Qualitative Analysis: The Paper 4 Identification Playbook

By Dr. Aisha Rahman, Ph.D. Chemistry·Updated April 18, 2026
Test tubes showing green, red-brown, and white precipitates from qualitative analysis tests.

What is the difference between testing for Zinc and Aluminum ions?

When you add a few drops of aqueous Sodium Hydroxide (NaOH), both Zinc (Zn2+) and Aluminum (Al3+) produce a white precipitate that dissolves in an excess of NaOH. To tell them apart, you MUST do the Ammonia test. Add aqueous Ammonia (NH3): the Zinc precipitate dissolves in excess, but the Aluminum precipitate is INSOLUBLE in excess.

Table of Contents

Paper 4 (Alternative to Practical) is notoriously fast-paced. You don't actually perform the experiments, but you are required to know the exact color changes, smells, and precipitates produced when reagents mix. CAIE provides a data sheet for this at the back of the exam, but this guide from our Ultimate O-Level Chemistry Guide teaches you the logic behind the tests so you don't waste time flipping pages.

1. Testing Cations (The Metal Precipitates)

To test for positive metal ions, we usually drop in two reagents: Aqueous Sodium Hydroxide (NaOH) and Aqueous Ammonia (NH3). We look for the color of the solid precipitate, and whether that solid dissolves when we add a massive excess of the reagent.

CationEffect of NaOHEffect of Aqueous NH3
Copper(II) (Cu2+)Light blue ppt. Insoluble in excess.Light blue ppt. Dissolves to form a dark blue solution.
Iron(II) (Fe2+)Green ppt. Insoluble in excess.Green ppt. Insoluble in excess.
Iron(III) (Fe3+)Red-brown ppt. Insoluble in excess.Red-brown ppt. Insoluble in excess.
Zinc (Zn2+)White ppt. Dissolves in excess (colorless).White ppt. Dissolves in excess (colorless).
Aluminum (Al3+)White ppt. Dissolves in excess (colorless).White ppt. Insoluble in excess.

2. Testing Anions (Halides, Sulfates, Carbonates)

Negative ions (anions) require specific reagants depending on the group.

The Halides (Cl-, Br-, I-)

Test: Add dilute Nitric Acid (HNO3) followed by Aqueous Silver Nitrate (AgNO3).
- Chloride (Cl-): Produces a White precipitate.
- Bromide (Br-): Produces a Cream precipitate.
- Iodide (I-): Produces a Yellow precipitate.

The Sulfate Ion (SO4 2-)

Test: Add dilute Nitric Acid followed by Aqueous Barium Nitrate (Ba(NO3)2).
Result: A White precipitate forms (Barium Sulfate).

3. Gas Identification and Litmus Paper

  • Hydrogen (H2): Use a lighted splint. Result: Burns with a "squeaky pop" sound.
  • Oxygen (O2): Use a glowing splint. Result: The splint relights.
  • Carbon Dioxide (CO2): Bubble through limewater. Result: Limewater turns milky/cloudy.
  • Ammonia (NH3): Use damp red litmus paper. Result: Turns blue (it's alkaline). Smells pungent.
  • Chlorine (Cl2): Use damp blue litmus paper. Result: Bleaches the paper white.
Dr. Aisha Rahman📋 From the Desk of Dr. Aisha Rahman
Whenever you use litmus paper to test for a gas (like Ammonia or Chlorine), you MUST state that the paper is DAMP. Dry litmus paper does not work because gases need water to form the aqueous ions that actually change the paper's color. "Use blue litmus paper" = 0 marks. "Use DAMP blue litmus paper" = 1 mark.

Frequently Asked Questions

How do you test for the presence of Zinc and Aluminum ions?▼
Both form white precipitates with NaOH that dissolve in excess. With Ammonia, Zinc's white precipitate dissolves in excess, but Aluminum's precipitate does not.
What is the test for a Carbonate?▼
Add an acid (like HCl). The resulting effervescence (CO2 gas) will turn limewater milky.
How do you distinguish between Iron(II) and Iron(III) ions?▼
Add NaOH. Fe2+ makes a green precipitate. Fe3+ makes a red-brown precipitate.
What is the chemical test for Chlorine gas?▼
It bleaches damp blue litmus paper white.

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