When analyzing an unknown substance, chemists follow a specific sequence of tests. This systematic approach ensures that tests do not interfere with each other and that conclusions are reliable. The main steps are:

1. Preliminary tests – Simple observations that give initial clues
2. Tests for anions (acid radicals) – Identifying negative ions
3. Tests for cations (basic radicals) – Identifying positive ions

Preliminary tests are performed first because they provide quick information without destroying much of the sample. These tests help narrow down the possibilities before more specific confirmatory tests are carried out.

1.1 Appearance and Physical State

Observe the color, texture, and physical form of the solid sample. Use the following guide:

• Colorless compounds – Usually contain no transition metals (e.g., NaCl, KCl, CaCO₃)
• Colored compounds – Often contain transition metal ions (e.g., Cu²⁺ gives blue/green, Fe³⁺ gives yellow-brown)
• Black substances – Often oxides or sulfides (e.g., CuO, PbS)
• White substances – Many salts are white (e.g., most chlorides, sulfates, carbonates)

1.2 Flame Test

The flame test identifies certain metal ions based on the characteristic color they produce when heated in a flame.

Procedure: Clean a platinum or nichrome wire by dipping it in dilute HCl and heating in a non-luminous flame. Dip the clean wire into the sample and place it back in the flame. Observe the color produced.

1.3 Solubility in Water

Test whether the sample dissolves in water:

• Soluble – Most nitrates, chlorides (except AgCl, PbCl₂), and sulfates (except BaSO₄, PbSO₄, CaSO₄) are soluble
• Insoluble – Most carbonates, phosphates, hydroxides, and sulfides are insoluble

1.4 Action on Litmus

Place a small amount of the solid in a dry test tube and gently heat. Hold moist litmus paper at the mouth of the tube to detect any gases evolved:

• Red litmus turns blue – Alkaline gas (e.g., NH₃) is present
• Blue litmus turns red – Acidic gas (e.g., HCl, SO₂, CO₂) is present
• No change – Neutral gas (e.g., H₂, N₂, O₂) is present

After preliminary tests, we test for negative ions (anions). The choice of test depends on the preliminary observations.

2.1 Test for Carbonate (CO₃²⁻)

1. Add dilute hydrochloric acid (HCl) or dilute sulfuric acid (H₂SO₄) to a solution of the sample.
2. Observe: effervescence (bubbling) indicates carbon dioxide gas is liberated.
3. Confirmatory test: Pass the gas through lime water. If the lime water turns milky, carbon dioxide is present, confirming carbonate ions.

$\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

2.2 Test for Chloride (Cl⁻)

1. Add dilute nitric acid (HNO₃) to the sample solution.
2. Add silver nitrate (AgNO₃) solution.
3. Observation: A white precipitate of silver chloride (AgCl) forms.
4. Confirmatory test: Add ammonia solution to the precipitate. If it dissolves, chloride is confirmed.

2.3 Test for Sulfate (SO₄²⁻)

1. Add dilute hydrochloric acid (HCl) to the solution.
2. Add barium chloride (BaCl₂) solution.
3. Observation: A white precipitate of barium sulfate (BaSO₄) forms. This precipitate does not dissolve in dilute acids.

$\text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4$

2.4 Test for Nitrate (NO₃⁻)

1. Add dilute sulfuric acid (H₂SO₄) to the solution.
2. Add a freshly prepared solution of iron(II) sulfate (FeSO₄).
3. Observation: A brown ring forms at the junction of the two liquids, indicating nitrate ions.

After identifying anions, we test for positive ions (cations). The tests below are performed on the solution obtained after acid radical analysis.

3.1 Action of Dilute Hydrochloric Acid (HCl)

Add dilute HCl to the solution:

• White precipitate forms – Indicates Pb²⁺ (lead ions), as lead chloride is insoluble
• No precipitate – Other cations present (Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄⁺)

3.2 Action of Dilute Sulfuric Acid (H₂SO₄)

Add dilute H₂SO₄:

• White precipitate forms – Indicates Pb²⁺, Ca²⁺, or Ba²⁺

3.3 Action of Sodium Hydroxide (NaOH)

Add NaOH solution drop by drop, then in excess:

3.4 Action of Aqueous Ammonia (NH₃·H₂O)

Add aqueous ammonia drop by drop, then in excess:

3.5 Confirmatory Tests for Specific Cations

After preliminary identification, confirm the cation with specific tests:

Let us apply the systematic approach to identify an unknown white salt. Assume the salt is lead(II) nitrate.

Step 1: Preliminary tests

• Appearance: White crystalline solid
• Flame test: Not distinctive (lead gives faint blue-green)
• Solubility in water: Soluble
• Action of heat: Decomposes to give brown nitrogen dioxide gas and oxygen

Step 2: Tests for anions

• Add dilute H₂SO₄ → No effervescence (no carbonate)
• Add AgNO₃ with dilute HNO₃ → No white precipitate (no chloride)
• Add BaCl₂ with dilute HCl → No white precipitate (no sulfate)

Step 3: Tests for cations

• Add dilute HCl → White precipitate forms (PbCl₂)
• Add NaOH → White precipitate forms, dissolves in excess (Pb²⁺)
• Add NH₃·H₂O → White precipitate forms, does not dissolve in excess

Conclusion: The salt is lead(II) nitrate, Pb(NO₃)₂

1. Use clean apparatus – Contamination can lead to false results
2. Add reagents in correct order – Some tests must be done before others
3. Use excess reagent – Ensure complete reaction
4. Warm gently when required – Use water bath, not direct flame
5. Observe carefully – Note color, texture, and timing of precipitates
6. Record all observations – Keep a systematic record of every test and result

In Tanzania, qualitative analysis is used in environmental monitoring, such as testing water from Lake Victoria or the Ruvuma River for contaminants like heavy metals (lead, copper) and harmful anions (nitrates, phosphates). For example, environmental scientists in Dar es Salaam use systematic qualitative analysis to detect if industrial effluents contain toxic metal ions before the water is used for irrigation or drinking. This helps protect public health and the environment.