Qualitative Analysis in Chemistry

Welcome to this comprehensive guide on Qualitative Analysis for the CXC Chemistry syllabus 2024-2025. This lesson covers all the essential aspects of identifying ions and compounds through chemical tests.

Table of Contents

Introduction to Qualitative Analysis

Qualitative analysis is the branch of chemistry concerned with identifying the components of a substance through chemical reactions. In the CXC syllabus, this primarily involves:

Unlike quantitative analysis, which determines the amount of substance present, qualitative analysis focuses on establishing what is present, not how much.

Why is this important? Qualitative analysis forms the foundation of analytical chemistry and has numerous applications in:

Tests for Cations

Cations are positively charged ions that form when an atom loses one or more electrons. The main methods for identifying cations in the CXC syllabus include:

Flame Tests

Flame tests involve introducing a compound into a flame and observing the characteristic color produced by specific metal ions.

Procedure for Flame Tests:

  1. Clean a platinum or nichrome wire by dipping it in concentrated HCl, then holding it in a blue Bunsen flame until no color is observed.
  2. Dip the clean wire into the test solution or solid.
  3. Place the wire in the hottest part of the flame (the edge of the blue cone).
  4. Observe the color of the flame and compare with known standards.

Sodium (Na+)
Golden Yellow

Potassium (K+)
Lilac

Calcium (Ca2+)
Brick Red

Barium (Ba2+)
Apple Green

Copper (Cu2+)
Blue-Green

Cation Flame Color
Sodium (Na+) Golden Yellow
Potassium (K+) Lilac (may be masked by sodium)
Calcium (Ca2+) Brick Red
Barium (Ba2+) Apple Green
Copper (Cu2+) Blue-Green

Note: Sodium contamination is common and its yellow color can mask other colors. When testing for potassium, the flame should be viewed through a blue cobalt glass to filter out the sodium yellow.

Sodium Hydroxide Tests

Adding sodium hydroxide (NaOH) solution to a solution containing metal ions can help identify cations through characteristic precipitates and their behavior with excess NaOH.

Cation Observation with NaOH Reaction with excess NaOH
Aluminum (Al3+) White gelatinous precipitate Precipitate dissolves to form a colorless solution (amphoteric)
Calcium (Ca2+) White precipitate Precipitate does not dissolve
Copper(II) (Cu2+) Blue precipitate Precipitate does not dissolve
Iron(II) (Fe2+) Green precipitate Precipitate does not dissolve, gradually oxidizes to brown in air
Iron(III) (Fe3+) Rusty brown/reddish-brown precipitate Precipitate does not dissolve
Lead(II) (Pb2+) White precipitate Precipitate dissolves in excess (amphoteric)
Magnesium (Mg2+) White precipitate Precipitate does not dissolve
Zinc (Zn2+) White precipitate Precipitate dissolves in excess (amphoteric)
Cu²⁺(aq) + NaOH(aq) Cu(OH)₂(s) Blue precipitate excess NaOH Cu(OH)₂(s) No change

Ammonia Tests

Adding aqueous ammonia (NH₃) solution to a solution containing metal ions can also help identify cations through characteristic precipitates and their behavior with excess NH₃.

Cation Observation with NH₃ (dilute) Reaction with excess NH₃
Aluminum (Al3+) White gelatinous precipitate Precipitate does not dissolve
Calcium (Ca2+) No precipitate No change
Copper(II) (Cu2+) Blue precipitate Precipitate dissolves to form deep blue solution [Cu(NH₃)₄]²⁺
Iron(II) (Fe2+) Green precipitate Precipitate does not dissolve, gradually oxidizes to brown
Iron(III) (Fe3+) Rusty brown/reddish-brown precipitate Precipitate does not dissolve
Lead(II) (Pb2+) White precipitate Precipitate does not dissolve
Magnesium (Mg2+) White precipitate Precipitate does not dissolve
Zinc (Zn2+) White precipitate Precipitate dissolves to form a colorless solution [Zn(NH₃)₄]²⁺
Cu²⁺(aq) + NH₃(aq) Cu(OH)₂(s) Blue precipitate excess NH₃ [Cu(NH₃)₄]²⁺ Deep blue solution

Key concept: The difference in behavior with excess NaOH versus excess NH₃ can be used to distinguish between certain cations, especially for copper(II) and zinc ions.

