Halogens and Chlorine - O-Level Chemistry

Introduction to Halogens

Halogens are Group 7 elements in the periodic table. They are nonmetals with seven valence electrons, making them highly reactive. They exist as diatomic molecules and form salts when combined with metals.

Elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At)
Reactivity decreases down the group
Color and boiling point increase down the group

Physical Properties of Halogens

Element	State	Color	Boiling Point (°C)
Fluorine	Gas	Pale yellow	−188
Chlorine	Gas	Greenish-yellow	−34
Bromine	Liquid	Reddish-brown	59
Iodine	Solid	Dark purple	184

Properties of Chlorine

Greenish-yellow gas with pungent odor
Denser than air
Soluble in water
Strong oxidizing agent

Reaction with metals:

2Na (s) + Cl₂ (g) → 2NaCl (s)

Reaction with hydrogen:

H₂ (g) + Cl₂ (g) → 2HCl (g)

Preparation of Hydrogen Chloride Gas

Method: React sodium chloride with concentrated sulfuric acid.

NaCl (s) + H₂SO₄ (l) → NaHSO₄ (s) + HCl (g)

HCl gas is collected by downward delivery due to its density.

Preparation of Hydrochloric Acid

Method: Dissolve hydrogen chloride gas in water.

HCl (g) → HCl (aq)

Hydrochloric acid is a strong acid used in cleaning and digestion.

Test for Halide Ions

Halide ions are tested using silver nitrate solution after acidifying with dilute nitric acid.

Halide Ion	Observation	Equation
Chloride (Cl⁻)	White precipitate of AgCl	Cl⁻ (aq) + Ag⁺ (aq) → AgCl (s)
Bromide (Br⁻)	Cream precipitate of AgBr	Br⁻ (aq) + Ag⁺ (aq) → AgBr (s)
Iodide (I⁻)	Yellow precipitate of AgI	I⁻ (aq) + Ag⁺ (aq) → AgI (s)

Confirmatory Test:

AgCl dissolves in dilute ammonia
AgBr dissolves in concentrated ammonia
AgI does not dissolve in ammonia