Advanced Topic 637 of 1102 Inorganic Chemistry
Alums
Double sulfates with formula M(I)M(III)(SO4)2 12H2O used in water purification.
In-depth explanation of Alums
Alums are double sulfates with the general formula M(I)M(III)(SO4)2 x 12H2O, where M(I) is a monovalent ion such as potassium or ammonium and M(III) is a trivalent ion such as aluminium or chromium. Potash alum, KAl(SO4)2 x 12H2O, forms large colourless octahedral crystals. The Al3+ ion hydrolyses in water and forms a gelatinous aluminium hydroxide floc. This floc traps suspended particles as it settles, which is why alum is used as a coagulant in water treatment. Alum also acts as a styptic and is used in dyeing to fix dyes on fabrics.
Going deeper: Alums
Key ideas: General formula M(I)M(III)(SO4)2.12H2O, for example potassium alum KAl(SO4)2.12H2O. It is a double salt that dissociates completely in water into K+, Al3+ and SO4 2-. Alums crystallize as octahedral crystals and are isomorphous (Cr, Fe in place of Al). Uses: water purification (flocculation), mordant in dyeing, styptic, tanning and fire retardants.
Common mistakes and connections: Students confuse double salts with coordination complexes: a double salt loses identity in solution, complexes keep it. Another error is writing 24 waters of crystallization instead of 12. The Al3+ hydrolysis produces gelatinous Al(OH)3 that traps suspended particles in treated water. Connects to hydrates, hydrolysis and colloid coagulation.
Visualization of Alums
Sulfate tetrahedra are the anion in alums, M(I)M(III)(SO4)2 with 12 water molecules; alum Al3+ hydrolyses to a gelatinous coagulating floc. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Adding potash alum to muddy water makes fine clay particles settle as a floc of aluminium hydroxide.
Frequently asked questions about Alums
What is Alums?
Double sulfates with formula M(I)M(III)(SO4)2 12H2O used in water purification.
Is Alums a beginner, intermediate or advanced topic?
Alums is a advanced topic in Inorganic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Alums?
Study these earlier topics in Inorganic Chemistry first: Borax, Boranes, Aluminium Chemistry.
What comes after Alums?
The next topics in Inorganic Chemistry are Extraction of Aluminium and Carbon Family.
Can I practise Alums with quizzes?
Yes. The interactive lesson includes a self-check question on Alums, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Alums fits in Inorganic Chemistry
Alums is topic 22 of 113 in Inorganic Chemistry.