alkalis

Question

for each of the folowing pairs of substances, suggest a simple chemical test to distinguish one substance from the other and state the expected observation( Flame test is not accepted)

a zinc nitrate and magnesium nitrate

- 如果我加入NaOH- into zinc nitrate and magnesium nitrate, 2者就會有white ppt. 啦~
咁如果再加入excess NaOH- zinc nitrate 係唔係會變成colourless?
咁如果再加入 excess NaOH- magnesium nitrate 係唔係都係white ppt.?

Answer 1

To distinguish between Zn(NO3)2 and Mg(NO3)2 solutions, add separately sodium hydroxide solution to each of the two solutions, dropwise until in excess.

Firstly, both solutions give white precipitates.
Zn2+(aq) + 2OH-(aq) → Zn(OH)2(s)
Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

However, the Zn(OH)2 dissolves in excess sodium hydroxide solution, but Mg(OH)2 hydroxide does not.
Zn(OH)2(s) + 2OH-(aq) → [Zn(OH)4]2-(aq)

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You may use ammonia solution instead of sodium hydroxide solution. Ammonia is a weak alkali which is ionized in water to give hydroxide ions.
NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)

Firstly, both solutions give white precipitates.
Zn2+(aq) + 2OH-(aq) → Zn(OH)2(s)
Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

However, the Zn(OH)2 dissolves in excess ammonia solution, but Mg(OH)2 hydroxide does not.
Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]2+(aq) + 2OH-(aq)