In IMDG Code we can see the terms hydrous and anhydrous. Hydrous means a chemical compound combined with water molecules and anhydrous means all water removed especially water of crystallization.

Photograph of a sample of anhydrous chromium(I...
Photograph of a sample of anhydrous chromium(III) chloride, CrCl 3 . Sample from Aldrich (99%, sublimed), CAS # 10025-73-7. Scene lit with natural sunlight. (Photo credit: Wikipedia)

Below are two examples of hydrous and anhydrous form of the same chemicals

Hydrated       =  UN 2672 AMMONIA SOLUTION Class 8)

Anhydrous     = UN 1005, AMMONIA, ANHYDROUS Class 2.3 / 8

Hydrated       = UN 1789 HYDROCHLORIC ACID Class 8

Anhydrous     = 1050 HYDROGEN CHLORIDE,   ANHYDROUS Class 2.3 / 8

Tin(II) oxide hydrate. It is pale yellow while...
Tin(II) oxide hydrate. It is pale yellow while the anhydrous form is blue-black (Photo credit: Wikipedia)

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By Shashi Kallada

35 years in Merchant Shipping, Last 23 years working on IMDG Code. Ex Sailor, Ex Manager Global Dangerous Goods Maersk Line.

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