Tests Cations And Flame Tests

Question 1

Aluminium Al3+

Answer 1

Add aqueous sodium hydroxide dropwise and then to a separate sample add aqueous ammonia dropwise. With sodium hydroxide a white precipitate is formed which dissolved in excess. With ammonia, a white precipitate is formed which does NOT dissolve in excess.

Al3+(aq)+3OH-(aq) -> Al(OH)3(s)
Al(OH)3(s)+OH- -> Al(OH)4-(aq)

Question 2

Ammonium NH4+

Answer 2

Add aqueous sodium hydroxide. A pungent smell of the gas ammonia, which turns red litmus paper blue, indicates ammonium ion.

NH4+ + OH- -> NH3(g) + H2O(l)

Question 3

Calcium Ca2+

Answer 3

Add concentrated aqueous sodium hydroxide. A thin white precipitate of calcium hydroxide is formed if at all. Note that the calcium hydroxide is slightly soluble (limewater).

Ca2+(aq) + 2OH- -> Ca(OH)2(s)

Question 4

Copper(II) Cu2+

Answer 4

Add aqueous sodium hydroxide and then to a separate sample add aqueous ammonia dropwise. A pale blue gelatinous precipitate is formed first with both substances. In the case of ammonia only, with excess ammonia, a deep blue coloured solution is formed.

Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s)
Cu(OH)2(s) + 4NH3(aq) -> Cu(NH3)4^2+(aq)

Question 5

Iron(II) Fe2+

Answer 5

Add aqueous sodium hydroxide and then to a separate sample add aqueous ammonia. A pale green gelatinous precipitate is formed in both cases, which does not dissolve in excess.

Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s)

Question 6

Iron(III) Fe3+

Answer 6

Add aqueous sodium hydroxide and then to a separate sample add aqueous ammonia. A red-brown gelatinous precipitate is formed in both cases.

Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)

Question 7

Zinc Zn2+

Answer 7

Add aqueous sodium hydroxide dropwise and then to a separate sample add aqueousamonia. A white precipitate is first formed with both substances. In both cases, with excess reagent, a colourless solution is formed.
with sodium hydroxide:
Zn2+(aq) + 2OH-(aq) -> Zn(OH)2(s)
Zn(OH)2(s) + 2OH-(aq) -> (Zn(OH)4)^2-(aq)

with ammonia
Zn2+(aq) + 2OH-(aq) -> Zn(OH)2(s)
Zn(OH)2(s) + 4NH3(aq) -> Zn(NH3)4^2+(aq)

Question 8

Chromium Cr3+

Answer 8

NaOH - Green precipitate formed which is soluble in excess NaOH and forms a dark green solution.

Cr3+(aq) + 3OH-(aq) -> Cr(OH)3(s)
Cr(OH)3(s) + OH-(aq) -> Cr(OH)4^1-(aq)

NH3 - Grey green precipitate which is insoluble in excess solution
Cr3+(aq) + 3NH3(aq) + 3H2O(l) <==> Cr(OH)3(s) + 3NH4^1+(aq)

Question 9

Barium flame test

Answer 9

Pale green

Question 10

Calcium flame test

Answer 10

Yellow-red

Question 11

Copper flame test

Answer 11

Green-blue

Question 12

Lithium flame test

Answer 12

Red

Question 13

Sodium flame test

Answer 13

Orange

Question 14

Potassium flame test

Answer 14

Lilac