Answer is: can dissolve in water.
Magnesium oxide (MgO):
1) does not have molecular structure.
2) it is not higly reactive, it is physically and chemically stable at high temperatures.
3) has very high boiling point (around 3600°C), because strong ionic bond between magnesium cations (Mg²⁺) and oxygen anion (O²⁻).
4) dissolve in water, because it has ionic bond.
Hydrogen peroxide (H₂O₂):
1) have molecular structure.
2) it is higly reactive, it is not physically and chemically stable.
3) has low boiling point (around 150°C), because weakcovalent bonds.
4) dissolve in water, because it has polar bond.