Identification of ions and gases

• Test for carbonate ions

  1. Add dilute hydrochloric acid to test sample

  2. Bubble gas produced through lime water

  3. If lime water turns cloudy, it indicates presence of carbonate ions
• Test for sulfate ions

  1. Add dilute hydrochloric acid to remove carbonate and sulfite ions

  2. Add barium chloride solution

  3. White precipitate indicates presence of sulfate ions
• Test for halide ions (chloride, bromide, iodide)


  1. Add dilute nitric acid to remove carbonate and sulfite ions

  2. Add silver nitrate solution

  3. White precipitate = chloride, cream precipitate = bromide, yellow precipitate = iodide
• The tests rely on the reactions of the anions (negatively charged ions) with other reagents to produce observable results
• The tests are used to check if a solution contains carbonate, sulfate or halide ions
• Impurities like carbonate and sulfite ions need to be removed first to avoid interference with the tests
• The color of the precipitate in the halide test indicates which halide ion is present
• Test for chlorine


  1. Get a test tube of sample gas

  2. Take a piece of blue litmus paper and dampen it

  3. Put the damp blue litmus paper into the test tube

  4. If chlorine is present, the paper will turn from blue to white
• The paper might briefly turn red before turning white, as the chlorine dissolves in the water on the damp paper and forms hydrochloric acid, which is acidic and turns the blue litmus paper red
• Chlorine is poisonous, so you'd want to wear a mask or do the experiment in a fume cupboard
• Test for oxygen


  1. Get a glowing splint (without a flame but still glowing red)

  2. Place the splint into the tube of sample gas

  3. If the gas is oxygen, the glowing splint will relight
• Test for hydrogen


  1. Get a test tube of hydrogen gas

  2. Get a burning splint

  3. Move the test tube and burning splint close together

  4. You'll hear a squeaky pop as the hydrogen burns with the oxygen in the air to form water
• Test for carbon dioxide


  1. Get a sample of the gas

  2. Get an aqueous solution of calcium hydroxide (lime water)

  3. Bubble the gas sample through the calcium hydroxide

  4. If the gas is carbon dioxide, the solution will go cloudy as the CO2 and calcium hydroxide react to form calcium carbonate (a solid)
• Tests for gases

  • Chlorine
  • Oxygen
  • Hydrogen
  • Carbon dioxide
• Hydrogen gas (H2) is colorless, odorless, tasteless, non-toxic, flammable, and lighter than air.
• Hydrogen gas (H2) is colorless, odorless, tasteless, non-toxic, flammable, and lighter than air.
• Nitrogen gas (N2) is colorless, odorless, tasteless, non-toxic, heavier than air, and does not support combustion.
• Oxygen gas (O2) is colorless, odorless, tasteless, non-toxic, heavier than air, and supports combustion.
• 5 gases to test for

  • Carbon dioxide
  • Hydrogen gas
  • Oxygen gas
  • Chlorine
  • Ammonia
• Test for hydrogen gas


  1. Use a lit splint

  2. Observe a squeaky pop sound
• Test for oxygen gas


  1. Use a glowing splint

  2. Observe the splint relighting
• Test for chlorine gas


  1. Use damp blue litmus paper

  2. Observe it turning red and then bleaching
• Test for ammonia gas

  1. Use damp red litmus paper

  2. Observe it turning blue
• Test for carbon dioxide

  1. Add acid to marble chips

  2. Observe bubbles

  3. Bubble gas through limewater

  4. Observe limewater turning cloudy
• Halide ions to test for

  • Chloride
  • Bromide
  • Iodide
• Test for halide ions

  1. Add nitric acid

  2. Add silver nitrate

  3. Observe white, cream, or yellow precipitate
• GCSE flame test colours

  • Lithium (red)
  • Sodium (yellow)
  • Potassium (lilac)
  • Calcium (brick red)
  • Copper (green)
• Flame test procedure


  1. Use a nichrome wire loop

  2. Wash in acid to remove previous chemicals

  3. Heat wire in flame and observe colour
• Sodium hydroxide precipitate test

  1. Add sodium hydroxide to metal ion solutions

  2. Observe green, blue, or brown precipitates
• Test for sulfate ion

  1. Add hydrochloric acid

  2. Add barium chloride

  3. Observe white precipitate
• Test for ammonium ion

  1. Add sodium hydroxide

  2. Warm the solution

  3. Observe bubbles and blue litmus paper
• Test for carbonate ion

  1. Add acid

  2. Observe bubbles