What is the molecular formula of alum

Potassium aluminum sulfate
Potassium aluminum sulfate dodecahydrate
Potassium aluminum sulfate
Potassium aluminum sulfate dodecahydrate
258.205 g / mol
474.389 g / mol
not specified
1.72 g / cm3
+92 ° C
+780 ° C (decomposes anhydrous)
100g H2Dissolve O 6.01 g (L)

White or colorless, crystalline powder

Special notes for schools
Potassium aluminum sulphate is not classified as a hazardous substance. Safety glasses must be worn when working with all chemicals. Dodecahydrate is mostly used in schools.
Potassium aluminum sulfate is also known by the name of alum or dodecahydrate Potash alum known. The substance forms colorless crystals according to the cubic system. The minerals potash alum and alunite are recognized minerals. While the dodecahydrate has a water content, the alunite is made up of basic, anhydrous potash alum.

Much more potassium alum dissolves in warm water.

When finely ground, the crystalline powder appears white. The crystals are readily soluble in water, forming an acidic solution. The solubility increases sharply when the water is heated: At 20 ° C, 100 grams of water dissolve about 6.01 grams of dodecahydrate, at 40 ° C it is 13.6 grams, at 60 ° C 33.3 grams and at 80 ° C even 72 grams. For this reason, the salt can easily be presented in pure form from a hot solution by crystallizing it out. Colorless, octahedral crystals crystallize out of the sour-tasting, aqueous alum solution. Occasionally, the cube surface shows up at the edges of the crystals. When heated to over 92 ° C, all of the water of crystallization escapes and is created Burnt alum, which is also called Alumen ustum is known.

Potash crystals in a petri dish

Octahedron grown using the thread technique
In ancient times, alum was extracted from the mineral alunite. From 1500 on, alum could also be extracted from black slate or clay slate. The rock was piled up in long rows. Through weathering processes, sulfuric acid was formed, which reacted with the clay minerals it contained to form alum. The process could be accelerated by firing. The rock turned reddish in color due to the iron content. The iron was removed by leaching with water. The alum was deposited as a precipitate.

In the laboratory, potassium alum is obtained by combining an aluminum sulfate solution and a potassium sulfate solution; the pure potassium aluminum sulfate is then obtained by crystallization. In technical production, clay or kaolin is converted into aluminum sulfate by heating it with sulfuric acid and then the potassium sulfate is added:

Al2(SO4)3 + K2SO4   2 KAl (SO4)2
In chemistry classes and in crystal growing kits, potassium aluminum sulfate is the classic salt for growing crystals. The ancient Egyptians used alum as a flame retardant for wood. In dyeing, alum is used to pickle textiles. The paper industry uses it to glue paper and the tannery to tan leather. In medicine and in razor stones, alum is used as an astringent to stop blood.

An alum razor stone is used to stop blood.

Further information and media
Instructions for growing crystals
Research assignments: growing crystals
Color project: dyeing with stain dyes
Manufacture of a pigment from vegetable dyes