1 ) 

Be reacts with ${O_2}$ when heated in air; Mg does not.

2 ) 

$Be{\text{ }}reacts{\text{ }}with{\text{ }}aqueous{\text{ }}{H_2}S{O_4}{\text{ }}to{\text{ }}form{\text{ }}a{\text{ }}metal{\text{ }}sulfate{\text{ }}and{\text{ }}{H_2}{\text{ }};{\text{ }}Mg{\text{ }}does{\text{ }}not$

3 ) 

$Be{\left( {N{O_3}} \right)_2}{\text{ }}decomposes{\text{ }}on{\text{ }}heating{\text{ }}to{\text{ }}form{\text{ }}a{\text{ }}metal{\text{ }}oxide,{\text{ }}N{O_2}{\text{ }}and{\text{ }}{O_2}{\text{ }};{\text{ }}Mg{\left( {N{O_3}} \right)_2}{\text{ }}does{\text{ }}not.{\text{ }}$

4 ) 

$BeC{l_2}{\text{ }}reacts{\text{ }}with{\text{ }}water{\text{ }}to{\text{ }}form{\text{ }}fumes{\text{ }}of{\text{ }}HCl{\text{ }};{\text{ }}MgC{l_2}{\text{ }}does{\text{ }}not.{\text{ }}$