Editing Alum (section)

==Chemical properties==
Aluminium-based alums have a number of common chemical properties. They are soluble in [[water (molecule)|water]], have a sweetish taste, react as [[acid]] by turning blue [[litmus]] to red, and [[crystal]]lize in regular [[octahedron|octahedra]]. In alums each metal ion is surrounded by six water molecules. When heated, they liquefy, and if the heating is continued, the [[water of crystallization]] is driven off, the salt froths and swells, and at last an amorphous powder remains.<ref name=Chisholm-1911/>{{rp|page=766|}} They are [[astringent]] and acidic.

===Crystal structure===
Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums. The first X-ray crystal structures of alums were reported in 1927 by [[James M. Cork]] and [[Lawrence Bragg]], and were used to develop the [[phase retrieval]] technique [[isomorphous replacement]].<ref name=Cork-1927-10-LED-Phil-Mag/>

===Solubility===
The solubility of the various alums in water varies greatly, sodium alum being soluble readily in water, while [[caesium]] and [[rubidium]] alums are only slightly soluble. The various solubilities are shown in the following table.<ref name=Chisholm-1911/>{{rp|page=767}}

At temperature {{mvar|T}}, 100&nbsp;parts water dissolve:
: {| class="wikitable" style="text-align:right;"
|-
! {{mvar|T}} !! Ammonium {{small|<br/>alum}} !! Potassium {{small|<br/>alum}} !! Rubidium {{small|<br/>alum}} !! Caesium {{small|<br/>alum}}
|-
|   0&nbsp;°C ||  2.62     ||   3.90 ||  0.71     || 0.19{{0}}
|-
|  10&nbsp;°C ||  4.50     ||   9.52 ||  1.09     || 0.29{{0}}
|-
|  50&nbsp;°C || 15.9{{0}} ||  44.11 ||  4.98     || 1.235
|-
|  80&nbsp;°C || 35.20     || 134.47 || 21.60     || 5.29{{0}}
|-
| 100&nbsp;°C || 70.83     || 357.48 || -  || - 
|}