a lab

1. Name: Niobe Nottingham Date: 11.03.24 Lab #: 22
Title: Analyzing substance Z
Aim: To determine the ions present in sample Z.
Apparatus and Materials:
• Sample Z
• Test tubes
• 6% H2O2
• Bench reagents - NH3(aq), NaOH, BaCL, HCl
• Materials for testing gas
• White tile
Method:
1. To a few cm3 of sample Z in a test tube, aqueous sodium hydroxide was added
drop by drop until in excess.
2. To a few cm3 of sample Z in the 2nd test tube, aqueous ammonia was added drop
by drop until in excess.
3. To a few cm3 of sample Z in the 3rd test tube, H2O2 was added drop by drop
until there was no further change. The solution was kept for test 4.
4. To the solution from test 3, dilute hydrochloric acid was added followed by
barium chloride.
5. To a few cm3 of sample Z in the 4th test tube, a little nitric acid was added
followed by aqueous silver nitrate. Aqueous ammonia was then added.

2. Results:
TABLE SHOWING QUALITATIVE RESULTS OBTAINED
Test Observations Inference
1. To a few cm3 of sample Z,
add aqueous sodium
hydroxide drop by drop
until in excess.
Greyish - green precipitate.
Soluble in excess
Cr3+ ions present
2. To a few cm3 of sample Z,
add aqueous ammonia
drop by drop until in
excess.
Greyish - green precipitate.
Insoluble in excess
Cr3+ ions present
3. To a few cm3 of sample Z,
add H2O2 drop by drop
until there is no further
change. (Keep solution
for test 4)
Yellow solution seen I2 ions present in solution
4. To the solution from test
3, add dilute
hydrochloric acid
followed by barium
chloride.
Orange or yellow - orange
precipitate can be seen.
Ba2+ ions present
5. To a few cm3 of sample Z,
add a little nitric acid
followed by aqueous
silver nitrate. Then add
aqueous ammonia.
White precipitate. Soluble
in NH3(aq)
Cl- ions present
Discussion:
When sample Z was reacted with aqueous NaOH, the following reaction occurred:
Cr3 + (aq) + 3OH-(aq) ~ CP(OH)3(s)

3. The above reaction is a precipitation reaction i.e when two soluble compounds, typically
aqueous solutions, react to form an insoluble solid compound, called a precipitate. When
H2O2 was added to sample Z, a yellow solution was seen. It was deduced that the
solution was this colour due to the I2 ions present within the solution. This is because
when the H2O2 was added, it oxidized the I- ions:
2l (aq)+H2O2(aq) l2(aq)+2OH (aq)
Based on tests 1 and 2, the cation present in sample Z can be deduced as Cr3+. Both tests
produced the same observation, greyish - green precipitate, indicating the consistent
presence of Cr3+ ions. Based on the results from test 5, the anion present in sample Z can
be determined as Cl-. The formation of a white precipitate upon the addition of silver
nitrate followed by the solubility of the precipitate in aqueous ammonia strongly suggests
the presence of Cl- ions.
Precautions:
1. Ensure to label test tubes to avoid contamination.
2. Measure the same amount of sample Z for all tests.
Source of Error: Some ions or compounds present in the sample or reagents may
interfere with the intended reactions, leading to misleading results.
Reflection: In forensic investigations, qualitative analysis techniques are used to analyze
trace evidence found at crime scenes. By identifying ions and compounds present in
substances such as powders, liquids, and residues, forensic scientists can provide
evidence to support criminal investigations. Also, in clinical chemistry, qualitative
analysis is used to identify ions and compounds present in biological samples such as
blood, urine, and tissue samples. This information is valuable for diagnosing diseases,
monitoring patient health, and assessing the effectiveness of treatments.
Conclusion: The ions present in sample Z were found to be Cl- and Cr3+.