1. Practical Scope & Target Ions
Required Practical 4 requires candidates to perform systematic qualitative tests using dropper bottles, test tubes, and heating apparatus to identify unknown inorganic salts containing:
- Cations: Group 2 metal ions (Mg2+, Ca2+, Sr2+, Ba2+) and the ammonium ion (NH4+).
- Anions: Halide ions (Cl-, Br-, I-), sulfate ions (SO4^2-), and carbonate / hydrogencarbonate ions (CO3^2- / HCO3-).
2. Group 2 Cation Identification & Solubility Trends
The four alkaline earth metal cations are distinguished by exploiting the opposing solubility trends of their hydroxides and sulfates:
Hydroxide Solubility Trend (Down Group 2)
Solubility INCREASES down Group 2:
- Mg2+: Sparingly soluble → forms a thick white precipitate of Mg(OH)2.
- Ca2+: Slightly soluble → forms a faint white suspension of Ca(OH)2 (limewater).
- Sr2+: Moderately soluble → forms a very slight white haze.
- Ba2+: Highly soluble → no precipitate forms (solution remains completely clear).
Sulfate Solubility Trend (Down Group 2)
Solubility DECREASES down Group 2:
- Mg2+: Highly soluble → no precipitate forms (MgSO4 is soluble).
- Ca2+: Slightly soluble → forms a faint white precipitate of CaSO4.
- Sr2+: Sparingly soluble → forms a dense white precipitate of SrSO4.
- Ba2+: Completely insoluble → forms a thick, heavy white precipitate of BaSO4.
| Group 2 Cation | Addition of dilute NaOH(aq) | Addition of dilute H2SO4(aq) | Diagnostic Deduction |
|---|---|---|---|
| Magnesium (Mg2+) | Thick white precipitate (Mg(OH)2) | Colourless solution (no precipitate) | Only Group 2 ion giving thick precipitate with NaOH and none with H2SO4. |
| Calcium (Ca2+) | Faint white suspension (Ca(OH)2) | Faint white precipitate (CaSO4) | Intermediate solubility with both reagents; confirms with brick-red flame test. |
| Strontium (Sr2+) | Very slight white haze (almost clear) | Dense white precipitate (SrSO4) | Confirms with crimson-red flame test. |
| Barium (Ba2+) | Colourless solution (no precipitate) | Thick white precipitate (BaSO4) | Only Group 2 ion giving thick precipitate with H2SO4 and none with NaOH. Confirms with apple-green flame. |
3. Test for the Ammonium Cation (NH4+)
The ammonium ion does not form precipitates with most standard reagents. Its identification relies on generating volatile ammonia gas (NH3):
2. Add approximately 1 cm3 of dilute sodium hydroxide (NaOH) solution.
3. Warm the test tube gently using a hot water bath (avoid boiling or spattering).
4. Hold a piece of damp red litmus paper over the mouth of the test tube (do not touch the glass walls).
5. Positive Observation: Damp red litmus paper turns blue due to alkaline ammonia gas:
NH4+(aq) + OH-(aq) → NH3(g) + H2O(l). A characteristic pungent smell of ammonia is also detected.
In written examinations, stating "hold red litmus paper over the tube" loses the mark. Dry red litmus paper cannot detect ammonia because NH3 gas is unreactive without water. It must dissolve in moisture on the paper to produce hydroxide ions (NH3 + H2O ⇔ NH4+ + OH-) that cause the indicator color transition from red to blue.
4. Halide Anion Identification with Acidified AgNO3 & Ammonia
Halide anions (Cl-, Br-, I-) are differentiated using silver nitrate followed by testing precipitate solubility in dilute and concentrated aqueous ammonia:
| Halide Ion | Addition of Acidified AgNO3(aq) | Ionic Precipitation Equation | Effect of Dilute NH3(aq) | Effect of Concentrated NH3(aq) |
|---|---|---|---|---|
| Chloride (Cl-) | White precipitate | Ag+(aq) + Cl-(aq) → AgCl(s) |
Dissolves (forms colourless [Ag(NH3)2]+ complex) | Dissolves completely |
| Bromide (Br-) | Cream precipitate | Ag+(aq) + Br-(aq) → AgBr(s) |
Insoluble (precipitate remains) | Dissolves (forms [Ag(NH3)2]+ complex) |
| Iodide (I-) | Yellow precipitate | Ag+(aq) + I-(aq) → AgI(s) |
Insoluble | Insoluble (precipitate remains) |
A classic Paper 5 question asks: "Explain why the sample must be acidified with dilute nitric acid before adding silver nitrate."
