Identification Of Ions & Gases

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Notes

Identification of Anions

**Carbonate (CO₃²⁻)**: Add dilute acid → CO₂ gas → bubble through **limewater** → turns **cloudy/milky** (white CaCO₃ precipitate).
**Halide (Cl⁻, Br⁻, I⁻)**: Acidify with **dilute HNO₃**, add **AgNO₃(aq)** → coloured precipitate: **Cl⁻ = white** (AgCl), **Br⁻ = cream** (AgBr), **I⁻ = yellow** (AgI).
**Nitrate (NO₃⁻)**: Add **aq. NaOH + Al foil**, warm gently → **ammonia gas** (turns damp red litmus blue).
**Sulfate (SO₄²⁻)**: Acidify with **dilute HNO₃**, add **BaCl₂(aq)** or Ba(NO₃)₂ → **white precipitate** (BaSO₄).
**Sulfite (SO₃²⁻)**: Add **dilute acid**, warm, bubble gas through **acidified KMnO₄** → purple to **colourless**.

Identification of Cations – NaOH Test

Add **few drops aq. NaOH** → observe precipitate colour; then **excess NaOH** → note solubility.
**Al³⁺**: white ppt, **soluble in excess** NaOH → colourless solution.
**NH₄⁺**: **ammonia gas** on warming (turns damp red litmus blue).
**Ca²⁺**: white ppt, **insoluble** in excess NaOH.
**Cr³⁺**: **green** ppt, **soluble** in excess NaOH.
**Cu²⁺**: **light blue** ppt, **insoluble** in excess NaOH.
**Fe²⁺**: **green** ppt, insoluble in excess, surface turns **brown** on standing.
**Fe³⁺**: **red-brown** ppt, insoluble in excess.
**Zn²⁺**: white ppt, **soluble** in excess NaOH → colourless solution.

Identification of Cations – Ammonia Test

Add **few drops aq. NH₃** → observe precipitate; then **excess NH₃** → note solubility.
**Al³⁺**: white ppt, **insoluble** in excess NH₃.
**Ca²⁺**: no ppt or very slight white ppt; no change with excess.
**Cr³⁺**: **green** ppt, insoluble in excess NH₃.
**Cu²⁺**: light blue ppt, **soluble** in excess NH₃ → **dark blue** solution.
**Fe²⁺**: green ppt, insoluble in excess, surface turns brown.
**Fe³⁺**: red-brown ppt, insoluble in excess.
**Zn²⁺**: white ppt, **soluble** in excess NH₃ → colourless solution.

Flame Tests for Metal Cations

Clean **nichrome or platinum wire** by dipping in conc. HCl and heating until no colour.
Dip wire into sample and hold in **blue Bunsen flame**.
**Li⁺** = red; **Na⁺** = yellow; **K⁺** = lilac; **Ca²⁺** = orange-red; **Ba²⁺** = light green; **Cu²⁺** = blue-green.

Identification of Gases

**Ammonia (NH₃)**: damp **red** litmus turns **blue**; sharp choking smell.
**Carbon dioxide (CO₂)**: bubble through **limewater** → turns **cloudy/milky**.
**Chlorine (Cl₂)**: damp **blue** litmus turns **red then bleached white**; toxic – use fume cupboard.
**Hydrogen (H₂)**: **burning splint** → **squeaky pop** (rapid combustion).
**Oxygen (O₂)**: **glowing splint** → **relights** (or glows brightly).
**Sulfur dioxide (SO₂)**: bubble through **acidified KMnO₄** → purple to **colourless**; sharp choking smell.

Key Tips & Common Mistakes

For carbonate test: describe the test (bubble gas through limewater), not just the result.
Halide test: use **HNO₃** to acidify, not HCl (Cl⁻ would interfere).
Sulfate test: add HNO₃ first to remove carbonates that also give white ppt.
Add NaOH/NH₃ **slowly** – some ppts dissolve in excess, so initial ppt may be missed.
Distinguish **colourless** (no colour) vs **clear** (transparent, may be coloured).
Flame test: sample must be **pure** (single cation) to avoid mixed colours.

Particle arrangement in solids, liquids, and gases. Useful for understanding gas tests (e.g., gas particles are far apart).

Practice questions

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1.What is the test for carbonate ions?
Easy
- AAdd dilute acid and bubble the gas through limewater; limewater turns cloudy.
- BAdd silver nitrate solution; a white precipitate forms.
- CAdd barium chloride solution; a white precipitate forms.
- DAdd sodium hydroxide and warm; damp red litmus turns blue.
2.Which gas is produced when ammonium ions are warmed with sodium hydroxide solution?
Easy
- AAmmonia
- BNitrogen
- CHydrogen
- DOxygen
3.What is the colour of the precipitate formed when chloride ions are tested with silver nitrate solution?
Easy
- AWhite
- BCream
- CYellow
- DNo precipitate
4.A solution gives a white precipitate with barium nitrate solution after adding dilute nitric acid. Which ion is present?
Medium
- ASulfate
- BCarbonate
- CChloride
- DNitrate
5.A green precipitate forms when sodium hydroxide is added to a solution. The precipitate dissolves in excess sodium hydroxide. Which cation is present?
Medium
- AChromium(III)
- BIron(II)
- CCopper(II)
- DAluminium
6.Which gas relights a glowing splint?
Medium
- AOxygen
- BHydrogen
- CCarbon dioxide
- DChlorine
7.A solution contains a mixture of chloride and bromide ions. After adding dilute nitric acid and silver nitrate, what colour precipitate would you expect?
Hard
- AWhite
- BCream
- CYellow
- DA mixture of white and cream, likely appearing as an off-white or pale cream
8.A student adds dilute hydrochloric acid to a solid and bubbles the gas through limewater. The limewater turns cloudy. What can be concluded?
Hard
- AThe solid contains carbonate ions.
- BThe solid contains sulfite ions.
- CThe solid contains hydrogencarbonate ions.
- DThe solid contains carbonate or hydrogencarbonate ions.

