(I once asked a class, "What keeps your drinking water safe?" They shrugged. Then I told them: a green gas called chlorine. It is added to water at very low concentrations to kill bacteria that would otherwise cause diseases like cholera and typhoid. It is one of the greatest public health advances in human history. And yet, the same gas was used as a weapon in the First World War. Same element. Different dose. That is the story of chlorine — powerful, useful, and dangerous all at once. Let us understand it properly.) Turn on your tap and fill a glass of water. That water is probably safe to drink because of chlorine. Look at the bottle of bleach under your sink — that is a chlorine compound. The PVC pipes that carry water into your home are made using chlorine. The antiseptic you use to clean a cut is often a chlorine compound. Chlorine is everywhere — in our water, our medicines, our plastics, and our cleaning products. It is a greenish-yellow gas with a sharp, choking smell, and it is one of the most reactive non-metals in the periodic table. In this chapter, we are going to learn about chlorine — how it is prepared, what its properties are, how it reacts, and how it is used. We will also look at its compounds: hydrogen chloride, hydrochloric acid, and the chlorides. By the end, you will understand why this green gas is both a lifesaver and a hazard — and why chemists must use it with great care. Let us begin.
Chlorine is too reactive to exist as a free element in nature. It is always found combined with other elements, most commonly as:
Seawater is the largest source of chlorine, containing about 2% chloride ions by mass.
Chlorine is prepared in the laboratory by oxidising concentrated hydrochloric acid with an oxidising agent such as manganese(IV) oxide (MnO₂).
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
Steps:
Safety: Chlorine is toxic. The preparation must be done in a fume cupboard or well-ventilated area. Avoid breathing the gas.
(Chlorine is one of those gases that you must respect. It is heavier than air, so it sinks and collects in low places. Its sharp, choking smell is a warning — but by the time you smell it strongly, you have already been exposed. Always use a fume cupboard.)
| Property | Description |
|---|---|
| Appearance | Greenish-yellow gas |
| Smell | Sharp, choking, pungent |
| Density | Denser than air (about 2.5 times) |
| Solubility | Moderately soluble in water (about 2 volumes in 1 volume of water) |
| Boiling point | -34°C |
| Melting point | -101°C |
| Toxicity | Highly toxic — irritates the respiratory system |
Chlorine is a very reactive non-metal. It reacts with:
Chlorine reacts with metals to form metal chlorides.
2Na + Cl₂ → 2NaCl
2Fe + 3Cl₂ → 2FeCl₃
Cu + Cl₂ → CuCl₂
Chlorine reacts with hydrogen to form hydrogen chloride, and with other non-metals to form chlorides.
H₂ + Cl₂ → 2HCl (in sunlight, explosive)
2P + 3Cl₂ → 2PCl₃
2S + Cl₂ → S₂Cl₂
Chlorine dissolves in water to form chlorine water, which contains hydrochloric acid and hypochlorous acid.
Cl₂ + H₂O → HCl + HOCl
Hypochlorous acid (HOCl) is a strong oxidising agent and is responsible for the bleaching and disinfecting properties of chlorine.
Chlorine reacts with cold dilute alkalis to form chlorides and hypochlorites, and with hot concentrated alkalis to form chlorides and chlorates.
Cl₂ + 2NaOH → NaCl + NaClO + H₂O (cold dilute)
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (hot concentrated)
Chlorine bleaches by oxidation. When chlorine dissolves in water, it forms hypochlorous acid (HOCl), which is a strong oxidising agent. This oxidises coloured substances, turning them colourless.
Difference from sulphur dioxide:
| Feature | Chlorine | Sulphur Dioxide |
|---|---|---|
| Mechanism | Bleaches by oxidation | Bleaches by reduction |
| Effect on litmus | Bleaches litmus permanently (turns it white) | Reduces litmus to a colourless compound, but colour returns on exposure to air |
| Type of bleaching | Permanent | Temporary |
| Use | Bleaching cotton, linen, paper, and water treatment | Bleaching wool, silk, and paper |
(Here is the key difference: chlorine bleaches by adding oxygen to the coloured substance, while sulphur dioxide bleaches by removing oxygen. One is oxidation, the other is reduction. Remember that, and you will never mix them up.)
Hydrogen chloride is a colourless gas with a sharp, pungent smell. It is prepared in the laboratory by reacting a chloride with concentrated sulphuric acid.
NaCl + H₂SO₄ → NaHSO₄ + HCl
The gas is collected by downward delivery (it is denser than air) or over mercury (it is very soluble in water).
| Property | Description |
|---|---|
| Appearance | Colourless gas |
| Smell | Sharp, pungent |
| Density | Denser than air |
| Solubility in water | Extremely soluble (about 450 volumes in 1 volume of water) |
| Reaction with water | Dissolves to form hydrochloric acid |
| Reaction with ammonia | Forms white fumes of ammonium chloride (test for HCl) |
| Effect on litmus | Wet blue litmus turns red (acidic in water) |
Hydrochloric acid is a solution of hydrogen chloride gas in water. It is a strong acid — it completely dissociates in water to form H⁺ and Cl⁻ ions.
