Identification Of Ions And Gases

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Notes

Identification of Anions

**Carbonate (CO₃²⁻)**: Add dilute acid; bubble gas through **limewater** – turns cloudy (white precipitate of CaCO₃).
**Halide ions (Cl⁻, Br⁻, I⁻)**: Acidify with **nitric acid**, then add **AgNO₃** solution. Precipitate colour: Cl⁻ = **white** (AgCl), Br⁻ = **cream** (AgBr), I⁻ = **yellow** (AgI).
**Nitrate (NO₃⁻)**: Add **NaOH(aq)** and **aluminium foil**; warm gently. Releases **ammonia gas** – turns damp red litmus **blue**.
**Sulfate (SO₄²⁻)**: Acidify with **nitric acid**, then add **barium nitrate** solution. **White precipitate** of BaSO₄ forms.
**Sulfite (SO₃²⁻)**: Add dilute acid; gas released turns **acidified KMnO₄** from purple to colourless (not in syllabus but common).

Identification of Cations – NaOH & Ammonia Tests

Add **few drops NaOH(aq)** then **excess**; observe precipitate colour and solubility.
**Ca²⁺**: white precipitate, insoluble in excess NaOH and excess ammonia.
**Cu²⁺**: light blue precipitate, insoluble in excess NaOH, dissolves in excess ammonia to form **dark blue solution**.
**Fe²⁺**: **green** precipitate, insoluble in excess NaOH and ammonia.
**Fe³⁺**: **brown** precipitate, insoluble in excess NaOH and ammonia.
**Zn²⁺**: white precipitate, **dissolves** in excess NaOH and excess ammonia to form colourless solution.
**NH₄⁺**: warm with NaOH – releases **ammonia gas** (damp red litmus turns blue).

Flame Tests for Metal Cations

Clean nichrome/platinum wire loop by dipping in **concentrated acid** and heating in blue Bunsen flame.
Dip loop into sample and hold in **edge of blue flame**; observe colour.
**Li⁺** → **crimson**; **Na⁺** → **yellow**; **K⁺** → **lilac**; **Cu²⁺** → **blue-green**.
Avoid wire glowing red (confuses colour); use blue flame for strong heating.

Identification of Gases

**Ammonia (NH₃)**: damp **red** litmus turns **blue**; sharp choking smell.
**Carbon dioxide (CO₂)**: bubble through **limewater** – turns **cloudy/milky** (white CaCO₃ precipitate).
**Chlorine (Cl₂)**: damp **blue** litmus turns **red then bleached white**; toxic – use fume cupboard.
**Hydrogen (H₂)**: **lighted splint** at mouth of tube gives **‘squeaky pop’** (rapid combustion).
**Oxygen (O₂)**: **glowing splint** relights (or glows brightly).

Key Tips & Common Mistakes

For carbonate test, say ‘bubble gas through limewater’ – not just ‘limewater turns cloudy’.
Halide test: use **nitric acid** not HCl (Cl⁻ would interfere).
Sulfate test: add nitric acid first to remove carbonates that also give white precipitate.
Add NaOH/ammonia **slowly** – some precipitates dissolve in excess and may be missed.
Distinguish **colourless** (no colour) from **clear** (transparent, may have colour).

Benedict's test for reducing sugars – colour change from blue to green/yellow/orange/red.

Iodine test for starch – blue-black colour indicates starch.

Biuret test for protein – purple colour indicates protein.

Emulsion test for lipids – white emulsion indicates lipid.

