Chemistry · Chemistry of the elements · Grade 8

Chlorine and hydrochloric acid

What disinfects swimming-pool water, and why does hydrogen chloride shoot up as a “fountain”?

The situation

In summer the water in a swimming pool has a distinctive “chlorine” smell, and tap water is chlorinated too. In our stomach there is hydrochloric acid, which helps digest food. Chlorine is a yellow-green poisonous gas, and hydrogen chloride dissolves in water so well that water shoots into a flask like a “fountain”. How is chlorine made, why does it kill germs, and how do we know that chloride ions are present in a solution?

Topic

Chlorine, hydrogen chloride and hydrochloric acid. Swimming-pool water and gastric juice: chlorine gas, the hydrogen chloride “fountain” and hydrochloric acid — a white precipitate with silver nitrate

Key ideas

Preparation of chlorine
HCl fountain
AgCl — a qualitative test

Formulas

MnO_{2} + 4 HCl \overset{}{\to} MnCl_{2} + Cl_{2} ↑ + 2 H_{2} O

Preparing chlorine in the laboratory (by heating). In industry — electrolysis of a sodium chloride solution: $2 NaCl + 2 H_{2} O \overset{}{\to} 2 NaOH + H_{2} ↑ + Cl_{2} ↑$ .

Cl_{2} + H_{2} O \overset{}{⇌} HCl + HClO

Chlorine water: hypochlorous acid HClO bleaches and kills germs.

NaCl + H_{2} SO_{4} \overset{}{\to} NaHSO_{4} + HCl ↑

Preparing hydrogen chloride in the laboratory (table salt + concentrated sulfuric acid).

H_{2} + Cl_{2} \overset{}{\to} 2 HCl

Industrial synthesis of hydrogen chloride; the gas is absorbed in water to give hydrochloric acid.

Ag^{+} + Cl^{-} \overset{}{\to} AgCl ↓

Qualitative test for the chloride ion: a white curdy precipitate, insoluble in nitric acid.

Theory

Chlorine: preparation and properties (§ 23)

Chlorine $Cl_{2}$ is a yellow-green, pungent, poisonous gas; it is ≈ 2.5 times heavier than air ( $M$ = 71 g/mol, air — 29; density at STP 3.2 g/L) and liquefies at −34 °C. It is slightly soluble in water: at 20 °C 1 volume of water dissolves ≈ 2.3 volumes (7.3 g/L) — chlorine water. In the laboratory it is made by heating concentrated hydrochloric acid with an oxidising agent: $MnO_{2} + 4 HCl \overset{}{\to} MnCl_{2} + Cl_{2} + 2 H_{2} O$ (with KMnO₄ it also goes without heating). The gas is cleaned of HCl in a wash bottle with water and of moisture in concentrated sulfuric acid, and is collected in a vessel with its mouth upwards — the heavy gas sinks to the bottom of the vessel and pushes the air upwards. In industry chlorine is made by electrolysis of a sodium chloride solution: $2 NaCl + 2 H_{2} O \overset{}{\to} 2 NaOH + H_{2} + Cl_{2}$ . Chlorine is an active oxidising agent: with metals it forms chlorides ( $2 Fe + 3 Cl_{2} \overset{}{\to} 2 FeCl_{3}$ ), and it combines with hydrogen explosively in light.

Water chlorination and bleaching

Chlorine reacts partially with water: $Cl_{2} + H_{2} O \overset{}{⇌} HCl + HClO$ . Hypochlorous acid HClO is a strong oxidising agent: it decolourises dyes and kills germs. That is why dry chlorine does not bleach dry cloth — moisture is needed. Drinking water and swimming-pool water are chlorinated; in drinking water 0.3–0.5 mg/L of free residual chlorine should remain after 30 minutes (more in the “Water purification and softening” experiment). Absorbing chlorine in alkali gives a bleaching solution: $Cl_{2} + 2 NaOH \overset{}{\to} NaCl + NaClO + H_{2} O$ . The toxicity of chlorine: on 22 April 1915 near Ypres it was used for the first time as a chemical weapon.

