Colour by Design Notes

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  • Created on: 17-04-13 19:19
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Colour by Design CD
The Chemistry of Colour
o Substances are coloured if they absorb energy that is in the visible frequency of the
electromagnetic spectrum.
o The energy absorbed causes electronic changes; electrons are promoted from their ground
state to a higher energy level (they become excited).
o The difference in energy between the ground state and the excited state is equal to the
quantum of energy absorbed and so determines the wavelength of light absorbed.
o It is the outermost electrons that are excited; these are the electrons involved in bonding or
in lone pairs.
o The inner electrons are held tightly by the positively charged nucleus; the energy needed to
promote these is very large.
o Not all electronic transitions are brought about by visible light; some transitions require more
energy and are brought about by absorbing light in the ultraviolet region of the spectrum.
o If only light from the ultraviolet region of the spectrum was absorbed, the compound would
appear colourless.
o The energy needed to excite an electron in a coloured compound is referred to as
the excitation energy:
Coloured Organic Compounds
o These often contain unsaturated groups such as C=C, C=O or -N=N-.

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Colour by Design CD
o These can form part of an extended delocalised electron system called a chromophore.
o Electrons in double bonds are more spread out than those in single bonds, and therefore
require less energy to become excited; this means that the energy absorbed is within the
visible region rather than in the ultraviolet region of the electromagnetic spectrum.…read more

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Colour by Design CD
o Paints contain pigments of different colours mixed with a liquid which carries the pigment
(the medium).
o In water colours, the finely ground pigment is suspended in water; when the water
evaporates it leaves behind the pigment on the page.
o In spray paints, the medium is a volatile organic substance that quickly evaporates.
o It takes a long time for oil paints to dry.…read more

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Colour by Design CD
o When restoring historical art work, it is important that the new colours used in retouching are
a good match of the original colours.
o Chemists can identify the colours and select the most appropriate pigments using a variety of
Reflectance Spectra
o The surface of a painting is never smooth, and as a result light is reflected in all directions.…read more

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Colour by Design CD
o Once the reflectance spectrums for the different pigment colours are produced, they can be
used to select another pigment that is the same colour; for example the reflectance
spectrum for a blue-red pigment was measured:
o The reflectance spectrum for ultramarine is as follows:
o These reflectance spectra are very similar and so the colour of the pigments will also be
similar. By comparing the spectra of different pigments, the chemists can find the most
accurate match of colour.…read more

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Colour by Design CD
Laser Microspectral Analysis (LMA)
o This technique is used to identify the different elements present in a pigment.
o A pulse of high power laser light is focused onto a small sample of paint, resulting in the
vaporisation of the metal compounds (which have high boiling points).…read more

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Colour by Design CD
o Glycerol can react with three palmitic acid molecules to form a triester:
o The triesters found in natural oils are usually mixed triesters (the three fatty acid groups are
7…read more

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Colour by Design CD
Fats and Fatty Acids
o The carboxylic acids in fats and oils are usually unbranched, containing an even number of
carbon atoms ranging from C4 to C24.
o The fatty acids can either be fully saturated (all single bonds) or they can
be unsaturated (contain one or more carbon-carbon double bond).
o Some common fatty acids are shown below:
Solid or Liquid?
o Natural oils and fats contain a mixture of many triesters.…read more

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Colour by Design CD
o The following diagram shows the structure of an unsaturated triglyceride:
o The unsaturated triglyceride molecules can not pack so closely together due to the kink
caused by the presence of the cis double bonds; the intermolecular forces are therefore
weaker and so less energy is needed to separate the molecules, so they have a lower
melting point and are liquid at room temperature.
o Oils can be converted to fat in a hydrogenation reaction; this is how margarine is produced.…read more

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Colour by Design CD
Hydrolysis of Fats
o Like all esters, the ester linkage holding the molecules together can be hydrolysed.
o When this is done with concentrated sodium hydroxide solution, a soap is made.
o Soaps are the sodium or potassium salts of fatty acids and they are made by heating the oils
and fats with sodium or potassium hydroxide.
o The free fatty acids are then released from the sodium salts by adding a dilute acid.…read more



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