Practice questions

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1.Which row correctly describes the test for each gas?
Easy
- AA: oxygen - lighted splint pops; hydrogen - glowing splint relights; chlorine - damp blue litmus turns red then white
- BB: oxygen - glowing splint relights; hydrogen - lighted splint pops; chlorine - damp blue litmus turns red then white
- CC: oxygen - glowing splint relights; hydrogen - lighted splint pops; chlorine - damp red litmus turns blue then white
- DD: oxygen - lighted splint pops; hydrogen - glowing splint relights; chlorine - damp red litmus turns blue then white
2.Halide ions in solution are detected using silver nitrate solution. Which row correctly describes the colour of the precipitate for each halide?
Easy
- AA: chloride - cream; bromide - white; iodide - silver
- BB: chloride - yellow; bromide - cream; iodide - white
- CC: chloride - white; bromide - cream; iodide - yellow
- DD: chloride - white; bromide - cream; iodide - silver
3.A solution containing an unknown substance was analysed. On the addition of acidified silver nitrate solution a yellow precipitate formed. A flame test produced a lilac coloured flame. What is the identity of the unknown solid?
Easy
- AA: Lithium iodide
- BB: Lithium bromide
- CC: Potassium iodide
- DD: Potassium bromide
4.Metal cations can be detected using flame tests. Which row correctly describes the colour of the flame for each metal cation?
Easy
- AA: Li+ - lilac; K+ - green; Cu2+ - yellow
- BB: Li+ - red; K+ - lilac; Cu2+ - green
- CC: Li+ - green; K+ - lilac; $Cu2+ - red$
- DD: Li+ - red; K+ - orange; Cu2+ - blue
5.A student tested copper sulfate solution and potassium iodide solution using flame tests. The student's method did not produce the distinct lilac flame colour using potassium iodide solution. Which statement would not cause the lack of lilac flame?
Easy
- AA: The metal wire loop being overheated
- BB: Using the safety flame of the Bunsen burner
- CC: The same metal wire loop being used for each experiment
- DD: The metal wire loop being dipped in concentrated acid and burned between flame tests
6.The results of two tests on a solid are shown below. Test 1: Acidified silver nitrate added - cream precipitate formed. Test 2: Aqueous sodium hydroxide added - red-brown precipitate formed. What is the solid?
Medium
- AA: Iron(II) bromide
- BB: Iron(III) bromide
- CC: Iron(II) chloride
- DD: Iron(III) chloride
7.Which reaction would result in the production of a gas that would produce a squeaky pop with a lit splint?
Medium
- AA: Iron(II) oxide + sulfuric acid
- BB: Methane + oxygen during complete combustion
- CC: Magnesium + hydrochloric acid
- DD: Carbon dioxide and water in the presence of light and chlorophyll
8.The results of different chemical tests on a solution of compound Y are shown. Test 1: Acidified barium nitrate added - white precipitate formed. Test 2: Aqueous sodium hydroxide added - white precipitate formed, soluble in excess. Test 3: Aqueous ammonia added - white precipitate formed, insoluble in excess. What is compound Y?
Medium
- AA: Aluminium sulfate
- BB: Aluminium chloride
- CC: Zinc sulfate
- DD: Zinc chloride

Slides

Identification of Ions & Gases

This lesson covers tests for common anions, cations, and gases.
You will learn the test procedures and expected observations.
Use these tests to identify unknown substances.

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 (white precipitate of CaCO₃).
Effervescence (fizzing) is also observed.

What is the observation when carbonate ions react with dilute acid and the gas is bubbled through limewater?

Limewater turns cloudy
Limewater turns blue
Limewater turns red
Limewater remains colourless

Test for Halide Ions (Cl⁻, Br⁻, I⁻)

Acidify the sample with dilute nitric acid.
Add silver nitrate solution (AgNO₃).
A silver halide precipitate forms:
Chloride (Cl⁻) → white precipitate (AgCl)
Bromide (Br⁻) → cream precipitate (AgBr)
Iodide (I⁻) → yellow precipitate (AgI)

A solution gives a cream precipitate with acidified silver nitrate. Which halide ion is present?

Chloride
Bromide
Iodide
Fluoride

Test for Sulfate Ions (SO₄²⁻)

Acidify the sample with dilute nitric acid.
Add a few drops of barium nitrate solution.
A white precipitate of barium sulfate (BaSO₄) confirms sulfate ions.
Nitric acid removes carbonates that could also give a white precipitate.

To test for sulfate ions, add dilute $____ and$ then barium nitrate solution. $A ____$ precipitate indicates sulfate.

Test for Nitrate Ions (NO₃⁻)

Add aqueous sodium hydroxide and aluminium foil.
Warm the mixture gently.
Test the gas released with damp red litmus paper.
If nitrate is present, ammonia gas is produced, turning damp red litmus paper blue.

Which gas is produced when nitrate ions are warmed with sodium hydroxide and aluminium foil?

Hydrogen
Oxygen
Ammonia
Carbon dioxide

Test for Ammonium Ions (NH₄⁺)

Warm the sample with sodium hydroxide solution.
Ammonia gas is released.
Ammonia turns damp red litmus paper blue.
It also has a characteristic sharp, choking smell.

Which statements about the ammonium ion test are true?

Sodium hydroxide is added and the mixture is warmed.
Ammonia gas turns damp blue litmus paper red.
The gas produced has a sharp, choking smell.

Flame Tests for Metal Ions

Clean a nichrome wire loop by dipping in concentrated acid and heating.
Dip the loop into the sample and hold it in a blue Bunsen flame.
Observe the flame colour:
Lithium (Li⁺) → crimson
Sodium (Na⁺) → yellow
Potassium (K⁺) → lilac
Copper (Cu²⁺) → blue-green

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

Lithium
Sodium
Potassium
Copper

Metal Hydroxide Precipitate Tests

Add a few drops of NaOH or ammonia solution to the sample.
Observe precipitate colour and solubility in excess reagent.
Key results:
Cu²⁺ → light blue ppt, insoluble in excess NaOH, dissolves in excess NH₃ to give dark blue solution.
Fe²⁺ → green ppt, insoluble in excess.
Fe³⁺ → brown ppt, insoluble in excess.
Zn²⁺ → white ppt, dissolves in excess NaOH and excess NH₃.
Ca²⁺ → white ppt, insoluble in excess.

