Page 7 - Curriculum Visions Dynamic Book

Light causes halogens to decompose
and change colour
These two test tubes below right show white silver chloride shortly after it has been prepared as a precipitate, and the same tube after it has been exposed to light for a few minutes and has darkened. (These changes are the basis of how a photographic film works. Silver bromide is used as it is the most sensitive to light.)
EQUATION: Silver halides and sunlight
Silver chloride ➪ silver + chlorine 2AgCl(s) ➪ 2Ag(s) + Cl2(g)
Halogens dissolve better in organic solvents than in water
In the photographs below, iodine, bromine and chlorine have been introduced to a tube containing two liquids in which other substances commonly dissolve (solvents). The bottom one is water and above it is an organic liquid (methyl benzene). Each of the halogens
is more soluble in the organic solvent, and it has turned the liquid dark brown. They are barely soluble in water, and so the water is only slightly coloured.
decompose: to break down a substance (for example by heat or with the aid
of a catalyst) into simpler components. In such a chemical reaction only one substance is involved.
organic substance: a substance that contains carbon.
solvent: the main substance in a solution (e.g. water in salt water).
 Halogens have been dissolved in methyl benzene.
Iodine
Silver chloride before exposure to light
Bromine
Silver chloride after exposure to light
Chlorine
7
7