Chlorine

Chlorine (Cl2) is the most reactive halogen you meet in detail at IGCSE, and it is the halogen that wins every displacement contest in the course. It links Group VII trends, redox, water treatment and the electrolysis of brine.

Where chlorine appears in 0620

• Group VII trends: chlorine is the most reactive of Cl2/Br2/I2 and the top of the displacement order.
• Redox and displacement: chlorine is reduced while the halide is oxidised.
• Water treatment: chlorination kills microbes to make water potable.
• Electrolysis of brine: chlorine is the anode product.
• Analysis: chlorine gas bleaches damp litmus; chloride ions give a white silver chloride precipitate.

Position in the periodic table

• Symbol: Cl
• Atomic number: 17
• Group 17 (VII), Period 3
• Electron configuration: 2, 8, 7
• Forms Cl- ions by gaining 1 electron
• Diatomic molecule: Cl2, with one covalent bond

Physical properties

Property	Detail
State at room temperature	Gas
Colour	Green-yellow
Smell	Pungent, choking
Density	Denser than air (Mr = 71)
Solubility	Slightly soluble in water
Toxicity	Toxic

Reactivity and the halogen trend

Reactivity decreases down Group VII: F2 > Cl2 > Br2 > I2. Going down, atoms have more electron shells and are larger, so an incoming electron is further from the nucleus and more shielded. The attraction is weaker, making it harder to gain an electron — so the halogen is less reactive.

Displacement reactions

A more reactive halogen displaces a less reactive halide. Chlorine displaces both bromide and iodide:

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

Observation: colourless solution turns orange-brown.

Cl2(aq) + 2KI(aq) -> 2KCl(aq) + I2(aq)

Observation: colourless solution turns brown.

These are redox reactions: chlorine is reduced (Cl2 + 2e- -> 2Cl-) and the halide is oxidised (e.g. 2Br- -> Br2 + 2e-).

Water treatment

Chlorine is added to water supplies to kill bacteria, making potable water safe. See water and water treatment.

• Advantage: an effective disinfectant even at very low concentrations, with residual protection along the pipes.
• Disadvantage: chlorine is a toxic gas that must be handled carefully; some people object to the taste.

Electrolysis of brine

Chlorine is made industrially by electrolysing concentrated sodium chloride solution (brine):

Anode: 2Cl-(aq) -> Cl2(g) + 2e-

The other products are hydrogen (at the cathode) and sodium hydroxide (in solution).

Two tests to keep separate

• Test for chlorine gas: it bleaches damp litmus paper (blue or red litmus turns white). The bleaching is the key observation.
• Test for chloride ions: add dilute nitric acid then silver nitrate. A white precipitate forms: Ag+(aq) + Cl-(aq) -> AgCl(s), which is soluble in dilute ammonia.

Key facts

• Symbol: Cl — proton number: 17
• Position: Group VII, Period 3
• Electron configuration: 2, 8, 7 (forms Cl-)
• Key property: green-yellow toxic gas; most reactive of the studied halogens
• Gas test: bleaches damp litmus paper
• Main uses: water treatment, bleach (NaClO), making hydrochloric acid

Compounds

Compound	Formula	Notes
Hydrogen chloride	HCl(g)	Steamy fumes in moist air
Hydrochloric acid	HCl(aq)	Strong acid
Sodium chloride	NaCl	Common salt
Sodium hypochlorite	NaClO	Bleach

Common exam mistakes

• Saying chlorine “turns litmus red”. Damp litmus first reddens then is bleached white — the bleaching is what identifies chlorine.
• Confusing the two chlorine tests. Bleaching damp litmus tests for chlorine gas; the white silver chloride precipitate tests for chloride ions.
• Reversing displacement. A less reactive halogen never displaces a more reactive one, so bromine cannot displace chloride.
• Forgetting the acid in the chloride test. Silver nitrate must be acidified with nitric acid, or carbonates/hydroxides give a false precipitate.

Worked exam questions

Chlorine water is added to potassium iodide solution. State the observation, write an equation, and explain why the reaction happens. (3 marks)

Mark scheme

• The colourless solution turns brown [1]
• Cl2 + 2KI -> 2KCl + I2 [1]
• Chlorine is more reactive than iodine, so it displaces iodine [1]

Examiner note: the brown colour is the iodine released. Balance the equation and make sure the displaced element is I2, not I.

Explain why chlorine is more reactive than bromine. (3 marks)

Mark scheme

• Chlorine has fewer electron shells / a smaller atom than bromine [1]
• Its outer shell is closer to the nucleus / there is less shielding [1]
• So there is a stronger attraction for an incoming electron and chlorine gains an electron more easily [1]

Examiner note: halogen reactivity is about gaining an electron. Answers written in terms of losing electrons describe metals and score nothing.