NATS 1870 Lecture 11: SC NATS 1870 LECTURE 11

19 views3 pages
12 Aug 2016
of 3
Tuesday, November 3, 2015
Chemistry and Colour
Colour as an information giver
Colour can be a temperature indicator (not chemistry)
Can indicate pH level (acidity) in a solution
Fruit ripens, fruit goes bad
Metals can oxidize
Colour change as information
Levels of oxidation
Food cooking changes colours
Change of seasons (leaves change colour)
Colour change can be a visible probe of change at the microscopic level
Chemical reactions can happen
a) New molecules can be created = visible colour change
b) Molecules can have their structure change = visible colour change
Key for Colour:
The way a molecule it put together
Spacing of energy levels ( structure, bonds determine spacing of energy levels/
Types of bonds:
Ionic-> One atom donates e- to neighbour
-This is a strong bond
-Need UV photo to raise e- to higher level
-Materials are often colourless
find more resources at
find more resources at
Tuesday, November 3, 2015
Covalent bonds-> equally shared
-These bonds tend to be present in colorants (dyes, pigments)
Molecules have 2 ways electrons can change internal energies
1. Electron transitions -> An electron jumps from one energy level to another
2. Vibrational transitions-> Bonds between atoms can vibrate, Range of energies
3. Rotational transitions-> 3D molecule can rotate in various orientations-each has
different energy
There is a range of rotational+vibrational energies within each (electronic) energy level.
Common Molecular structures in colorants:
1. Double Covalent
-1st e- pair that is shared is tightly bound
-2nd e- pair can readily absorb visible wavelengths
2. Auxochromes- addition of extra atoms to molecular structures charges (lowers)
energy level spacing making invisible molecules (benzene) changed into a substance
with colour (MARITUS YELLOW)
3. Conjugated bonds (chains of alternating single and double covalent bonds):
4. Porphyrins:
-Chlorophyll- Contains Magnesium
-Phthalo blue- Contains Cooper
Changing leaf colour
-Summer: green- from chlorophyll
-Fall: Cooler days, shorter nights no more chlorophyll production
-Can reveal yellow leaves-> from carotenes already present
find more resources at
find more resources at
Tuesday, November 3, 2015
-Can change to red-> from anthocyanin - a chemical made in a leaf, from a reaction
between sugar+sunlight
find more resources at
find more resources at