Final answer:
The lack of expected color changes in complexation reactions for nickel and iron(III) ions after separating black precipitate suggests that either these ions were not present, the concentrations were too low, or the reactions failed due to issues like incomplete dissolution, interfering substances, incorrect reagent concentrations, or contamination.
Step-by-step explanation:
When attempting to identify metal ions in a mixture using precipitation reactions, the student possibly encountered an issue where the expected precipitates did not form upon adding specific reagents. In the first half of the liquid, addition of NH3 (ammonia) and DMG (dimethylglyoxime) typically indicates a test for nickel ions, which form a red complex with DMG. If no color change occurred, the nickel ions might be absent, or the concentration could be too low. On the other hand, adding acetone and potassium thiocyanate to the other half of the liquid is indicative of a test for iron(III) ions, which would typically form a blood-red complex. The absence of a color change suggests the absence of iron(III) ions or insufficient reagent concentration or interaction time.
One possible reason for the unexpected results could be that the black precipitate did not exclusively contain the targeted ions, or that the liquefaction process in HCl and HNO3 was incomplete. If the precipitate was incompletely dissolved, this might lead to a failure in forming the expected complexes upon the addition of later reagents. Another possibility is that interfering substances could be present in the solution, which prevents the expected reactions or the formation of complexes. Incorrect reagent concentrations, improper pH, or contamination could also contribute to the lack of expected results.