Reactions of oxides with water

Question 1

Na2O

Answer 1

• Appears to dissolve completely in water
• Vigorous & exothermic, even in cold water
• Forms strongly alkaline colourless solution around pH 13

Na2O (s) + H2O (l) —> 2Na+ (aq) + OH- (aq)

Question 2

MgO

Answer 2

• very slow reaction and limited solubility in water
• some Mg(OH)2 is formed but it is sparingly soluble so not all OH- ions are released into the solution (pH9)

MgO (s) + H2O (l) —> Mg(OH)2 (s)
Mg(OH)2 (s) — Mg2+ (aq) + 2OH-(aq)

Question 3

Al2O3 & Al(OH)3

Answer 3

No reaction, insoluble

Question 4

SiO2

Answer 4

No reaction, insoluble

Question 5

P4O10

Answer 5

• reacts violently to give a strongly acidic solution around pH 2

P4O10 (s) + 6 H2O (l) —> 4H3PO4 (aq)

Question 6

SO3

Answer 6

• violent and very exothermic reaction, producing a very acidic mist of H2SO4 droplet
• forms strongly acidic solution around pH1

SO3 (s) + H2O (l) —> H2SO4 (aq)