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.
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
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