🧪 Chapter 12.5: Identification of Ions and Gases

Welcome to one of the most practical and exciting parts of Chemistry! This chapter is your guide to becoming a chemical detective. When you have an unknown substance in the lab, these tests are the tools you use to figure out exactly which ions and gases are present.

This knowledge is essential not just for your exams (especially Paper 5 or 6), but also for understanding how chemical analysis works in the real world—from checking water quality to diagnosing chemical components in manufacturing.

Don't worry if there are a lot of names and colours; we will use some simple tricks and memory aids to help you master these tests!

🔬 Section 1: Identifying Anions (Negative Ions)

Anions are the negative ions in a compound. We usually identify them by adding specific reagents (chemicals) and observing a distinctive result, like bubbling or forming a coloured solid (a precipitate).

1. Carbonate Ion (\(CO_3^{2-}\))

The Test: Add a small volume of dilute acid (like nitric acid or hydrochloric acid) to the solid or solution.

The Observation/Result:

  • Immediate effervescence (fizzing) is seen.
  • The gas produced is collected and passed into limewater (aqueous calcium hydroxide).
  • If carbonate is present, the limewater turns milky (cloudy).

Why it works: Carbonates react with acid to produce carbon dioxide gas:

\(CO_3^{2-}(aq) + 2H^+(aq) \rightarrow CO_2(g) + H_2O(l)\)

Quick Tip: Always remember that fizzing with acid means CO₂ is being released, which almost always points to a carbonate.

2. Halide Ions (Chloride (\(Cl^-\)), Bromide (\(Br^-\)), Iodide (\(I^-\)))

Halides are ions from Group VII (the halogens). We test these using aqueous silver nitrate.

The Test:

  1. Add dilute nitric acid (\(HNO_3\)) to the solution. This is a crucial step! It removes any other potentially interfering anions (like carbonates or sulfites) that might also form a precipitate with silver ions.
  2. Add a few drops of aqueous silver nitrate (\(AgNO_3(aq)\)).

The Observations/Results: A coloured precipitate forms, which helps distinguish them:

  • Chloride (\(Cl^-\)): Forms a white precipitate (AgCl).
  • Bromide (\(Br^-\)): Forms a cream precipitate (AgBr).
  • Iodide (\(I^-\)): Forms a yellow precipitate (AgI).

Analogy: Imagine the three halides are wearing wedding dresses for their marriage to Silver: Chlorine wears White, Bromine wears Cream, and Iodine wears Yellow.

3. Nitrate Ion (\(NO_3^-\))

Testing for nitrates is a bit more complex, as it requires a reduction reaction to convert the nitrate into ammonia gas.

The Test (Reduction Test):

  1. Add aqueous sodium hydroxide (NaOH) to the solution.
  2. Add small pieces of aluminium foil (this is the reducing agent).
  3. Warm the mixture carefully.
  4. Test the gas given off using damp red litmus paper.

The Observation/Result: The damp red litmus paper turns blue. This confirms the presence of ammonia gas (\(NH_3\)), which must have come from the original nitrate ion.

4. Sulfate Ion (\(SO_4^{2-}\))

The Test:

  1. Add dilute nitric acid (\(HNO_3\)) (again, to remove interferences like carbonates).
  2. Add aqueous barium nitrate (\(Ba(NO_3)_2(aq)\)).

The Observation/Result: A heavy white precipitate forms (barium sulfate, BaSO₄).

Key Takeaway: If you see a white precipitate after adding acidified barium nitrate, you have a sulfate.

5. Sulfite Ion (\(SO_3^{2-}\))

Sulfite ions are often tested using their ability to act as a reducing agent.

The Test: Add a small volume of acidified aqueous potassium manganate(VII).

The Observation/Result: The acidified aqueous potassium manganate(VII) solution changes colour from purple to colourless.

Why it works: The purple manganate(VII) ion is a strong oxidising agent. It gets reduced (loses its purple colour) while it oxidises the sulfite ion.

🔬 Section 2: Identifying Aqueous Cations (Positive Metal Ions)

Cations are identified primarily using two common reagents: aqueous sodium hydroxide (NaOH) and aqueous ammonia (\(NH_3\)). We look for the colour of the precipitate and whether that precipitate dissolves when we add *excess* reagent.

1. Testing with Aqueous Sodium Hydroxide (NaOH)

Most metal ions form insoluble metal hydroxides when NaOH is added, appearing as precipitates. We look for colour and solubility in excess NaOH.

Key Observations using Aqueous NaOH

(Precipitate = ppt)

  • Aluminium (\(Al^{3+}\)): White ppt, soluble in excess (gives a colourless solution).
  • Calcium (\(Ca^{2+}\)): White ppt, insoluble in excess.
  • Chromium(III) (\(Cr^{3+}\)): Green ppt, soluble in excess (gives a colourless solution).
  • Copper(II) (\(Cu^{2+}\)): Light blue ppt, insoluble in excess.
  • Iron(II) (\(Fe^{2+}\)): Green ppt, insoluble in excess. (Note: The green ppt turns brown near the surface as it is oxidised by air).
  • Iron(III) (\(Fe^{3+}\)): Red-brown ppt, insoluble in excess.
  • Zinc (\(Zn^{2+}\)): White ppt, soluble in excess (gives a colourless solution).

