SOLUBILITYSuppose, for example, that 100 g of hexane is mixed with 100 g of water at room tem-
perature. The liquids are almost immiscible, but the hexane layer would actually contain
about 0.01 g of water, and the water layer would contain about 0.001 g of hexane.
Justification of the rule that ‘like dissolves like’
The information contained in Table 11.2 confirms the rule that ‘like dissolves like’.
This rule is based upon experimental observation. In order to explain the rule, we
rely upon a knowledge of the forces between ions, atoms and molecules:
1.Inorganic salts (such as Na, Cl(s)) are polar by definition. The solubility of
sodium chloride in water is due to the strong attractive forces between the Naand
Clions, and the polar ends (O–H+) of the water molecules. Salts such as
sodium chloride are also fairly soluble in ethanol and methanol (which, although
less polar than water, still give rise to fairly strong ion–molecule forces).
2.Salts are immiscible with non-polar solvents such as hexane and tetrachloro-
methane. The forces between ions (such as Naand Cl) and molecules of non-polar
solvents are London dispersion forces. Such forces are much weaker than the attrac-
tions between opposite ions which hold the sodium chloride lattice together.
3.Solvents such as hexane and tetrachloromethane are soluble in similar solvents
(such as hydrocarbons) because the London dispersion forces between the two
different kinds of molecules are roughly equal to (or greater than) the London dis-
persion forces between the same molecules. For example, hexane dissolves in hep-
tane because the forces between the heptane and hexane molecules are at least equal
to those between heptane and heptane or between hexane and hexane molecules. If
water is added to heptane or hexane, the water molecules ‘stick together’ and form a
separate phase because the hydrogen bonds between the water molecules are much
stronger than the forces between the water and hydrocarbon molecules.
4.Even large covalent molecules may be soluble in water if they contain sufficiently
polar bonds. This is the case for sucrose (table sugar, C 12 H 22 O 11 ) which contains
eight polar O–H+groups per molecule and so is able to link with the water
molecules using hydrogen bonding. However, the general rule is that within a series
of organic compounds, solubility in water decreases as the number of carbon atoms
in the molecular chain increases. For example, consider ethanoic and hexanoic acids:
173An oil slick. Crude oil is
largely immiscible with water
and floats on its surface. It
then washes ashore. It is
particularly dangerous to birds
since it is miscible with the
oils on a bird’s body.