Coloured substances, both natural and synthetic, have long been of interest to
the human race. For example, there are over 130 minerals that have been used as
The corundum minerals, which include rubies and sapphires, are mainly
aluminium oxide. Rubies have some of the aluminium ions replaced by
chromium(III) ions and they range in colour from pink to deep red. Sapphires occur in
many different colours including blue, yellow, purple, orange and green.
Diamond is an allotrope of carbon. It is the hardest naturally occurring substance.
Diamonds are often colourless but the presence of nitrogen atoms in natural
diamonds gives them a brown or yellow colour, whilst the presence of boron gives
them a blue colour.
Dyes and pigments have been developed as colouring materials. Dyes are applied as a
solution, whilst pigments are solids that are ground into a fine powder and made into
a suspension with a suitable liquid.
Methyl red is an azo dye and is used as an indicator. Its structure is
Prussian Blue, , (discovered in 1704) is a deep blue colour and was one of
the first synthetic pigments. It is used as a pigment in paints.
Colour can arise in many different ways; for example, the colours seen in flame tests
and in transition metal compounds.
Explain why complex ions containing chromium(III) ions are coloured.
d-subshell splits / d-orbitals split (in
energy) / d energy level(s) split(s)
(by the ligands) (1)
Electron(s) promoted / excited
(from lower) to higher energy
levels / electron(s)move (from
lower) to higher energy d orbitals
ALLOW d-d transitions (1)
Absorbing photons / energy of a
certain frequency (in visible region)
ALLOW absorbing light (1)
Transmitted / remaining light is
ALLOW complementary colour is seen
ALLOW reflected / transmitted /
remaining light is seen (1)
IGNORE “opposite” colour / reference
to electrons relaxing / dropping to
the ground state