Generate chlorine gas and investigate its reactions with water or halide ions in this class practical

In this experiment, students can use microscale apparatus to generate chlorine gas safely themselves in the open laboratory, investigating how it reacts with water and halide ions in solution.

The activity should take about 20 minutes.



  • Eye protection (goggles)
  • Student sheet with instructions and diagram of apparatus (available for download below)
  • Clear plastic sheet – eg OHP film
  • Plastic Petri dish and lid, 9 cm
  • Plastic teat pipette (see note 10 below)
  • Scissors
  • Spatula (optional)


  • Access to solutions contained in plastic pipettes:
    • Sodium chlorate(I) solution, 10–14% w/v chlorine (CORROSIVE) – also known as sodium hypochlorite (see note 11 below)
    • Dilute hydrochloric acid, 1 M
    • Sodium hydroxide solution, 1 M (CORROSIVE)
    • Potassium (or sodium) chloride solution, 0.2 M
    • Potassium (or sodium) bromide solution, 0.2 M
    • Potassium (or sodium) iodide solution, 0.2 M
  • Optional (see Teaching notes):
    • Silver nitrate solution, 0.1 M, a few drops
    • Zinc oxide powder (DANGEROUS FOR THE ENVIRONMENT), spatula tip
    • Zinc sulfide powder, spatula tip
    • Blue litmus or universal Indicator paper, about 1 cm

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection (goggles) throughout. Ensure laboratory is well ventilated.
  • Sodium chlorate(I) solution, Also known as sodium hypochlorite NaOCl(aq), (CORROSIVE) – see CLEAPSS Hazcard HC089 and CLEAPSS Recipe Book RB081. 
  • Chlorine, Cl2(g), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC022a.  Chlorine gas is produced in small quantities in this experiment, however, care should be taken if large numbers of students are carrying out this experiment simultaneously. The lab should be well ventilated and students with asthma should be warned not to inhale the gas. Chlorine should be generated for no longer than is necessary to observe the results.
  • Dilute hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.
  • Sodium hydroxide solution, NaOH(aq), (CORROSIVE) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085. 
  • Halide solutions: potassium chloride, KCl(aq), potassium bromide, KBr(aq), potassium iodide, KI(aq) – see CLEAPSS Hazcard HC047b.
  • Silver nitrate solution, AgNO3(aq) – see CLEAPSS Hazcard HC087 and CLEAPSS Recipe Book RB077. 
  • Zinc oxide, ZnO(s) (DANGEROUS FOR THE ENVIRONMENT) and zinc sulfide, ZnS(s) – see CLEAPSS Hazcard HC108b.  
  • The ‘reaction vessel’ for the microscale apparatus is the hemispherical dome cut from the top of a plastic teat pipette. Students may cut this themselves with suitable scissors, or it can be provided.
  • Commercial chlorine-based bleach solutions can be used instead of sodium chlorate(I) solution supplied by laboratory suppliers but they may not be sufficiently concentrated to generate enough chlorine. They are often less than 5% even when fresh. Some commercial bleaches now also contain detergents, which foam when chlorine is generated. They should not be used. Household ‘bleaches’ based on peroxide are becoming more widely available and do not contain chlorine, therefore they should not be used.


  1. Cover the worksheet containing the diagram of the microscale setup with the plastic sheet.
  2. Place the Petri dish directly over the circle on the worksheet.
  3. Use the microscale reaction vessel provided or make one by cutting the hemispherical top off the teat part of a plastic pipette. Place this in the centre of the Petri dish, as shown in the diagram.
  4. At the corners of the triangle on the worksheet place two to three drops of the test solutions indicated on the diagram. Moisten the small piece of indicator paper and place it in the space between any two of the test solutions, along the side of the triangle.
  5. Place two drops of bleach solution in the reaction vessel and add three drops of dilute hydrochloric acid. Quickly place the lid on the Petri dish to prevent any chlorine escaping.
  6. Record your observations over the next 10 minutes. The greenish-yellow colour of chlorine gas may be visible in the Petri dish, especially if viewed from the side. The indicator paper turns red and then becomes bleached. The sodium chloride solution is unaffected. The potassium bromide solution gradually turns pale yellow due to the formation of bromine. The potassium iodide solution turns yellow-brown due to the liberation of iodine by the chlorine.
  7. When the reactions have finished, add three drops of sodium hydroxide solution to the reaction vessel to stop the generation of chlorine and replace the lid.

Teaching notes

The effect of chlorine gas on the moist indicator paper shows that it dissolves to some extent in water and reacts to produce an acidic and strongly bleaching solution. The reaction is the reverse of the reaction used to generate the gas from bleach. Acidifying bleach solution produces chloric(I) acid, HOCl, which decomposes to produce chlorine:

HOCl(aq) + HCl(aq) → Cl2(g) + H2O(l)

When chlorine dissolves in water, it reacts to form the strong acid, HCl, and the weak but strongly oxidising acid, HOCl, which is responsible for the bleaching properties.

The displacement reactions involving chlorine and the solutions containing halide show that chlorine displaces bromine and iodine from solution:

Cl2(g) + 2KX(aq) → 2KCl(aq) + X2(aq), where X = Br or I.

or Cl2(g) + 2X(aq) → 2Cl(aq) + X2(aq)

This establishes the trend in reactivity of the halogens down Group 17, which could be extended by investigating the reaction of bromine water with halide ions in solution. The reactivity is related to the oxidising power of the halogens, which decreases down the group.

The tendency of halogen atoms to act as oxidising agents by accepting an electron can be related to their atomic radius. The smaller the halogen atom, the stronger the attraction of the nucleus on the electrons in the outer shell. Thus fluorine attracts an extra electron to complete its outer shell, most strongly, and is therefore the most powerful oxidising agent in the Group.

This microscale apparatus can be used in a similar way to investigate other chlorine reactions. The solutions at the three corners of the triangle can be replaced by silver nitrate solution, solid zinc sulfide and zinc oxide respectively. The silver nitrate solution becomes cloudy as the reaction of chlorine with water produces chloride ions in solution (see above), which then forms a silver chloride precipitate.

The zinc sulfide takes on a yellowish tinge due to the formation of elemental sulfur:

Cl2(g) + ZnS(s) → ZnCl2(s) + S(s)

Chlorine is reacting as an oxidising agent again. The zinc oxide shows no change although some oxygen gas is probably produced in a similar reaction.