Alum. Characteristics of an Alum

It is called whole alum salt inorganic which presents its constitution represented by the following general formula:

X₂ONLY₄.Y₂(ONLY₄)₃.24H₂O

From this general formula, we can conclude that all alum has:

• Presence of two sulfate anions (SO₄);

• Presence of a metal whose NOX is +1 (X), that is, a monovalent cation;

• Presence of a metal whose NOX is +3 (Y), that is, a trivalent cation;

• Presence of 24 water molecules.

Here are some examples:

• K₂ONLY₄.Cr₂(ONLY₄)₃.24H₂O

In this alum, we have the presence of the monovalent cation Potassium (K⁺¹) and the trivalent cation Chromium (Cr⁺³).

• Ag₂ONLY₄.Al₂(ONLY₄)₃.24H₂O

In this alum, we have the presence of the monovalent cation Silver (Ag⁺¹) and the aluminum trivalent cation (Al⁺³).

• At₂ONLY₄.Bi₂(ONLY₄)₃.24H₂O

In this alum, we have the presence of the monovalent cation Sodium (Na⁺¹) and the trivalent Bismuth cation (Bi⁺³).

• Ass₂ONLY₄.ww₂(ONLY₄)₃.24H₂O

In this alum, we have the presence of the monovalent cation Copper (Cu⁺¹) and the aluminum trivalent cation (Au⁺³)

• The nomenclature of an alum salt is made according to the following rule:

Alum + de + Trivalent cation name + e + monovalent cation name

Look rule application examples of alum nomenclature:

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• K₂ONLY₄.Cr₂(ONLY₄)₃.24H₂O

Chromium III Alum and Potassium

The Roman numeral III is used in front of the trivalent Chromium cation because it does not belong to the IA, IIA and IIIA families, and is neither Silver nor Zinc.

• Ag₂ONLY₄.Al₂(ONLY₄)₃.24H₂O

silver alum

In this example, the name of the monovalent cation was only used in the nomenclature because the trivalent cation is Aluminum and this name refers to the name alum.

• At₂ONLY₄.Bi₂(ONLY₄)₃.24H₂O

Bismuth III alum and sodium

The Roman numeral III is used in front of the trivalent Bismuth cation because it does not belong to the IA, IIA and IIIA families, and is neither Silver nor Zinc.

• Ass₂ONLY₄.ww₂(ONLY₄)₃.24H₂O

Alum of gold III and copper I

The Roman numerals III and I are used in front of the trivalent cation Gold and the monovalent Copper because they do not belong to the families IA, IIA and IIIA, and are neither Silver nor Zinc.

The main features of alum, generally speaking, they are:

• Solid at room temperature;

• Soluble in water and alcohol;

• ionic compound.

By Me. Diogo Lopes Dias

Would you like to reference this text in a school or academic work? Look:

DAYS, Diogo Lopes. "Alum"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/alumen.htm. Accessed on June 27, 2021.