Bromine

Bromine (Br2) is the middle Group VII halogen and the only non-metal that is liquid at room temperature. It is the element examiners use to test whether you understand the direction of halogen displacement, because it sits between chlorine and iodine.

Where bromine appears in 0620

• Group VII trends (The Periodic Table): bromine is the reference point for the reactivity order Cl2 > Br2 > I2.
• Displacement / redox: bromine is displaced by chlorine but itself displaces iodine, so it appears on both sides of displacement questions.
• Organic chemistry: the bromine water test for a C=C double bond.
• Analysis: the cream silver bromide precipitate in the halide test.

Position in the periodic table

• Symbol: Br
• Atomic number: 35
• Group 17 (VII), Period 4
• Electron configuration: 2, 8, 18, 7
• Forms Br- ions by gaining 1 electron
• Diatomic molecule: Br2

Physical properties

Property	Detail
State at room temperature	Liquid
Colour	Orange-brown (liquid and vapour)
Boiling point	59 C
Density	Dense liquid (3.1 g/cm3)
Smell	Pungent, unpleasant
Toxicity	Toxic and corrosive

Bromine’s liquid state reflects its position between chlorine (gas) and iodine (solid). Going down Group VII, melting and boiling points increase because the molecules are larger and the intermolecular (van der Waals) forces are stronger.

Displacement reactions

A more reactive halogen displaces a less reactive halide from solution. Bromine sits in the middle, so it wins one contest and loses the other.

Chlorine displaces bromine (chlorine is more reactive):

Cl2(aq) + 2KBr(aq) -> 2KCl(aq) + Br2(aq)

Observation: the colourless/pale solution turns orange-brown.

Bromine displaces iodine (bromine is more reactive than iodine):

Br2(aq) + 2KI(aq) -> 2KBr(aq) + I2(aq)

Observation: the orange-brown solution darkens to brown.

Bromine does not displace chlorine from a chloride solution — it is less reactive than chlorine.

Bromine water test for alkenes

Bromine water is the standard test to tell alkenes from alkanes:

• Alkene: bromine water is decolourised (orange-brown to colourless) as bromine adds across the C=C bond in an addition reaction
• Alkane: bromine water stays orange-brown (no reaction under these conditions)

Example: C2H4 + Br2 -> C2H4Br2 (1,2-dibromoethane)

Testing for bromide ions

Add dilute nitric acid, then silver nitrate solution:

Ag+(aq) + Br-(aq) -> AgBr(s)

A cream precipitate of silver bromide forms — the colour sits between white silver chloride and yellow silver iodide.

Comparison within Group VII

Property	Cl2	Br2	I2
State (room temp)	Gas	Liquid	Solid
Colour	Green-yellow	Orange-brown	Grey-black (purple vapour)
Reactivity	Most reactive	Moderate	Least reactive
AgX precipitate	White	Cream	Yellow

Key facts

• Symbol: Br — proton number: 35
• Position: Group VII, Period 4
• Electron configuration: 2, 8, 18, 7
• Key property: the only non-metal liquid at room temperature (orange-brown)
• Reactivity rank: middle halogen — displaces iodine, displaced by chlorine
• Main test: decolourises bromine water (C=C); gives a cream AgBr precipitate

Compounds

Compound	Formula	Notes
Hydrogen bromide	HBr	Gas, forms hydrobromic acid in water
Potassium bromide	KBr	White solid, soluble
Lead(II) bromide	PbBr2	White solid, electrolysed in the molten state

Common exam mistakes

• Writing that bromine water goes “clear”. It goes colourless — “clear” only means transparent, and orange-brown bromine water is already clear.
• Saying an alkane also decolourises bromine water. Alkanes have no C=C bond and give no quick colour change; only the alkene decolourises it.
• Getting the AgBr colour wrong. It is cream, not white (that is AgCl) and not yellow (that is AgI).
• Reversing the displacement direction. Bromine cannot displace chloride ions, because a less reactive halogen never displaces a more reactive one.

Worked exam questions

Chlorine water is added to colourless potassium bromide solution. Describe and explain the observation, and write an equation. (3 marks)

Mark scheme

• The solution turns from colourless to orange-brown [1]
• Chlorine is more reactive than bromine / higher in Group VII, so it displaces bromine [1]
• Cl2(aq) + 2KBr(aq) -> 2KCl(aq) + Br2(aq) [1]

Examiner note: name the new colour (orange-brown from bromine), not just “it changes colour”. The equation must be balanced and show the displaced element as Br2, not Br.

Describe how bromine water is used to distinguish between ethane and ethene. (3 marks)

Mark scheme

• Add bromine water to each and shake [1]
• With ethene the bromine water is decolourised / turns from orange-brown to colourless [1]
• With ethane the bromine water stays orange-brown / no change [1]

Examiner note: you must state what happens with both compounds. A common lost mark is describing only the positive (ethene) result.