Answers

2016-05-07T19:39:29+05:30
MgO is hard 
because its molecule is smaller then CaO and thus the atomic strength of MgO is more then CaO....
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2016-05-07T19:51:32+05:30
MgO is hard because it is smaller in size as compared to CaO and thus the atomic strength of MgO is more then CaO and the charge in case of MgO is more than CaO

therefore MgO is harder than CaO
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