NaCl → ionic solid → soluble in POLAR
SOLVENTS
 Iodine → non-polar covalent → soluble in
NON-POLAR SOLVENTS
 PCl3 → polar covalent → soluble in POLAR
SOLVENTS
3. Discuss each substance in
terms of STRUCTURE, and
relate the structure to the
PROPERTY (in this case
solubility, but could be
conductivity or mpt)
NaCl (s)
STRUCTURE
GENERALISATION
 Ionic substances are soluble in polar
solvents. Sodium chloride is an ionic
solid. It consists of positive and negative
IONS in a 3D lattice structure, the ions
held by ionic bonds. The attraction of
polar water molecules for the ions is
greater than the water molecules for
Related to
each other or the ions for each other, so
the water can attract the ions out of their
lattice and surround the ions in a
hydration shell, therefore NaCl is soluble
PROPERTY
in water, but not non-polar hexane.
Iodine
GENERALISATION
• Non-polar substances are soluble in non-polar
solvents like hexane. Iodine is a non-polar
covalent solid. It consists I2 molecules, in which
both atoms have identical EN, therefore all
non-polar bonds, therefore I2 is a non-polar
molecule. The attraction between non-polar
hexane molecules and non-polar iodine
molecules is greater than the attraction of the
solid I2 molecules for each other, therefore
hexane can attract the non-polar iodine
molecules out of their lattice and surround the
molecules in a hydration shell, and so iodine is
soluble in hexane but not water.
PCl3
GENERALISATION –
most important part
 PCl3 is a polar covalent molecular
compound. It is soluble in polar
solvents like water. PCl3 is
composed of polar molecules*.
Polar water molecules can attract to
and can surround polar PCl3
molecules in a hydration shell.
Therefore PCl3 is soluble in water
but not hexane.