P7
Water: “Uses”, Properties and Structures
Water is a major component of the hydrosphere and also exists in significant amounts
in the atmosphere and the lithosphere. Water is of fundamental importance to all living
things and has many unique chemical and physical properties.
Role of water on
Earth
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Necessary for all living things as a raw material, solvent,
transport medium and thermal regulator
Habitat for some life forms
Weathering and eroding agent
Natural resource for humans
Properties of water
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Surface tension
Viscosity
Shape of molecules
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Linear (carbon dioxide)
Bent (water)
Trigonal (boron trifluoride)
Pyramidal (ammonia)
Tetrahedral (methane)
Polar covalent bonds
Covalent bonds in which the electrons are not shared equally.
The atoms forming the covalent bond have different
electronegativities.
Dipole
A pair of equal and opposite charges separated in space.
Dipole molecules
Molecules that have a net dipole.
Bonds must be polar
Molecule must be asymmetrical.
Dipole-dipole
interactions
Electrostatic attractions between the positive end of one dipole
molecule and the negative end of another dipole molecule.
Dipole-dipole interactions lead to stronger intermolecular
forces and consequently high melting and boiling points.
Hydrogen bonds
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Surface tension
Particularly strong types of polar interactions
Occur in molecules where hydrogen is bonded to atoms
of either fluorine, oxygen or nitrogen
The strength of a hydrogen bond is typically one-tenth
that of a normal covalent bond
A molecule in the surface layer of a liquid experiences
intermolecular forces only from molecules below and beside
it; there are no molecules above it. These molecules result in a
downward force acting on surface molecules.
The stronger the intermolecular forces, the greater will be the
surface tension of the liquid.
Cohesion
The force with which a substance sticks together.
Adhesion
The force with which a substance sticks to another substance.
Capillary action
Adhesive forces draw the liquid up the walls of a capillary
tube until the weight of the liquid column being supported just
balances the adhesive forces.
Viscosity
Measures the resistance of liquid to flow through a tube.
Viscosity depends on
 The size and complexity of the molecules.
 The intermolecular forces in the substance
Freezing of water
Water expands when it freezes because the hydrogen bonding
between the water molecules creates the open structure of ice.
This open structure gives ice a lower density than water.
Hardness and
brittleness
The hydrogen bonds between water molecules in ice are
highly directional and give ice a very ordered structure. Ice is
hard and shatters rather than bends.
Water as a solvent of
ionic compounds
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Water is a polar solvent
Most ionic compounds are soluble in water
The energy to break up a crystal lattice comes from the
energy released when bonds form between the water
molecules and the positive and negative ions of the ionic
compound.
The negative end of the water molecule (O) is attracted to
the positive ions
The positive end of the water molecule (H) is attracted to
the negative ions
Water of
crystallisation
Water that becomes chemically bonded to a pure substance
during crystallisation
Anhydrous
Salts that do not contain water of crystallisation
Solubility of
substances
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Most ionic compounds are soluble in water
Some polar molecular compounds are soluble because of
hydrogen bonds
Some polar molecular substances are soluble in water
because they react with water and ionise
Many polar molecular substances are insoluble
A few non-polar molecular compounds are slightly
soluble
Most non-polar molecular substances are insoluble
Very large molecules are insoluble if highly structured
A few very large molecules are soluble.
Covalent lattice substances are insoluble
Metals are insoluble unless they react with water
Non-polar solvent
A non-polar solvent is a liquid that consists of non-polar
molecules
Like dissolves like
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Polar (including ionic) substances dissolve in polar
solvents such as water and do not dissolve in non-polar
solvents such as hexane.
Non-polar substances dissolve in non-polar solvents but
do not dissolve in polar ones.
Diffusion
A mixing process in which particles move from a region
where their concentration is higher to one where their
concentration is lower.
Concentration
Number of particles of the substance per unit volume.
Semi-permeable
membrane
Small molecules and ions are able to pass through the pores or
holes in the barrier while large molecules cannot.
Osmosis
Process in which a solvent passes through a semi-permeable
membrane from a solution of lower concentration into one of
higher solute concentration