Slides

Identification of Ions & Gases

You need to know tests for anions (negative ions) and cations (positive ions).
You also need to know tests for common gases like CO₂, NH₃, H₂, O₂, Cl₂, SO₂.
Tests often involve adding reagents and observing colour changes, precipitates, or gases.

Test for Carbonate Ions (CO₃²⁻)

Add dilute acid to the sample.
Bubble the gas produced through limewater (calcium hydroxide solution).
If carbonate is present, limewater turns cloudy (milky) due to formation of white CaCO₃ precipitate.

What is the positive result for the carbonate ion test?

Limewater turns blue
Limewater turns cloudy
A white precipitate forms in the sample
Effervescence only

Test for Halide Ions (Cl⁻, Br⁻, I⁻)

Acidify the sample with dilute nitric acid.
Add silver nitrate solution (AgNO₃).
Chloride gives a white precipitate (AgCl).
Bromide gives a cream precipitate (AgBr).
Iodide gives a yellow precipitate (AgI).

A solution gives a cream precipitate with silver nitrate after adding nitric acid. Which halide ion is present?

Chloride
Bromide
Iodide
Fluoride

Test for Sulfate Ions (SO₄²⁻)

Acidify the sample with dilute nitric acid.
Add barium chloride (or barium nitrate) solution.
A white precipitate of barium sulfate (BaSO₄) indicates sulfate ions.

To test for sulfate ions, add dilute ____ and then ____ solution. A white precipitate confirms sulfate.

Test for Nitrate Ions (NO₃⁻)

Add aqueous sodium hydroxide and aluminium foil.
Warm gently.
Ammonia gas is produced, which turns damp red litmus paper blue.

Which gas is released when testing for nitrate ions?

Nitrogen dioxide
Ammonia
Oxygen
Hydrogen

Test for Sulfite Ions (SO₃²⁻)

Add dilute acid and warm gently.
Bubble the gas through acidified potassium manganate(VII) solution.
The purple solution turns colourless if sulfite is present.

Which of these are true about the sulfite test?

The gas produced is sulfur dioxide.
Potassium manganate(VII) turns from purple to colourless.
The test uses limewater.

Test for Ammonium Ions (NH₄⁺)

Warm the sample with sodium hydroxide solution.
Ammonia gas is released.
Test with damp red litmus paper: it turns blue.

What is observed when ammonium ions are warmed with NaOH?

A white precipitate forms
Damp red litmus turns blue
Effervescence and limewater turns cloudy
A yellow precipitate

Metal Cations with NaOH

Add a few drops of NaOH, then excess.
Al³⁺: white ppt, soluble in excess.
Ca²⁺: white ppt, insoluble in excess.
Cu²⁺: light blue ppt, insoluble.
Fe²⁺: green ppt, turns brown on standing.
Fe³⁺: red-brown ppt, insoluble.
Zn²⁺: white ppt, soluble in excess.

A solution gives a white precipitate with NaOH that dissolves in excess. Which cation is likely?

Calcium
Aluminium
Copper
Iron(II)

Metal Cations with Ammonia

Add a few drops of NH₃, then excess.
Cu²⁺: light blue ppt, dissolves in excess to give dark blue solution.
Fe²⁺: green ppt, insoluble.
Fe³⁺: red-brown ppt, insoluble.
Al³⁺: white ppt, insoluble in excess.
Zn²⁺: white ppt, soluble in excess.

Copper(II) ions form a ____ precipitate with ammonia, which dissolves in excess to give a ____ solution.

Flame Tests

Clean a nichrome wire loop with concentrated acid.
Dip into sample and hold in blue Bunsen flame.
Li⁺: red; Na⁺: yellow; K⁺: lilac; Ca²⁺: orange-red; Ba²⁺: light green; Cu²⁺: blue-green.

A compound gives a lilac flame. Which metal ion is present?

Sodium
Potassium
Calcium
Barium

Test for Gases

Ammonia: damp red litmus turns blue.
Carbon dioxide: limewater turns cloudy.
Chlorine: damp blue litmus turns red then bleached white.
Hydrogen: burning splint gives a squeaky pop.
Oxygen: glowing splint relights.
Sulfur dioxide: acidified KMnO₄ turns from purple to colourless.

Which tests are correct?

Hydrogen gives a squeaky pop with a burning splint.
Oxygen relights a glowing splint.
Chlorine turns damp red litmus blue.

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