Adding excess ammonia to a solution containing copper(II) ions gives $a ____$ solution. The initial precipitate $is ____$ .

Test for Hydrogen Gas (H₂)

Hold a burning splint at the mouth of a test tube containing the gas.
If hydrogen is present, it burns with a loud 'squeaky pop'.
Do not insert the splint into the tube; the gas needs air to burn.

Which gas gives a 'squeaky pop' with a lighted splint?

Oxygen
Hydrogen
Carbon dioxide
Chlorine

Test for Oxygen Gas (O₂)

Place a glowing splint inside a test tube of the gas.
If oxygen is present, the splint relights (or glows brightly).
Remember: 'glOwing splint' for 'Oxygen'.

A glowing splint relights in a gas. What is the gas?

Hydrogen
Carbon dioxide
Oxygen
Chlorine

Test for Carbon Dioxide (CO₂)

Bubble the gas through limewater (calcium hydroxide solution).
If carbon dioxide is present, limewater turns cloudy (milky).
The cloudiness is due to formation of insoluble calcium carbonate.

Which statements about the carbon dioxide test are correct?

Limewater turns cloudy white.
A burning splint is extinguished.
The gas is bubbled through limewater.

Test for Chlorine Gas (Cl₂)

Damp blue litmus paper turns red then bleaches white.
Chlorine has a sharp, choking smell and is toxic (use fume cupboard).
The initial red colour is due to acid formation (HCl).

What happens when damp blue litmus paper is exposed to chlorine gas?

It turns blue
It turns red then white
It turns white only
It turns green

Flashcards

What is the test for carbonate ions (CO₃²⁻)?

Add dilute acid and bubble the gas through limewater. Limewater turns cloudy if carbonate is present.

What is observed when carbonate ions react with acid?

Fizzing/effervescence occurs as CO₂ gas is produced.

What is the test for halide ions (Cl⁻, Br⁻, I⁻)?

Acidify with nitric acid, then add silver nitrate solution. A silver halide precipitate forms.

What colour precipitate does chloride give with silver nitrate?

White precipitate of silver chloride (AgCl).

What colour precipitate does bromide give with silver nitrate?

Cream precipitate of silver bromide (AgBr).

What colour precipitate does iodide give with silver nitrate?

Yellow precipitate of silver iodide (AgI).

Why is nitric acid added before silver nitrate in the halide test?

To remove any carbonate ions that would also form a precipitate and interfere.

What is the test for nitrate ions (NO₃⁻)?

Add aqueous NaOH and aluminium foil, warm gently. Ammonia gas is released, which turns damp red litmus paper blue.

What is the test for sulfate ions (SO₄²⁻)?

Acidify with dilute nitric acid, then add barium nitrate solution. A white precipitate of barium sulfate forms.

What is the test for ammonium ions (NH₄⁺)?

Warm with sodium hydroxide solution. Ammonia gas is released, turning damp red litmus paper blue.

What colour precipitate does Cu²⁺ form with NaOH or ammonia (few drops)?

Light blue precipitate.

What happens to the Cu²⁺ precipitate in excess ammonia?

It dissolves to form a dark blue solution.

What colour precipitate does Fe²⁺ form with NaOH?

Green precipitate.

What colour precipitate does Fe³⁺ form with NaOH?

Brown precipitate.

What colour precipitate does Zn²⁺ form with NaOH or ammonia (few drops)?

White precipitate.

What happens to the Zn²⁺ precipitate in excess NaOH or excess ammonia?

It dissolves to form a colourless solution.

What is the flame colour for lithium (Li⁺)?

Crimson.

What is the flame colour for sodium (Na⁺)?

Yellow.

What is the flame colour for potassium (K⁺)?

Lilac.

What is the flame colour for copper (Cu²⁺)?

Blue-green.

How do you test for ammonia gas?

Damp red litmus paper turns blue.

How do you test for carbon dioxide gas?

Bubble through limewater; it turns cloudy/milky.

How do you test for chlorine gas?

Damp blue litmus paper turns red then bleached white.

How do you test for hydrogen gas?

Hold a lighted splint at the mouth of the test tube; it burns with a squeaky pop.

How do you test for oxygen gas?

Place a glowing splint inside the test tube; it relights.

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