Tests for Anions

Anions are negatively charged ions that form when an atom gains one or more electrons. The CXC syllabus focuses on tests for the following common anions:

Carbonate Test

Carbonates (CO₃²⁻) can be identified by adding dilute acid, which produces carbon dioxide gas.

Procedure for Carbonate Test:

  1. Add dilute hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) to the test compound.
  2. If bubbles form (effervescence), collect the gas.
  3. Pass the gas through limewater (calcium hydroxide solution).
  4. If the limewater turns milky/cloudy, carbon dioxide is confirmed.

Chemical equation: CO₃²⁻ + 2H⁺ → CO₂ + H₂O

Then: Ca(OH)₂ + CO₂ → CaCO₃ (white precipitate) + H₂O

CO₃²⁻ + HCl CO₂ gas limewater CaCO₃ Milky

Tests for Halides

Halide ions (Cl⁻, Br⁻, I⁻) can be identified using silver nitrate solution followed by ammonia.

Procedure for Halide Tests:

  1. Add dilute nitric acid (HNO₃) to the test solution (to remove interferences from carbonate ions).
  2. Add silver nitrate solution (AgNO₃).
  3. Observe the color and appearance of any precipitate formed.
  4. Add dilute ammonia solution, then if needed, concentrated ammonia, and observe changes.
Halide Observation with AgNO₃ Effect of dilute NH₃ Effect of concentrated NH₃
Chloride (Cl⁻) White precipitate Precipitate dissolves Already dissolved
Bromide (Br⁻) Cream/pale yellow precipitate Precipitate partially dissolves Precipitate dissolves completely
Iodide (I⁻) Yellow precipitate Precipitate does not dissolve Precipitate does not dissolve

Chemical equations:

Cl⁻ + Ag⁺ → AgCl (white)

Br⁻ + Ag⁺ → AgBr (cream)

I⁻ + Ag⁺ → AgI (yellow)

AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻ (dissolves in dilute ammonia)

AgBr + 2NH₃ → [Ag(NH₃)₂]⁺ + Br⁻ (dissolves in concentrated ammonia)

Sulfate Test

Sulfate ions (SO₄²⁻) can be identified using barium chloride solution.

Procedure for Sulfate Test:

  1. Add dilute hydrochloric acid (HCl) to the test solution.
  2. Add barium chloride solution (BaCl₂).
  3. A white precipitate of barium sulfate confirms the presence of sulfate ions.

Chemical equation: SO₄²⁻ + Ba²⁺ → BaSO₄ (white precipitate)

Nitrate Test

Nitrate ions (NO₃⁻) can be identified using the brown ring test.

Procedure for Nitrate Test (Brown Ring Test):

  1. Add freshly prepared iron(II) sulfate solution (FeSO₄) to the test solution.
  2. Carefully add concentrated sulfuric acid (H₂SO₄) down the side of the test tube to form a layer.
  3. A brown ring at the junction of the two solutions confirms the presence of nitrate ions.

Chemical principles: The concentrated H₂SO₄ reacts with nitrate ions to produce nitric oxide (NO). This reacts with Fe²⁺ to form the brown complex [Fe(H₂O)₅(NO)]²⁺.

