Identification of Ions and Gases — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 10Why acidify first

• Purpose
  • acidifying before adding silver nitrate or barium solutions removes ions such as carbonate that give false results.
• Choice of acid
  • use dilute nitric acid with silver nitrate, because hydrochloric acid would add chloride and form a white precipitate.
  • Barium tests
    • do not acidify with sulfuric acid because it would give a white precipitate of barium sulfate.
  • Rule
    • there is one test for the cation and another for the anion.

Section 2 of 10Tests for anions

Anion	Test	Positive result
Carbonate CO32-	Add dilute acid	Effervescence; gas turns limewater milky
Chloride Cl-	Acidify with dilute nitric acid, add silver nitrate	White precipitate
Bromide Br-	Same test	Cream precipitate
Iodide I-	Same test	Yellow precipitate
Nitrate NO3-	Add sodium hydroxide and aluminium foil and warm	Ammonia given off turns damp red litmus blue
Sulfate SO42-	Acidify with dilute nitric acid, add barium nitrate	White precipitate
Sulfite SO32-	Add dilute acid and warm	Sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless

Section 3 of 10Testing cations with alkalis

• Method
  • add sodium hydroxide a few drops at a time, then in excess, and note colours.
• Repeat
  • do the same with aqueous ammonia.
  • Slowly
    • add slowly or a soluble precipitate may dissolve as fast as it forms.
  • Precipitate
    • shows the hydroxide is insoluble in water.
  • Equation
    • ZnCl₂ + 2NaOH → Zn(OH)₂ + 2NaCl.

Section 4 of 10Cation results

Cation	With NaOH	With excess NaOH	With NH3	With excess NH3
Al3+	White precipitate	Dissolves, colourless solution	White precipitate	Insoluble
Ca2+	White precipitate	Insoluble	No precipitate or very slight	—
Cr3+	Green precipitate	Dissolves	Green precipitate	Insoluble
Cu2+	Light blue precipitate	Insoluble	Light blue precipitate	Dissolves to dark blue solution
Fe2+	Green precipitate	Insoluble	Green precipitate	Insoluble
Fe3+	Red-brown precipitate	Insoluble	Red-brown precipitate	Insoluble
Zn2+	White precipitate	Dissolves, colourless solution	White precipitate	Dissolves, colourless solution

• Distinguishing
  • Ca²⁺ from Zn²⁺ and Al³⁺ because calcium hydroxide stays in excess NaOH,
  • and Zn²⁺ from Al³⁺ because zinc hydroxide dissolves in excess ammonia.
• Ammonium
  • NH₄⁺ gives ammonia on warming with sodium hydroxide.
  • Words
    • colourless means no colour, clear means see-through, and a clear solution may be coloured.

Section 5 of 10Flame tests

1. Clean a nichrome or platinum wire loop in concentrated acid.
2. Heat it in the blue Bunsen flame until no colour shows.
3. Dip the loop into the sample.
4. Hold it at the edge of the blue flame and observe.

• Colours
  • lithium red, sodium yellow, potassium lilac, copper(II) blue-green, calcium orange-red, barium light green.
• Care
  • keep the wire from glowing red, and test only one metal ion at a time or colours mix.

Section 6 of 10Tests for gases

Gas	Test	Result
Hydrogen H2	Lighted splint	Squeaky pop
Oxygen O2	Glowing splint	Relights
Carbon dioxide CO2	Bubble through limewater	Turns milky
Ammonia NH3	Damp red litmus paper	Turns blue
Chlorine Cl2	Damp litmus paper	Bleached white
Sulfur dioxide SO2	Acidified potassium manganate(VII)	Purple to colourless

• Memory aid
  • a lighted splint has an H for hydrogen and a glowing splint has an O for oxygen.

Section 7 of 10Revision points

• Anion tests
  • acidify before adding silver nitrate or barium nitrate.
• Cation tests
  • use sodium hydroxide, then ammonia, adding in excess.
  • Gas tests
    • know the result for hydrogen, oxygen, carbon dioxide, ammonia, chlorine and sulfur dioxide.

Section 8 of 10Worked examples

1. Describe a test for sulfate ions. [3]3 marks

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2. A solution gives a white precipitate with NaOH that dissolves in excess, and with ammonia a white precipitate that dissolves in excess. Name the cation. [2]2 marks

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3. A gas relights a glowing splint. Name it. [1]1 mark

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4. Explain why hydrochloric acid cannot be used to acidify a solution before adding silver nitrate. [2]2 marks

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Section 9 of 10Key terms

Term	Meaning
Anion	A negatively charged ion.
Cation	A positively charged ion.
Precipitate	An insoluble solid formed in a solution reaction.
Flame test	A test identifying metal ions by flame colour.
Limewater	Calcium hydroxide solution used to test for carbon dioxide.
Effervescence	Bubbles of gas given off by a liquid.

Section 10 of 10Exam technique

1. Adding sodium hydroxide too quickly — the first precipitate may be missed.
2. Using hydrochloric acid in the silver nitrate test.
3. Confusing the splint tests for hydrogen and oxygen.
4. Calling a coloured solution not clear — clear and colourless are different.