Hydrogen chloride (§ 24)

Hydrogen chloride HCl is a colourless gas with a pungent smell; it “fumes” in damp air (an acid mist) and is heavier than air ( $M$ = 36.5). In the laboratory it is made by the action of concentrated sulfuric acid on table salt: $NaCl + H_{2} SO_{4} \overset{}{\to} NaHSO_{4} + HCl$ (with strong heating — $2 NaCl + H_{2} SO_{4} \overset{}{\to} Na_{2} SO_{4} + 2 HCl$ ). HCl dissolves in water extremely well: at 0 °C 82 g in 100 g of water, i.e. ≈ 500 volumes of gas in 1 volume of water (≈ 440 volumes at 20 °C). The fountain experiment: if a few drops of water are introduced into a flask filled with HCl, the gas dissolves in them and the pressure in the flask falls sharply; the outside atmospheric pressure drives the water from the beaker up the tube into the flask as a fountain. Methyl orange turns red — the solution is acidic.

Hydrochloric acid (§ 27)

Hydrochloric acid is a solution of hydrogen chloride in water. The concentrated acid is ≈ 36–38 %, with a density of ≈ 1.18–1.19 g/cm³, and fumes in air. It is a strong acid and reacts with many substances: metals (those above hydrogen): $Zn + 2 HCl \overset{}{\to} ZnCl_{2} + H_{2}$ ; basic oxides: $CuO + 2 HCl \overset{}{\to} CuCl_{2} + H_{2} O$ (the solution is blue-green); bases: $NaOH + HCl \overset{}{\to} NaCl + H_{2} O$ ; salts of weak acids: $CaCO_{3} + 2 HCl \overset{}{\to} CaCl_{2} + H_{2} O + CO_{2}$ . It does not react with copper. In industry hydrogen chloride is made in a synthesis furnace ( $H_{2} + Cl_{2} \overset{}{\to} 2 HCl$ ) and absorbed in water. In the stomach: the cells of the stomach wall secrete hydrochloric acid at ≈ 0.16 mol/L (≈ 0.5 %), and the pH of gastric juice is 1–3; the acid activates the enzyme pepsin and protects against germs.

Lab work 3 and 4: qualitative reactions

Halide ions give coloured precipitates with silver nitrate: $Ag^{+} + Cl^{-} \overset{}{\to} AgCl$ is a white curdy precipitate, $AgBr$ is pale yellow, $AgI$ is yellow. They do not dissolve in nitric acid, whereas silver carbonate and phosphate do — so the solution is first acidified with HNO₃: other ions then cannot interfere. In ammonia solution AgCl dissolves, AgBr partly, AgI not at all: the solubility products are $K_{s}$ (AgCl) = 1.8·10⁻¹⁰, $K_{s}$ (AgBr) = 5.4·10⁻¹³, $K_{s}$ (AgI) = 8.5·10⁻¹⁷ (25 °C). Iodine gives a deep blue colour with starch. Chlorides in a soil solution: 10 g of soil is shaken with 50 mL of distilled water and filtered, the filtrate is acidified with HNO₃, and AgNO₃ is added: a faint white cloudiness means few chlorides, a thick curdy precipitate means the soil is saline. In Uzbekistan nearly half of the irrigated land is saline to some degree, which is why the salts are washed out. Unknown KCl, KBr and KI can also be told apart with AgNO₃ (practical 1).

Safety and the model

Safety: work with chlorine and hydrogen chloride only in a fume cupboard — they burn the airways; wear safety goggles and gloves with concentrated acids; before putting out the spirit lamp, take the delivery tube out of the vessel; silver nitrate stains skin black. Do not taste substances. Model: all the chlorine is assumed to be collected in the vessel (dissolving in the water of the wash bottle is ignored), MnO₂ reacts completely (in reality the reaction stops once the acid has become dilute), and the rates are schematic; gas volumes are given at STP; while HCl is being collected, the gas in the flask is assumed to be uniformly mixed; fountain: ideal gas, 20 °C, HCl dissolves completely, flow through the tube follows Torricelli's law; the chloride contents of the soils are illustrative.

Did you know?

Chlorine was first obtained in 1774 by the Swedish chemist Carl Scheele, but he did not recognise it as an element: even afterwards chemists long regarded it as an oxygen compound. In 1810 Humphry Davy showed that it is an element and named it after its colour, from the Greek “chloros” — “pale green”.
The PVC complex launched at Navoiyazot at the end of 2019 is designed to produce ≈ 100 thousand tonnes of polyvinyl chloride and ≈ 75 thousand tonnes of sodium hydroxide a year; sodium hydroxide is made by electrolysis of a sodium chloride solution — and chlorine forms in the process too.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Hydrogen — the lightest gas — Grade 7
Water: composition and properties — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Non-metals and halogens — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8
Phosphorus and mineral fertilisers — Grade 8
Carbon and its compounds — Grade 9