Precipitation colours

Question 1

precipitate and equation of Cu2+ reacting with NaOH

Answer 1

goes from pale blue to forming a blue gelatinous precipitate
[Cu(H2O)6]2+ + 2OH- = [Cu(OH)2(H2O)4] + 2H20

Question 2

precipitate and equation of Fe2+ reacting with NaOH

Answer 2

goes from pale green solution containing Fe2+ to a green precipitate turning rust brown at its surface in standing air
Fe2+ + 20H- = Fe(OH)2

Question 3

precipitate and equation of Fe3+ reacting with NaOH

Answer 3

• pale yellow containing Fe3+ forms a rusty brown precipitate of iron(III) hydroxide
  Fe3+ + 3OH- = Fe(OH)3

Question 4

precipitate and equation of Mn2+ reacting with NaOH

Answer 4

brown precipitate forms which darkens in the air as it oxidises
[Mn(H20)6]2+ + 2OH- = [Mn(OH)2(H20)4] + 2H20
4Mn(OH)2 + O2 = 2Mn2O3 + 4H20

Question 5

precipitate and equation of Cr3+ reacting with NaOH

Answer 5

green precipitate forms which will then re-dissolve in excess hydroxide
[Cr(H2O)6]3+ + 3OH- = [Cr(OH)3(H2O)3] + 3H2O
[Cr(OH)3(H2O)3] + 3OH- = [Cr(OH)6]3- + 3H2O

Question 6

precipitate and equation of Cu2+ reacting with ammonia

Answer 6

blue gelatinous precipitate is formed which re-dissolves into a blue solution
[Cu(H20)6] 2+ + 2NH3 = [Cu(OH)2(H20)4] + 2NH4+

Question 7

precipitate and equation of Fe2+ reacting with ammonia

Answer 7

dark green precipitate is formed

[Fe(H20)6]2+ + 2NH3 = [Fe(OH)2(H20)4] + 2NH4+

Question 8

precipitate and equation of Fe3+ reacting with ammonia

Answer 8

rusty brown precipitate

[Fe(H20)6]3+ + 3NH3 = [Fe(OH)3(H20)3] + 3NH4+

Question 9

precipitate and equation of Mn2+ reacting with ammonia

Answer 9

brown precipitate forms which darkens in the air as it is oxidises
[Mn(H20)6]2+ + 2NH3 = [Mn(OH)2(H20)4] + 2NH4+

Question 10

precipitate and equation of Cr3+ reacting with ammonia

Answer 10

green precipitate forms which will re-dissolve in excess ammonia is added
[Cr(H20)6]3+ + 3NH3 = [Cr(OH)3(H20)3] + 3NH4+
THEN
[Cr(OH)3(H20)3] + 6NH3 = [Cr(NH3)6]3+ + 3H20 + 3OH-