Chlorine and Its Inorganic Compounds
Senior 3 · Chemistry
Chlorine and Its Inorganic Compounds
Lesson 2 of 4
25 min
In S1 you followed river water through a treatment works such as Kigali's Nzove plant and saw that the last stage, disinfection, usually uses chlorine to kill germs. In November 2025 the Minister of Infrastructure told the Senate that water leaving Rwanda's treatment plants meets safety standards, but that the public is still advised to boil it, because it can be contaminated in old and leaking pipes on the way to homes. To make tap water safe to drink everywhere, he said, the network would need sensors that watch chlorine levels and automatic systems that add chlorine wherever the level drops. Why does chlorine matter so much, and why does its level fall along the pipes? The answers lie in its chemical properties.
Chlorine atoms need only one more electron to fill their outer shell, so chlorine is a very reactive element and a strong oxidising agent: it takes electrons from metals, from hydrogen and from less reactive halide ions.
1. With water. Chlorine reacts with water to form a mixture of hydrochloric acid and hypochlorous acid:
Hypochlorous acid is the germ-killer and the bleach: it oxidises coloured dyes to colourless compounds, and the change is permanent. In sunlight it slowly breaks down and gives off oxygen, so chlorine water must be kept in dark bottles: .
2. With metals, forming chlorides. Hot sodium burns in chlorine with a bright yellow flame, forming white clouds of sodium chloride; hot iron wool glows and forms brown iron(III) chloride:
3. With hydrogen. A jet of hydrogen burns in chlorine with a white flame, forming hydrogen chloride; a mixture of the two gases explodes in sunlight: .
4. With alkalis. With cold, dilute sodium hydroxide it forms sodium chlorate(I), better known as sodium hypochlorite, the active chemical in household bleach. With hot, concentrated alkali it forms sodium chlorate(V):
5. Displacing less reactive halogens. Chlorine turns colourless bromide solutions orange (bromine) and iodide solutions brown (iodine): .
6. Oxidising other compounds. It turns pale green iron(II) chloride solution yellow-brown, as iron(II) is oxidised to iron(III): . With hydrogen sulfide it deposits yellow sulfur: .
Definition
Residual chlorine: the free chlorine (chlorine, hypochlorous acid and hypochlorite ions) deliberately left in drinking water after treatment, so that it keeps killing germs in reservoirs and pipes. The World Health Organization recommends at least 0.2 mg per litre at the point of delivery. Example: water leaving a treatment works may carry about 1 mg per litre.
Chlorine is added to drinking water because hypochlorous acid kills bacteria and viruses that cause cholera, typhoid and diarrhoea. A little more is added than is needed at once, so that some residual chlorine remains. As the water travels, this chlorine is used up: it reacts with germs, with dirt, and with rust on the inside of pipes. If dirty water gets into a leaking pipe, the chlorine can be used up completely, and then nothing protects the water that reaches the tap (Figure 2.1). That is why engineers want sensors along the network, and why boiling is still advised where pipes are old.
Figure 2.1: How residual chlorine can fall between the treatment plant and a far tap (illustrative values).
Table 2.1: Uses of chlorine
| Use | Chemical used | Property |
|---|---|---|
| treating drinking water and swimming pools | chlorine, or sodium or calcium hypochlorite | kills germs (forms HOCl) |
| household bleach and disinfectant | sodium hypochlorite solution | oxidising agent: bleaches and kills germs |
| PVC plastic for water pipes and cable covers | chlorine-containing plastic | strong, waterproof, does not rot |
| making hydrochloric acid | hydrogen chloride | forms from hydrogen and chlorine |
| medicines, pesticides and solvents | chlorine compounds | reactive starting material |
Warning
Bleach safety. Household bleach is sodium hypochlorite solution. Never mix bleach with acid, such as toilet cleaners that contain hydrochloric acid: the mixture gives off chlorine gas at once. Never mix bleach with ammonia or cleaners that contain it: they form poisonous gases called chloramines. Use bleach diluted as the label says, in a well-ventilated place, keep it in its original bottle out of the reach of children, and never put it in a drink bottle. If it splashes into the eyes or onto skin, rinse with plenty of clean water for at least 15 minutes; if it is swallowed, do not make the person vomit, and get medical help at once.
The reaction that makes mixing bleach and acid dangerous:
Worked example
Problem. WASAC's Kanzenze treatment plant in Bugesera, opened in 2021, can produce of water a day. Suppose chlorine is added at a dose of 1.5 mg per litre (an illustrative figure). What mass of chlorine is needed each day? ()
Solution. Volume .
About 60 kg of chlorine a day: a small amount for so much water, because only a few milligrams per litre are needed.
Worked example
Problem. A cleaner pours bleach containing 7.45 g of sodium hypochlorite into a toilet bowl that already holds an acid cleaner, in excess. (a) What volume of chlorine could be released, at room temperature and pressure? (b) Evaluate the danger. (NaOCl = 74.5; molar volume )
Solution. (a) . From , 1 mol gives 1 mol : volume .
(b) 2.4 litres of chlorine is enough to make the air in a small, closed toilet room dangerous to breathe, causing coughing, chest pain and breathing difficulty. Bleach and acid cleaners must never be used together; open windows, leave the room and get into fresh air if it happens.
Tip
HOCl does the work. In water, chlorine forms hypochlorous acid, HOCl: it bleaches (permanently, by oxidation) and it disinfects. Bleach is NaOCl; add acid and you get back.
FuseSchool: What Are The Reactions Of Halogens (5 min). Watch for the reaction of chlorine with cold sodium hydroxide to make bleach, and for the displacement reactions between halogens.
Practice
Sources and credits
