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Physical Properties
Topic 4.4
Physical Properties
• The nature of the bonds within a substance
can predict and explain some of its properties.
This is particularly true for physical properties.
• We’ll consider three of these properties:
– Melting and boiling points
– Solubility
– Electrical conductivity
Melting and Boiling Points
• Ionic compounds tend to have high melting
and boiling points as the forces of electrostatic
attraction between the ions in the lattice are
strong and thus require high energy to break.
• Ion-ion attractions are stronger than the
intermolecular forces: H-bonding, dipoledipole, and van der Waals’ forces.
Melting and Boiling Points
• Ionic substances with multiply charged ions
such as Al3+, S2-, etc. usually have higher
mp/bp than singly charged ions such as Na+,
Cl-, etc. When comparing ions with the same
charge, the ion having the smallest radii will
usually have higher mp/bp.
Melting and Boiling Points
• Macromolecular or giant covalent structures
also have high melting and boiling points as
covalent bonds must be broken for these
changes of state to occur.
• They also exist as solids at room temperature.
Melting and Boiling Points
• Covalent substances have lower melting and
boiling point than ionic compounds as the
forces needed to be overcome to separate the
molecules are the relatively weak
intermolecular forces. Consequently many
covalent substances are liquids or gases at
room temperature.
• Intermolecular forces increase with:
– Increasing molecular mass
– The extent of polarity within the bonds
Solubility
• Solubility refers to the ease with which a solid
(the solute) becomes dispersed through a
liquid (the solvent) and forms a solution.
• There are several factors involved but, in
general, solubility is determined by the degree
to which the particles of the solute are
attracted to the particles of the solvent.
Solubility
• Solubility trends are based on the nature of
the solute and solvent. The following rule of
thumb is used: Like Dissolves Like.
• Polar and ionic solutes tend to dissolve in
polar solvents and non-polar solutes tend to
dissolve in non-polar solvents.
• So, if two substances have similar
intermolecular forces, chances are that the
solute will have a high solubility in the solvent.
Solubility
• Consider an ionic compound being placed in
water:
– The partial charges of the water molecules are
attracted to the ions of opposite charge in the
lattice, which causes them to become dislodged
from their position.
– Ions are separated from the lattice in this way
become surrounded by water molecules and
become hydrated; when this happens, the solid is
dissolved.
Solubility
Dissolution of
sodium chloride
in water
Solubility
• Similarly, many polar compounds are soluble
in water because of the attraction between
their dipoles and those of the water
molecules.
• Non-polar substances are not readily soluble
in water (a polar solvent) because of the lack
of attraction between the water molecules
and the solute.
Solubility
• In a non-polar solvent, ionic solutes will not
dissolve because the ions are not attracted to
the solvent.
• Similarly, polar compounds have limited
solubility in non-polar solvents.
• Non-polar substances, however, are soluble in
non-polar solvents due to their ability to
interact with the non-polar solvent by van der
Waals’ forces.
Electrical Conductivity
• The ability of a compound to conduct
electricity depends on whether it contains
ions that are able to move and carry a charge.
• Ionic compounds are not able to conduct
electricity in the solid state as the ions are
firmly held within the lattice and can’t move.
• However, in the liquid state or dissolved in
water, ionic compounds can conduct
electricity because the ions are able to move.
Electrical Conductivity
• Covalent compounds do not contain ions and
so, are not able to conduct electricity in the
solid or liquid states.