it is possible maybe the crystalline material containing 30 g of barium chloride is a hydrated barium chloride. So when it was heated, the water bounded was evaporated and the remaining barium chloride is only 20 g. I hope this can help.
Explanation:
When 30 grams of barium chloride crystals when heated at 400 degree Celsius and again weighed again which comes to be 20 grams. This means that water of crystallization associated with the barium chloride crystal got vaporized when heated at 400 degree Celsius.
Water of crystallization: Water molecules associated to a crystal structure of a compound.
[tex]BaCl_2.2H_2O\overset{400^o C}\rightarrow BaCl_2+2H_2O[/tex]
Hence, the sample of barium chloride crystals was hydrated.
