Na2O, MgO and Al2O3 - all have high melting points because they form giant ionic lattices.
MgO has a higher melting point than Na2O because Mg forms 2+ ions which attract O^2- ions more strongly than 1+ Sodium ions in Na2O.
Al2O3 has a melting between Na2O and MgO because it's 3+ ions distort the oxygen's electron cloud making the bonds partially covalent.
SiO2 has a higher melting point than other non-metal oxides because it has a giant macromolecular structure. Strong covalent bonds hold it together, so lots of energy is needed to break the bonds and the melting temperature is high.
P4O10 and SO2 are covalent molecules. They have relatively low melting points because they form simple molecular structures. The molecules are held together by weak intermolecular forces (dipole-dipole and van der Waals), which take little energy to overcome.
Reactions with Water - Na and Mg
The oxides of Na and Mg dissolve in water to form hydroxides. They are both alkaline solutions but sodium hydroxide is more soluble in water so forms a more alkaline solution.
Na2O(s) + H2O(l) >>> 2NaOH(aq) pH = 12 - 14
MgO(s) + H2O(l) >>> Mg(OH)2(aq) pH = 9 - 10
Reactions with Water - P and S
The simple covalent oxides of the non-metals phosporus and sulfur form strong acidic solutions of pH = 0 - 2.
P4O10(s) + 6H2O(l) >>> 4H3PO4(aq) - phospuric(V) acid
SO2(g) + H2O(l) >>> H2SO3(aq) - sulfurous (or sulfuric(IV)) acid
SO3(l) + H2O(l) >>> H2SO4(aq) - sulfuric(VI) acid
Reactions with Water - Si and Al
The giant covalent structure of silicon dioxide means that it is insoluble in water. But it will react with bases to form salts so it is classed as an acid.
Aluminium oxide is also insoluble in water. But, it will react with acids and bases to form salts - it can act as an acid or a base so it is classed as amphoteric.
Reactions with Acids and Bases
Acid + Base >>> Salt + Water
Na and Mg are basic so will neutralise acids:
- Na2O(s) + 2HCl(aq) >>> 2NaCl(aq) + H2O(l)
- MgO(s) + H2SO4(aq) >>> MgSO4(aq) + H2O(l)
Silicon, phosphorus and sulfur oxides are acidic so will neutralise bases:
- SiO2(s) + 2NaOH(aq) >>>Na2SiO3(aq) + H2O(l)
- P4O10(s) + 12NaOH(aq) >>> 4Na3PO4(aq) + H2O(l)
- SO2(g) + 2NaOH(aq) >>> Na2SO3(aq) + H2O(l)
- SO3(g) + 2NaOH(aq) >>> Na2SO4(aq) + H2O(l)
Aluminium oxides are amphoteric so can neutralise acids or bases:
- Al2O3(s) + 3H2SO4(aq) >>> Al2(SO4)3(aq) + 3H2O(l)
- Al2O3(s) + 2NaOH(aq) + 3H2O(l) >>> 2NaAl (OH)4(aq)