Testing for Gases and Ions

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CognitoTests for gases and ions

Key facts & methods

Tests for common gases

Hydrogen (H2): burning splint held at mouth of test tube — squeaky pop.
Oxygen (O2): glowing splint held at mouth of test tube — splint relights.
Carbon dioxide (CO2): bubble through limewater (Ca(OH)2) — turns milky/cloudy.
Chlorine (Cl2): damp blue litmus paper — turns red then bleaches white.
Ammonia (NH3): damp red litmus paper — turns blue (ammonia is alkaline).

Flame tests for metal ions

Clean nichrome wire loop. Dip in sample. Hold in blue Bunsen flame.
Lithium (Li+): crimson/red flame.
Sodium (Na+): yellow/orange flame.
Potassium (K+): lilac/violet flame.
Calcium (Ca2+): orange-red flame.
Copper (Cu2+): green/blue-green flame.

NaOH tests for metal ions in solution

Add sodium hydroxide solution to the metal ion solution.
Al3+: white precipitate, dissolves in excess NaOH (amphoteric).
Ca2+: white precipitate, insoluble in excess NaOH.
Cu2+: blue precipitate.
Fe2+: pale green precipitate.
Fe3+: orange-brown precipitate.
Nitric acid added first to remove interfering ions.

Tests for negative ions (anions)

Carbonate (CO32-): add dilute acid — effervescence (CO2 gas produced, test with limewater).
Sulfate (SO42-): add dilute HCl then barium chloride solution — white precipitate of BaSO4.
Halides (Cl-, Br-, I-): add dilute HNO3 then silver nitrate solution.
Cl-: white AgCl precipitate (dissolves in dilute NH3).
Br-: cream AgBr precipitate (dissolves in concentrated NH3).
I-: yellow AgI precipitate (insoluble in NH3).

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markGas testAQA 8462 style

What is the test for carbon dioxide, and what is observed if CO2 is present?

A Hold burning splint at mouth of tube — squeaky pop

B Hold glowing splint at tube — splint relights

C Bubble gas through limewater — turns milky/cloudy

D Damp blue litmus paper held at tube — turns red then white

4 marksFlame test identificationAQA 8462 style

A student carries out flame tests on four unknown metal salt solutions. Solution A gives a yellow flame, solution B gives a crimson flame, solution C gives a lilac flame, and solution D gives a green flame. Identify the metal ion in each solution. (4 marks)

Hint: Learn the flame test colours: sodium=yellow, lithium=crimson, potassium=lilac, copper=green, calcium=orange-red.

+40 XP

3 marksDistinguish iron ionsAQA 8462 style

A solution contains either iron(II) ions or iron(III) ions. Describe how you would use sodium hydroxide solution to identify which ion is present. Include the expected observations for each. (3 marks)

Hint: Add NaOH to the solution. Observe precipitate colour. Fe2+ = pale green. Fe3+ = orange-brown. Name the precipitate formed.

+30 XP

1 markHalide identificationAQA 8462 style

Dilute nitric acid and then silver nitrate solution are added to an unknown solution. A cream precipitate forms. Which halide ion is present?

A Chloride (Cl-) — white precipitate

B Bromide (Br-) — cream precipitate

C Iodide (I-) — yellow precipitate

D Sulfate (SO42-) — white precipitate with BaCl2

Testing for Gases and Ions

Tests for common gases

Flame tests for metal ions

NaOH tests for metal ions in solution

Tests for negative ions (anions)

Module complete! 🎉