2. Testing with Aqueous Ammonia (\(NH_3(aq)\))

Ammonia often behaves differently, especially with copper and zinc ions.

Key Observations using Aqueous \(NH_3\)

(Precipitate = ppt)

  • Aluminium (\(Al^{3+}\)): White ppt, insoluble in excess. (This distinguishes it from zinc!)
  • Calcium (\(Ca^{2+}\)): No ppt or very slight white ppt.
  • Chromium(III) (\(Cr^{3+}\)): Green ppt, insoluble in excess.
  • Copper(II) (\(Cu^{2+}\)): Light blue ppt, soluble in excess (gives a deep, dark blue solution).
  • Iron(II) (\(Fe^{2+}\)): Green ppt, insoluble in excess. (Turns brown near the surface on standing).
  • Iron(III) (\(Fe^{3+}\)): Red-brown ppt, insoluble in excess.
  • Zinc (\(Zn^{2+}\)): White ppt, soluble in excess (gives a colourless solution).

3. The Special Case: Ammonium Ion (\(NH_4^+\))

The ammonium ion is not tested with ammonia solution (because it contains ammonium ions!).

The Test: Add aqueous sodium hydroxide (NaOH) to the solution containing the \(NH_4^+\) ion and warm carefully.

The Observation/Result: Ammonia gas (\(NH_3\)) is produced, which turns damp red litmus paper blue.

Why it works: This is an acid-base reaction where the alkali (NaOH) neutralises the ammonium ion:

\(NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l)\)


🔑 Quick Cation Summary (The Colour Code):

  • White ppt (Soluble in Excess NaOH): Aluminium (\(Al^{3+}\)) or Zinc (\(Zn^{2+}\)).
  • Green ppt: Iron(II) (\(Fe^{2+}\)) or Chromium(III) (\(Cr^{3+}\)).
  • Red-brown ppt: Iron(III) (\(Fe^{3+}\)).
  • Light Blue ppt: Copper(II) (\(Cu^{2+}\)).
  • If you see a White ppt that is Insoluble in BOTH excess NaOH and excess \(NH_3\): It’s most likely Calcium (\(Ca^{2+}\)).

🔥 Section 3: Identifying Metal Ions using Flame Tests

Some metal ions give off characteristic colours when heated strongly in a flame (usually using a piece of nichrome wire cleaned with acid). This is useful for identifying the metal in a salt.

Did you know? This happens because the heat excites the electrons in the metal ions. When the electrons fall back down to their original energy levels, they release energy in the form of light, producing a specific colour!

Flame Test Results
  • Lithium (\(Li^+\)): Red
  • Sodium (\(Na^+\)): Yellow (often described as bright orange-yellow)
  • Potassium (\(K^+\)): Lilac (pale purple)
  • Calcium (\(Ca^{2+}\)): Orange-red
  • Barium (\(Ba^{2+}\)): Light green
  • Copper(II) (\(Cu^{2+}\)): Blue-green

Memory Aid: Li, Na, K are the most important: Li-Red, Na-Yellow, K-Lilac.

💨 Section 4: Identifying Common Gases

Identifying gases is vital, especially when testing for carbonates, nitrates, or other compounds that produce gas during reactions.

Tests for Gases

| Gas | Test | Result |

|---|---|---|

| Ammonia (\(NH_3\)) | Use damp red litmus paper. | Turns damp red litmus paper blue. |

| Carbon Dioxide (\(CO_2\)) | Bubble through limewater (aqueous calcium hydroxide). | Turns limewater milky (cloudy). |

| Chlorine (\(Cl_2\)) | Use damp litmus paper (red or blue). | Bleaches the damp litmus paper (turns it white). |

| Hydrogen (\(H_2\)) | Put a lighted splint into the test tube. | Produces a characteristic 'pop' sound. |

| Oxygen (\(O_2\)) | Put a glowing splint (a splint with a glowing ember) into the test tube. | Relights the glowing splint. |

| Sulfur Dioxide (\(SO_2\)) | Bubble through acidified aqueous potassium manganate(VII). | Turns the solution from purple to colourless. |

Why \(SO_2\) is special: Like the sulfite ion (\(SO_3^{2-}\)), sulfur dioxide is a reducing agent, so it causes the purple oxidising agent (KMnO₄) to turn colourless.

🧠 Quick Review and Study Strategy

The secret to mastering "Identification of Ions and Gases" is practice and memorisation of the key reagents and colours.

📝 Study Tip for Challenging Students:

Instead of trying to memorise every test at once, group them by reagent:

  1. Limewater: Only tests for one gas: Carbon Dioxide (\(CO_2\)).
  2. Dilute Acid: Tests for only one anion (with limewater confirmation): Carbonate (\(CO_3^{2-}\)).
  3. Silver Nitrate (plus acid): Tests for the three Halides (Cl, Br, I).
  4. Aqueous Sodium Hydroxide (NaOH): This is the main test for Cations. Focus on the precipitate colours and solubility in excess. (e.g., *blue = copper; red-brown = iron(III)*).
  5. KMnO₄ (purple): Tests for reducing agents (Sulfite ion or Sulfur Dioxide gas). If it turns colourless, reduction has occurred.

Good luck! You’ve got this—these tests turn laboratory work into a fascinating puzzle!