HCl(aq) → H⁺(aq) + Cl⁻(aq)
| Property | Description |
|---|---|
| Appearance | Colourless liquid |
| Smell | Sharp, pungent |
| Acidity | Strong acid — completely dissociates |
| Reaction with metals | Produces hydrogen gas and a metal chloride |
| Reaction with bases | Neutralisation to form a salt and water |
| Reaction with carbonates | Produces a salt, water, and carbon dioxide |
| Reaction with silver nitrate | White precipitate of silver chloride (test for chloride ions) |
To test for a chloride:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
The precipitate is soluble in ammonia solution.
| Use | Explanation |
|---|---|
| Water treatment | Chlorine kills bacteria and viruses in drinking water and swimming pools. This is the most important use of chlorine. |
| Bleaching | Used to bleach cotton, linen, and paper. |
| Disinfectants | Used in household bleach and cleaning products. |
| PVC production | Chlorine is used to make polyvinyl chloride (PVC), a versatile plastic. |
| Solvents | Chlorinated solvents are used in dry cleaning and industry. |
| Medicines | Chlorine is used in the manufacture of many pharmaceuticals. |
| Pesticides | Some chlorine compounds are used as pesticides. |
(Chlorine is one of the greatest public health advances in history. Before chlorination, waterborne diseases like cholera and typhoid killed millions. Today, chlorinated water is one of the reasons we live longer, healthier lives. But chlorine is also toxic, and must be used carefully. It is a powerful servant, but a dangerous master.)
While chlorine is essential for water treatment, its use has environmental consequences:
Solutions: Replacing CFCs with safer alternatives, treating waste before disposal, and regulating the use of chlorinated compounds.
Question: Describe the laboratory preparation of chlorine from manganese(IV) oxide and concentrated hydrochloric acid.
Solution: Place manganese(IV) oxide in a round-bottomed flask. Add concentrated hydrochloric acid slowly through a dropping funnel. Gently warm the flask. Chlorine gas is released. Pass the gas through water to remove HCl vapour, then through concentrated sulphuric acid to dry it. Collect the dry chlorine by downward delivery. Equation: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂.
Question: Explain why chlorine bleaches by oxidation, while sulphur dioxide bleaches by reduction.
Solution: Chlorine dissolves in water to form hypochlorous acid (HOCl), which is a strong oxidising agent. It oxidises coloured substances, turning them colourless. Sulphur dioxide bleaches by reducing coloured substances — it removes oxygen from them, turning them colourless. When the reduced substance is exposed to air, it may be re-oxidised and the colour may return, so SO₂ bleaching is temporary. Chlorine bleaching is permanent because the coloured substance is oxidised.
Question: Describe a test to confirm that a gas is hydrogen chloride.
Solution: Hold a glass rod dipped in concentrated ammonia solution near the gas. If white fumes of ammonium chloride form, the gas is hydrogen chloride. This is the classic test.
Question: Write the equation for the reaction between chlorine and cold dilute sodium hydroxide solution.
Solution: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. The products are sodium chloride, sodium hypochlorite (bleach), and water. This is how household bleach is made.
1. Describe the laboratory preparation of chlorine.
Chlorine is prepared by oxidising concentrated hydrochloric acid with manganese(IV) oxide: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂. The gas is dried by passing through concentrated sulphuric acid and collected by downward delivery.
2. State the colour and smell of chlorine gas.
Chlorine is a greenish-yellow gas with a sharp, choking smell.
3. Explain how chlorine bleaches.
Chlorine dissolves in water to form hypochlorous acid (HOCl), which is a strong oxidising agent. It oxidises coloured substances, turning them colourless.
4. Describe the test for hydrogen chloride gas.
Hold a glass rod dipped in concentrated ammonia solution near the gas. White fumes of ammonium chloride form if hydrogen chloride is present.
5. State two uses of chlorine.
Any two of: water treatment, bleaching, disinfectants, PVC production, medicines, pesticides.
6. Describe the test for chloride ions in solution.
Add dilute nitric acid, then silver nitrate solution. A white precipitate of silver chloride confirms the presence of chloride ions.
7. What is the difference between the bleaching action of chlorine and sulphur dioxide?
Chlorine bleaches by oxidation (permanent); sulphur dioxide bleaches by reduction (temporary — colour may return on exposure to air).
Question 1 [10 marks]
(a) Describe the laboratory preparation of chlorine, including the equation. [4]
(b) State two physical properties of chlorine. [2]
(c) Describe how chlorine reacts with cold dilute sodium hydroxide solution. Write the equation. [4]
(a) Chlorine is prepared by oxidising concentrated hydrochloric acid with manganese(IV) oxide. Equation: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂. The gas is dried by passing through concentrated sulphuric acid and collected by downward delivery. (4 marks: 1 for reactants, 1 for equation, 1 for drying, 1 for collection.)
