• # question_answer Hydrated magnesium chloride cannot be dehydrated by heating.

The formula of the hydrated salt is $MgC{{l}_{2}}.6{{H}_{2}}O$. When heated, it gets hydrolysed to form MgO. $MgC{{l}_{2}}.6{{H}_{2}}O\xrightarrow{Heat}\,MgO+2HCl+5{{H}_{2}}O$ Therefore, the salt cannot be dehydrated by heating as such. The dehydration is done by passing $HCl$ gas through the hydrated salt at about 650 K. $MgC{{l}_{2}}.6{{H}_{2}}O\xrightarrow[650K]{HCl(g)}\underset{\text{(Anthydrous)}}{\mathop{MgC{{l}_{2}}}}\,+6{{H}_{2}}O$