Mark Scheme Answer: To react with and remove any carbonate (CO3^2-) or hydroxide (OH-) ions present as impurities. If not removed, silver carbonate (Ag2CO3) or silver oxide (Ag2O) would form white/brown precipitates, giving a false positive test for chloride.
5. Tests for Sulfate (SO4^2-) and Carbonate (CO3^2-) Anions
Sulfate Test (Acidified BaCl2)
Protocol:
- Acidify the sample with dilute hydrochloric acid (HCl) to destroy carbonate impurities.
- Add barium chloride solution (BaCl2).
- Observation: Formation of a thick white precipitate of barium sulfate:
Ba2+(aq) + SO4^2-(aq) → BaSO4(s).
Note: Do NOT acidify with sulfuric acid (H2SO4) because the reagent itself contains sulfate ions, causing an immediate false positive!
Carbonate Test (Dilute Acid + Limewater)
Protocol:
- Add dilute hydrochloric acid (HCl) or nitric acid (HNO3) to the solid or solution.
- Rapid effervescence (fizzing) occurs as CO2 gas is released:
CO3^2- + 2H+ → CO2(g) + H2O(l). - Bubble the gas through limewater (aqueous Ca(OH)2).
- Observation: Limewater turns cloudy / milky due to fine white CaCO3 precipitate.
6. Worked Inorganic Deduction Problem
Problem: Solid X is a white crystalline inorganic salt. A student carries out three qualitative tests:
- Test 1: A portion of solid X is dissolved in water. Addition of dilute sodium hydroxide produces no visible precipitate. Warming this mixture produces a gas that turns damp red litmus paper blue.
- Test 2: A separate sample of solution X is acidified with dilute nitric acid. Addition of silver nitrate solution produces a cream precipitate. When dilute ammonia solution is added to this precipitate, it remains insoluble; however, upon adding concentrated ammonia, the precipitate completely dissolves to form a colourless solution.
- Test 3: Addition of acidified barium chloride solution to solution X gives no precipitate.
Deduce the identity of the cation and anion in Salt X. Write ionic equations for Test 1 and Test 2.
Deduction of Cation:
Warming with NaOH produces gas turning damp red litmus blue → NH3 gas evolved.
Therefore, Cation is Ammonium (NH4+).
Ionic equation: NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)
Deduction of Anion:
Acidified AgNO3 gives a cream precipitate → silver bromide (AgBr).
Insoluble in dilute NH3 but soluble in concentrated NH3 confirms Bromide (Br-).
Ionic equation: Ag+(aq) + Br-(aq) → AgBr(s)
Final Formula of Salt X:
Salt X is Ammonium bromide (NH4Br).
Conclusion: NH4Br
7. Practice Exam Questions
Question 1: Which reagent can distinguish between aqueous solutions of magnesium chloride and barium chloride in a single test-tube step?
Show Answer & Explanation
Correct Answer: B
Adding sulfuric acid gives a thick white precipitate with barium ions (insoluble BaSO4) but leaves magnesium ions in clear solution (soluble MgSO4). Dilute NaOH could also distinguish them (giving white precipitate with Mg2+ but not Ba2+).
Question 2: Why must dilute hydrochloric acid NEVER be used to acidify a sample when testing for sulfate ions with barium chloride?
Show Answer & Explanation
Correct Answer: B
Careful reading of exam questions is vital! Dilute HCl is the standard, correct acid used to acidify before BaCl2. The prohibited acid is sulfuric acid (H2SO4), which contains sulfate ions and would cause an immediate false positive.