FeSO₄ H₂SO₄ Brown Ring NO₃⁻

Tests for Gases

Identifying gases is an important part of qualitative analysis. Here are the key gases and their tests in the CXC syllabus:

Gas Test Positive Result
Ammonia (NH₃) Damp red litmus paper Turns blue
Hydrogen chloride gas White smoke (NH₄Cl) forms
Carbon dioxide (CO₂) Limewater (Ca(OH)₂ solution) Turns milky/cloudy (white precipitate of CaCO₃)
Chlorine (Cl₂) Damp litmus paper Bleaches/turns white
Starch-iodide paper Turns blue-black
Hydrogen (H₂) Burning splint "Pop" sound
Hydrogen chloride (HCl) Ammonia gas White smoke (NH₄Cl) forms
Hydrogen sulfide (H₂S) Lead(II) acetate paper Turns black (PbS forms)
Smell Rotten egg odor (use with caution!)
Oxygen (O₂) Glowing splint Relights the splint
Sulfur dioxide (SO₂) Acidified potassium dichromate(VI) paper Orange to green color change
Acidified potassium permanganate solution Purple to colorless change

Safety Note: Never directly smell any gas in the laboratory. If instructed to identify a gas by smell, gently waft a small amount toward your nose using your hand.

Systematic Qualitative Analysis

Systematic qualitative analysis involves following a structured approach to identify both cations and anions in an unknown substance.

General Approach for Unknown Substances:

  1. Note physical properties (color, state, solubility).
  2. Test for gases evolved with acids (if relevant).
  3. Prepare a solution of the substance (if not already in solution).
  4. Test for cations using flame tests, NaOH, and NH₃ tests.
  5. Test for anions using the appropriate tests.
  6. Confirm findings with additional specific tests if needed.

Decision Tree for Qualitative Analysis:

When identifying an unknown compound, follow this general pattern:

  1. First determine if the substance is soluble in water.
  2. If soluble, prepare a solution for testing cations and anions.
  3. If insoluble, attempt to dissolve in dilute acid and proceed.
  4. Perform flame tests to identify possible metal ions.
  5. Confirm with sodium hydroxide and ammonia tests.
  6. Test for anions using appropriate reagents.
  7. Verify results by cross-checking reactions.

Self-Assessment Questions

Section A: Multiple Choice

  1. Which cation gives a blue precipitate with NaOH that does not dissolve in excess?
    1. Fe²⁺
    2. Cu²⁺
    3. Al³⁺
    4. Zn²⁺

    2. Answer: b) Cu²⁺

    Copper(II) ions form a blue precipitate with sodium hydroxide that does not dissolve in excess NaOH.

  2. What color flame is produced when a compound containing calcium ions is heated in a Bunsen flame?
    1. Yellow
    2. Lilac
    3. Brick red
    4. Green

    3. Answer: c) Brick red

    Calcium compounds produce a brick-red flame color when heated in a Bunsen flame.

  3. Which of the following anions produces a yellow precipitate with silver nitrate that does not dissolve in either dilute or concentrated ammonia?
    1. Cl⁻
    2. Br⁻
    3. I⁻
    4. SO₄²⁻

    4. Answer: c) I⁻

    Iodide ions form a yellow precipitate (AgI) with silver nitrate that is insoluble in both dilute and concentrated ammonia.

  4. Which gas turns limewater milky?
    1. Oxygen
    2. Hydrogen
    3. Carbon dioxide
    4. Ammonia

    5. Answer: c) Carbon dioxide

    Carbon dioxide reacts with limewater (calcium hydroxide solution) to form a white precipitate of calcium carbonate, giving a milky appearance.

  5. Which cation gives a white precipitate with both NaOH and NH₃ that dissolves in excess NaOH but not in excess NH₃?
    1. Al³⁺
    2. Zn²⁺
    3. Pb²⁺
    4. Ca²⁺

    6. Answer: a) Al³⁺

    Aluminum ions form a white precipitate with both reagents but only dissolve in excess sodium hydroxide due to the formation of aluminate ions [Al(OH)₄]⁻.

Section B: Short Answer Questions

  1. Describe the test and expected results to distinguish between solutions containing bromide and iodide ions.

    2. Add dilute nitric acid followed by silver nitrate solution to separate samples of each solution:

    Alternatively, add chlorine water followed by a non-polar solvent like hexane:

  2. A student performs the Brown Ring Test and observes a brown ring. What ion does this confirm, and what are the chemical principles behind this test?