(b) Any two of: greenish-yellow gas, sharp choking smell, denser than air, moderately soluble in water. (2 marks: 1 for each.)
(c) Chlorine reacts with cold dilute sodium hydroxide to form sodium chloride, sodium hypochlorite, and water: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. This is the basis of household bleach. (4 marks: 2 for products, 2 for equation.)
Question 2 [10 marks]
(a) Describe the preparation and properties of hydrogen chloride gas. [4]
(b) Describe the test for hydrogen chloride gas. [2]
(c) Explain why hydrogen chloride gas is very soluble in water, and what happens when it dissolves. [4]
(a) Hydrogen chloride is prepared by reacting a chloride (e.g., sodium chloride) with concentrated sulphuric acid: NaCl + H₂SO₄ → NaHSO₄ + HCl. It is a colourless gas with a sharp, pungent smell, denser than air. (4 marks: 2 for preparation and equation, 2 for properties.)
(b) Hold a glass rod dipped in concentrated ammonia solution near the gas. White fumes of ammonium chloride form. (2 marks: 1 for method, 1 for result.)
(c) Hydrogen chloride is very soluble in water because the polar HCl molecule forms strong hydrogen bonds with water molecules. When it dissolves, it ionises completely to form H⁺ and Cl⁻ ions, producing hydrochloric acid. (4 marks: 2 for solubility explanation, 2 for ionisation.)
Question 3 [10 marks]
(a) Describe how chlorine bleaches, and explain how this differs from the bleaching action of sulphur dioxide. [5]
(b) State three uses of chlorine. [3]
(c) Explain why chlorine is added to drinking water. [2]
(a) Chlorine bleaches by oxidation. It dissolves in water to form hypochlorous acid (HOCl), a strong oxidising agent that oxidises coloured substances, turning them colourless. This bleaching is permanent. Sulphur dioxide bleaches by reduction — it removes oxygen from coloured substances, turning them colourless. This bleaching is temporary because the reduced substance may be re-oxidised on exposure to air, and the colour may return. (5 marks: 2 for chlorine mechanism, 2 for SO₂ mechanism, 1 for comparison.)
(b) Any three of: water treatment, bleaching, disinfectants, PVC production, medicines, pesticides. (3 marks: 1 for each.)
(c) Chlorine is added to drinking water to kill bacteria and viruses that cause waterborne diseases like cholera and typhoid. (2 marks: 1 for "kills bacteria", 1 for "prevents disease".)
Challenge 1: Explain why chlorine is both beneficial and dangerous, using examples from the chapter.
Chlorine is beneficial because it is used to treat drinking water, killing bacteria and viruses that cause diseases like cholera and typhoid. It is also used to make PVC, bleach, disinfectants, and many medicines. Without chlorine, modern water treatment and many medical advances would not be possible. However, chlorine is also dangerous. It is a toxic gas that irritates the respiratory system and can be fatal in high concentrations. It was used as a chemical weapon in the First World War. Chlorinated organic compounds such as CFCs damage the ozone layer, and some, like dioxins, are highly toxic and persistent in the environment. So chlorine is a powerful tool that must be used with care and respect — beneficial when used wisely, dangerous when misused.
Challenge 2: Explain why hydrogen chloride gas cannot be collected over water, but chlorine gas can be collected by downward delivery.
Hydrogen chloride gas is extremely soluble in water — about 450 volumes of the gas dissolve in 1 volume of water. If it were collected over water, it would dissolve completely and be lost. That is why it must be collected by downward delivery (since it is denser than air) or over mercury (which does not dissolve it). Chlorine, on the other hand, is only moderately soluble in water (about 2 volumes in 1 volume) and is denser than air. So it can be collected by downward delivery — the gas sinks and displaces air from the collection vessel. Both gases are collected by downward delivery, but for different reasons: HCl because it is too soluble for water collection, and Cl₂ because it is denser than air and reacts with water.
Challenge 3: Describe how you would distinguish between three unlabelled gas jars containing chlorine, hydrogen chloride, and sulphur dioxide.
First, observe the colour. Chlorine is greenish-yellow; the other two are colourless. So the green gas is chlorine. To distinguish between hydrogen chloride and sulphur dioxide: hold a glass rod dipped in concentrated ammonia solution near each gas. Hydrogen chloride will form white fumes of ammonium chloride; sulphur dioxide will not. Alternatively, test with acidified potassium dichromate(VI) paper: sulphur dioxide turns it from orange to green, while hydrogen chloride does not. Another test: bubble each gas through limewater. Sulphur dioxide turns limewater milky; hydrogen chloride does not (it just acidifies it). Using these simple tests, all three gases can be identified.
"Chlorine — the green gas — is both a lifesaver and a hazard. Now you know how to tell the difference — and why chemistry demands respect."
