Tests for Cations, Anions, and Functional Groups

Introduction to Qualitative Analysis

Qualitative analysis involves identifying the chemical constituents of a substance. In this section, we focus on tests for cations, anions, gases, and functional groups.

Tests for Cations

Cations are positively charged ions. Here are some common tests:

• Sodium (Na⁺): Flame test gives a bright yellow color.
• Potassium (K⁺): Flame test gives a lilac color.
• Calcium (Ca²⁺): Flame test gives a brick-red color.
• Copper (Cu²⁺): Flame test gives a blue-green color. Adding NaOH gives a blue precipitate.

Tests for Anions

Anions are negatively charged ions. Here are some confirmatory tests:

• Chloride (Cl^-): Add silver nitrate (AgNO₃), a white precipitate of silver chloride forms, which is soluble in ammonia.
• Sulfate (SO₄^2-): Add barium chloride (BaCl₂), a white precipitate of barium sulfate forms.
• Carbonate (CO₃^2-): Add dilute acid, effervescence occurs, and carbon dioxide gas is released.

Tests for Gases

Identifying gases is crucial in qualitative analysis:

• Hydrogen (H₂): A lighted splint makes a 'pop' sound.
• Oxygen (O₂): A glowing splint relights.
• Carbon Dioxide (CO₂): Turns limewater milky.

Functional Group Tests

Functional groups determine the properties of organic compounds. Here are some characteristic tests:

• Alcohols: Add acidified potassium dichromate. Primary and secondary alcohols will turn the solution from orange to green.
• Aldehydes: Add Tollens' reagent. A silver mirror forms on the test tube.
• Carboxylic Acids: Add sodium carbonate. Effervescence occurs, releasing carbon dioxide gas.