Page 36 - Curriculum Visions Dynamic Book

Aluminium sulphate (alum)
Alum is a compound of aluminium sulphate. It is used as a mordant, that is a substance that will absorb dyes and so allow some natural fabrics to be coloured. The dye-absorbing property of alum is shown here.
For many thousands of years, people knew that clay-rich rocks contained a useful substance, even though they could not extract it properly or find out what it was. So
they called it the metal of clay. Alum was eventually obtained by heating suitable rocks, often clay-rich
rocks called shales, in sulphuric acid. Today alum is still used as a mordant
to help fix dyes in cotton fabric. The fabrics are first dipped in a solution
containing alum and then in a solution of alkali so that a precipitate of aluminium
hydroxide (the real mordant) is formed between the fibres. As the cotton fabric
dries, the tiny particles of aluminium hydroxide remain in the fabric, and the
dye sticks to them.
Alum is also used as a filler in
paper and to purify water.
 Freshly precipitated aluminium hydroxide, a nearly colourless gelatinous solid, shown here
in a water suspension.
EQUATION: Precipitating aluminium hydroxide mordant
 A purple vegetable dye
Aluminium sulphate (alum) + ammonia solution + water ➪ aluminium hydroxide + ammonium sulphate Al2(SO4)3(aq) + 6NH3(aq) + 6H2O(l) ➪ 2Al(OH)3(s) + 3(NH4)2SO4(aq)
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