metal reactivity既問題..

Potassium, sodium and calcium are very reactive metal. They can react with water or steam to give metal hydroxides. These metal hydroxides are thermally stable which are not decomposed when heating.
2K(s) + 2H2O(l/g) → 2KOH(aq) + H2(g)
2Na(s) + 2H2O(l/g) → 2NaOH(aq) + H2(g)
Ca(s) + 2H2O(l/g) → Ca(OH)2(aq) + H2(g)

Magnesium reacts very slowly with cold water to give magnesium hydroxide, but reacts with steam to give magnesium oxide.
Mg(s) + 2H­2­­O(l) → Mg(OH)2(s) + H2­(g)
Mg(s) + H2O(g) → MgO(s) + H2(g)
This is because magnesium hydroxide is thermally unstable which is decomposed when heating.
Mg(OH)2(s) → MgO(s) + H2O(g)

Zinc and iron does not react with cold water, but reacts with steam to give metal oxides.
Zn(s) + H2O(g) → ZnO(s) + H2(g)
3Fe(s) + 4H2O(g) = Fe3O4(s) + 4H2(g) {reversible}
The products are not metal hydroxides. This is because the oxides of zinc and iron are thermally unstable which are decomposed when heating.
Zn(OH)2(s) → ZnO(s) + H2O(l)
Fe(OH)2(s) → FeO(s) + H2O(l)
2Fe(OH)3(s) → Fe2O3(s) + 3H2O(l)

鉀potassium、鈉sodium、鈣calcium比較活潑most reactive,所以即使係溫度比較低既水都會有反應
鎂magnesium、鋁aluminium、鋅zinc、鐵iron冇咁活潑decrease in reactivity,所以要用溫度較高既steam先可以有反應
而鉛lead、銅copper、汞mercury、銀silver、鉑platinum、金gold唔活潑least reactive,所以即使係steam都唔會有反應....