    3. The brown ring confirms the presence of nitrate ions (NO₃⁻).

    Chemical principles:

    Equation: NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O

    Then: Fe²⁺ + NO → [Fe(NO)]²⁺ (brown complex)

  3. Explain how you would distinguish between solutions containing aluminum ions and zinc ions using only sodium hydroxide and ammonia solutions.

    4. Using sodium hydroxide (NaOH):

    Using ammonia solution (NH₃):

    Therefore, the key distinguishing test is adding excess ammonia solution. If the white precipitate dissolves, the solution contains zinc ions. If it remains, the solution contains aluminum ions.

Section C: Structured Questions

  1. A student is given a white crystalline solid X for analysis. The following observations were made:

    Identify the cation and anion present in X and write the formula of compound X.

    2. Analysis:

    Answers:

    Thermal decomposition equation: CaCO₃ → CaO + CO₂

    Then: CaO + H₂O → Ca(OH)₂ (compound Y dissolved in water)

  2. A student was given four unlabeled test tubes containing solutions of the following salts: sodium chloride, sodium bromide, sodium iodide, and sodium sulfate. Describe how the student could identify each solution using only silver nitrate solution, dilute nitric acid, dilute ammonia solution, concentrated ammonia solution, and barium chloride solution.

    3. Systematic approach:

    1. First test: Add dilute nitric acid followed by barium chloride to a small portion of each solution
      • Sodium sulfate will form a white precipitate (BaSO₄) that is insoluble in excess acid
      • Halide solutions (NaCl, NaBr, NaI) will show no reaction
    2. Second test: For the remaining three solutions, add dilute nitric acid followed by silver nitrate
      • Sodium chloride: Forms a white precipitate (AgCl)
      • Sodium bromide: Forms a cream/pale yellow precipitate (AgBr)
      • Sodium iodide: Forms a yellow precipitate (AgI)
    3. Third test: Add dilute ammonia to the silver halide precipitates
      • AgCl precipitate will dissolve completely
      • AgBr precipitate will partially dissolve
      • AgI precipitate will not dissolve
    4. Fourth test (if needed): Add concentrated ammonia to any remaining precipitates
      • AgBr precipitate will dissolve completely
      • AgI precipitate will remain insoluble

    This systematic approach will uniquely identify all four solutions.

Glossary

Qualitative Analysis
The branch of chemistry concerned with identifying the components of a substance through chemical reactions.
Cation
A positively charged ion formed when an atom loses one or more electrons during chemical reactions.
Anion
A negatively charged ion formed when an atom gains one or more electrons during chemical reactions.
Precipitate
An insoluble solid that forms in a solution during a chemical reaction.
Amphoteric
Having both acidic and basic properties; can react as either an acid or a base. In qualitative analysis, refers to hydroxides that dissolve in excess sodium hydroxide.
Flame Test
A procedure used to detect the presence of certain metal ions based on the characteristic color they impart to a flame when heated.
Effervescence
The bubbling of a solution due to the escape of gas.
Limewater
A solution of calcium hydroxide, Ca(OH)₂, used to test for the presence of carbon dioxide.
Brown Ring Test
A test used to detect the presence of nitrate ions, which produces a brown ring at the junction of two liquids.
Complex Ion
An ion containing a central metal atom or ion bonded to one or more molecules or ions, such as [Cu(NH₃)₄]²⁺.
Litmus Paper
Paper impregnated with litmus dye, used as an indicator to test for acidity or alkalinity.
Colorimetry
The measurement of color intensity to determine the concentration of a substance in solution.
Confirmatory Test
A specific test that provides definitive evidence for the presence of a particular ion or substance.
Preliminary Test
An initial test conducted to narrow down the possibilities before proceeding to more specific confirmatory tests.

Exam Tips for Qualitative Analysis: