colours of inorganic ions and complexes

Question 1

[Cu(H2O)6]2+

Answer 1

from CuSO4•5H2O(s)
pale blue

Question 2

Cu(OH)2(s)

Answer 2

NH3 dropwise to Cu2+(aq)
pale blue precipitate

Question 3

[Cu(NH3)4(H2O)2 ]2+

Answer 3

excess NH3 to Cu2+(aq)
dark blue

Question 4

CuCl42–

Answer 4

excess HCl to Cu2+(aq)
yellow

Question 5

Cu+

Answer 5

reduction of Cu2+ with I–
white precpitate (CuI)
and brown I2

Question 6

Cu2+ / Cu

Answer 6

Disproportionation of Cu+
brown solid (Cu) and
blue solution (CuSO4)

Question 7

CoCl42–

Answer 7

from CoCl2.6H2O(s) plus
water and excess HCl
blue

Question 8

[Cr(H2O)6]3+

Answer 8

from KCr(SO4)2.12H2O(s)
pale purple
(heat -> green)
Due to impurities, it is
common for Cr(III) to
appear green in solution

Question 9

Cr(OH)3(s)

Answer 9

NH3 dropwise to Cr3+(aq)
dark green precipitate

Question 10

[Cr(NH3)6]3+

Answer 10

excess NH3 to Cr2+(aq)
purple

Question 11

[Cr(OH)6]3–

Answer 11

excess OH– to Cr(OH)3(s)
dark green

Question 12

Cr2O72–

Answer 12

from K2Cr2O7(s) in
0.1mol dm-3 H2SO4
orange

Question 13

CrO42–

Answer 13

oxidation of Cr3+ with
hot alkaline H2O2
yellow

Question 14

[Mn(H2O)6]2+

Answer 14

from MnSO4•4H2O(s)
pale pink

Question 15

Mn(OH)2(s)

Answer 15

NaOH dropwise to Mn2+(aq)
pale brown precipitate

Question 16

[Fe(H2O)6]2+

Answer 16

from FeSO4.7H2O(s)
pale green

Question 17

Fe(OH)2(s)

Answer 17

NaOH dropwise to Fe2+(aq)
dark green precipitate

Question 18

[Fe(H2O)6]3+

Answer 18

from FeCl3.6H2O(s)
yellow

Question 19

Fe(OH)3(s)

Answer 19

NaOH dropwise to Fe3+(aq)
orange-brown precipitate

Question 20

MnO4–/Fe2+ Mn2+/Fe3+

Answer 20

purple (MnO4–) to pale pink (Mn2+)
(in titrations, so dilute that it is
practically colourless)

Question 21

I–/Fe3+ I2/Fe2+

Answer 21

orange brown (Fe3+) to brown (I2)