Iodine

Iodine (I2) is the least reactive of the three Group VII halogens studied at IGCSE, so it is the halogen that is always displaced and never displaces. It is also the syllabus’s headline example of sublimation.

Where iodine appears in 0620

• Group VII trends: iodine is the bottom of the reactivity order, displaced by both chlorine and bromine.
• Changes of state: iodine sublimes — solid straight to purple vapour and back.
• Analysis: the yellow silver iodide precipitate identifies iodide ions.
• Food/biology tests: the iodine starch test (blue-black).

Position in the periodic table

• Symbol: I
• Atomic number: 53
• Group 17 (VII), Period 5
• Electron configuration: 2, 8, 18, 18, 7
• Forms I- ions by gaining 1 electron
• Diatomic molecule: I2

Physical properties

Property	Detail
State at room temperature	Solid
Colour	Grey-black crystals (shiny, almost metallic)
Vapour colour	Purple
Melting point	114 C
Solubility in water	Slightly soluble (dissolves better in KI solution)
Key behaviour	Sublimes when heated gently

Sublimation

Iodine is the classic IGCSE example of sublimation. When gently heated:

1. Grey-black crystals turn directly into purple vapour, with no liquid seen
2. On cooling, the vapour deposits back as solid crystals on a cold surface

This bypasses the liquid state entirely, and it can be used as a separation technique. The colour change makes it easy to see the change of state happen.

Reactivity

Iodine is the least reactive of the studied halogens: Cl2 > Br2 > I2. Its atoms have the most electron shells, so an incoming electron is furthest from the nucleus and most shielded. The attraction is weakest, so iodine gains an electron least easily.

Displacement reactions

Iodine is displaced from iodide solutions by both chlorine and bromine:

Cl2(aq) + 2KI(aq) -> 2KCl(aq) + I2(aq) (solution turns brown)

Br2(aq) + 2KI(aq) -> 2KBr(aq) + I2(aq) (solution turns brown)

Iodine does not displace chloride or bromide ions — it is less reactive than both.

Testing for iodide ions

Add dilute nitric acid, then silver nitrate solution:

Ag+(aq) + I-(aq) -> AgI(s)

A yellow precipitate of silver iodide forms.

Halide	Precipitate colour	Solubility in dilute ammonia
Cl-	White	Soluble
Br-	Cream	Slightly soluble
I-	Yellow	Insoluble

The starch test

Iodine solution (brown) turns blue-black in the presence of starch. It is used in food tests and as the endpoint indicator in iodine titrations.

Key facts

• Symbol: I — proton number: 53
• Position: Group VII, Period 5
• Electron configuration: 2, 8, 18, 18, 7 (forms I-)
• Key property: grey-black solid that sublimes to a purple vapour
• Reactivity rank: least reactive halogen studied — always displaced
• Signature tests: yellow AgI precipitate; blue-black with starch

Uses

Use	Detail
Antiseptic	Iodine solution kills bacteria
Starch indicator	Detecting starch in food and biology
Thyroid health	Iodine is essential for thyroid hormones

Common exam mistakes

• Saying iodine “melts” when heated. Gently heated iodine sublimes — solid to vapour with no liquid in between.
• Muddling the two colours. The solid is grey-black and shiny; the vapour is purple. State the right colour for the right state.
• Claiming iodine displaces bromide or chloride. It is the least reactive halogen studied, so it cannot displace either.
• Confusing the two iodine results. Blue-black is the starch test; the yellow precipitate is the iodide-ion test with silver nitrate.

Worked exam questions

Iodine crystals are gently heated in a beaker covered with a cold watch glass. Describe and name what is observed. (3 marks)

Mark scheme

• Purple vapour forms as the grey-black crystals are heated [1]
• No liquid is seen — the solid changes directly to gas (sublimation) [1]
• Shiny grey-black crystals re-form on the cold watch glass as the vapour cools [1]

Examiner note: the term “sublimation” (or “sublimes”) is expected. Describing melting or a liquid stage contradicts the observation and loses marks.

Describe a test to distinguish between potassium chloride and potassium iodide solutions. (3 marks)

Mark scheme

• Add dilute nitric acid, then silver nitrate solution, to each [1]
• Potassium chloride gives a white precipitate (AgCl) [1]
• Potassium iodide gives a yellow precipitate (AgI) [1]

Examiner note: both results are needed for full marks. Naming only the yellow iodide result, without the white chloride result, caps the answer.