13. Qualitative Analysis (Self Study)

Qualitative Analysis Introduction

• Lesson 13: A0001F Applied Science

• Centre for Foundational Studies (CFS)

• REPUBLIC POLYTECHNIC

• Theme: DISCOVER. TRANSFORM. ACHIEVE.

Self-Study Instructions (Page 2)

• This is a self-study lesson preparing for Lesson 15 (Qualitative analysis lab).

• Allocate 1 hour for the following activities:

  • Go through slides independently and attempt examples before checking answers.

  • Complete practice questions and clarify doubts with the lecturer during Lesson 15 in the lab.

• By the end of this lesson, understand that all substances are tested for cations and anions to identify dissolved substances.

Learning Outcomes (Page 3)

• Suggest appropriate apparatus for simple experiments, including gas collection and reaction rate measurement.

• Describe tests to identify ions and gases.

Recap: Understanding Salts (Page 4)

• Definition: A salt forms when hydrogen ions of an acid are replaced by a metal ion or ammonium ion (E.g., MgO + 2 HCl → MgCl2 + H2O).

• Types of Salts:

  • Soluble Salts: Dissolve to form an aqueous solution (e.g., NaCl forms colorless NaCl solution).

  • Insoluble Salts: Do not dissolve in water; known as precipitates (e.g., PbCl2).

  • General reaction: Base + Acid → Salt + Water.

Solubility of Salts (Page 5)

• Soluble Salts:

  • All nitrate compounds.

  • All ammonium compounds.

  • Group I metal compounds (e.g., sodium, potassium).

  • Most chlorides, bromides, iodides (exceptions: AgCl, PbCl2).

• Insoluble Salts:

  • Most carbonate compounds (exceptions: ammonium and Group I).

  • Most metal hydroxides (exceptions: ammonium and Group I).

Scenario Application (Page 6)

• At a crime scene, a colorless solution is found beside the victim.

• Actions to verify if it caused death:

  • Conduct tests for cations and anions from the solution and the victim's blood.

  • Compare results.

• Importance of water quality testing as regulated by EPH (Quality of Piped Drinking Water) Regulations 2008.

  • Essential for preventing lead poisoning.

Qualitative Analysis Overview (Page 7)

• Qualitative analysis: An experimental method to identify unknown chemical substances.

• Cation: Positive ion (e.g., Ag+ in AgCl; K+ in K2SO4).

• Anion: Negative ion (e.g., Cl- in AgCl; SO4^2- in K2SO4).

• Testing for unknown salt involves identifying both cation and anion.

Anions Testing Methods (Page 8)

• Anions include:

  • Carbonate (CO32-), Sulfate (SO42-), Chloride (Cl-), Bromide (Br-), Iodide (I-), Nitrate (NO3-).

• Two experiment types:

  1. Precipitate test: Add chemicals to form a precipitate; identify by color.

  2. Gas test: Add chemicals to produce gas; identify gas produced.

  • Note: Unknown substance must be in aqueous state for clear observation.

Carbonate Anion Testing (Page 9)

• All CO32- are generally insoluble; gas test is appropriate instead of a precipitate test.

• Unique precipitate tests for SO42-, Cl-, Br-, I- due to certain exceptions.

• All NO3- are soluble, hence gas tests are applicable.

Tests for Anions (Page 10)

• Testing Procedures:

  • Carbonate (CO32-): Add dilute acid (produces effervescence).

  • Sulfate (SO42-): Acidify with nitric acid, add barium nitrate/chloride (white precipitate = BaSO4).

  • Chloride (Cl-), Bromide (Br-), Iodide (I-): Use silver nitrate after acidifying with nitric acid.

  • Nitrate (NO3-): Add sodium hydroxide, aluminum foil, and warm gently.

• Acidification with nitric acid is crucial to prevent confusion from other precipitates.

Example: Confirming Carbonate Presence (Page 12)

• Suspected sodium carbonate:

  • Add dilute nitric acid and observe effervescence (CO2).

  • Gas turns limewater white (indicating CaCO3 production).

  • Conclusion: Presence of carbonate ions confirmed.

Distinguishing Between Hydrochloric and Sulfuric Acid (Example 2, Page 13)

• Test for chloride ions using silver nitrate (white precipitate indicates HCl).

• Test for sulfate ions using barium nitrate (white precipitate indicates H2SO4).

Cation Testing Procedures (Page 14)

• Cation testing requires aqueous forms; reactions with water or acid prepare solutions.

• Testing involves color and solubility of formed precipitates using:

  • Sodium hydroxide (NaOH): Precipitate color and solubility.

  • Ammonium hydroxide (NH4OH): Similar method as NaOH.

Examples of Cation Tests (Pages 15-16)

• Multiple cations tested using either NaOH or NH4OH. Results include:

  • Alkali metals (e.g., Sodium): No precipitate in either test.

  • Calcium: White precipitate in NaOH, none in NH4OH.

  • Zinc: Forms white precipitate with both.

  • Iron (Fe2+/Fe3+): Distinct precipitate colors noted in both tests.

Testing Strategies for Zinc and Lead Carbonates (Page 17-18)

• Direct cation tests on powders are not recommended; dissolve powder to proceed.

• Stepwise method involves:

  • Adding dilute nitric acid to powder to form solutions.

  • Testing with aqueous ammonia or sodium hydroxide for precipitate observations.

Gas Testing (Page 19-20)

• Conduct gas tests under conditions of effervescence, heating, or chemical addition.

• Gases classified by acidic/neutral/alkaline properties.

• Identified gases include:

  • Oxygen: Glowing splint relights.

  • Hydrogen: Burning splint pops.

  • Chlorine: Bleaching litmus paper.

  • Carbon Dioxide: Forms white precipitate with limewater.

Example of Identification (Page 21)

• Given reactions led to identifying solids A (AgNO3), B (AgCl), and gas C (NH3).

Example: Analysis of Substance X (Page 22-23)

• A mixture of compound A (soluble) and element B (insoluble).

• Detailed identification based on precipitation and solution tests leading to the conclusion that compound A is Calcium iodide and element B is Zinc.

Reminder for Next Lesson (Page 24)

• Dress code requirements for Lab Lesson 14: Salts Preparation.

• Mandatory attire includes lab coat, goggles, covered shoes, and long pants/skirts.

• Strict enforcement: Non-compliance results in absence marked.