Anion, Cation, Gas & Flame Tests

Question 1

Tests To Identify The Anions
- CO3 2-,
- Cl-, Br-, I-
- NO3 -
- SO4 2-
- SO3 2–,

Answer 1

(a) Carbonate (CO3^2–):
Reacts with dilute acid to produce carbon dioxide gas (CO2), which turns limewater milky upon bubbling through it.

(b) Chloride (Cl–), Bromide (Br–), and Iodide (I–):
Acidification with dilute nitric acid followed by addition of aqueous silver nitrate (AgNO3) results in the formation of white precipitates for silver chloride (AgCl), cream precipitate for silver bromide (AgBr), and yellow precipitate for silver iodide (AgI).

(c) Nitrate (NO3–):
Reduction with aluminum foil and aqueous sodium hydroxide (NaOH) leads to the detection of ammonia gas (NH3), which can be detected by its distinctive odor or using damp red litmus paper, turning it blue.

(d) Sulfate (SO4^2–):
Acidification with dilute nitric acid followed by addition of aqueous barium nitrate (Ba(NO3)2) causes the formation of a white precipitate of barium sulfate (BaSO4).

(e) Sulfite (SO3^2–):
Reacts with acidified aqueous potassium manganate(VII) (KMnO4), leading to the decolorization of the purple color of potassium manganate(VII), indicating reduction to colorless manganese(II) ions (Mn^2+).

Question 2

Tests To Identify The Aqueous Cations (8)

• Al 3+
• NH4 +
• Ca 2+
• Cr 3+
• Cu 2+
• Iron (II) 2+
• Iron (III) 3+
• Zn 2+

Answer 2

(a) Aluminium Al 3+:

With aqueous sodium hydroxide: Forms white gelatinous precipitate of aluminium hydroxide (Al(OH)3).

With aqueous ammonia: Precipitate of aluminium hydroxide initially forms, then re-dissolves in excess ammonia to form colorless complex ion, [Al(OH)4]^−.

(b) Ammonium, NH4^+:
with dilute sodium hydroxide: gently heating it, NH4+ will be converted to ammonia gas if present. Ammonia has a distinct odor. It also turns red litmus to blue.

(c) Calcium, Ca^2+:
With aqueous sodium hydroxide: Forms white precipitate of calcium hydroxide (Ca(OH)2), soluble in excess NaOH.

With aqueous ammonia: Forms white precipitate of calcium hydroxide

(d) Chromium(III), Cr^3+:
With aqueous sodium hydroxide: Forms green precipitate of chromium(III) hydroxide (Cr(OH)3).

With aqueous ammonia: Forms green precipitate of chromium(III) hydroxide

(e) Copper(II), Cu^2+:
With aqueous sodium hydroxide: Initially forms pale blue precipitate of copper(II) hydroxide (Cu(OH)2), then oxidizes to form dark blue precipitate of copper(II) oxide (CuO).

With aqueous ammonia: Forms deep blue precipitate of tetraamminecopper(II) hydroxide complex, [Cu(NH3)4(OH)2].

(f) Iron(II), Fe^2+:
With aqueous sodium hydroxide: Forms green precipitate of iron(II) hydroxide (Fe(OH)2), rapidly oxidizes to form brown precipitate of iron(III) hydroxide (Fe(OH)3).

With aqueous ammonia: Initially forms green precipitate of iron(II) hydroxide, then oxidizes to form brown precipitate of iron(III) hydroxide, similar to reaction with NaOH.

(g) Iron(III), Fe^3+:
With aqueous sodium hydroxide: Forms brown precipitate of iron(III) hydroxide (Fe(OH)3).

With aqueous ammonia: Forms brown precipitate of iron(III) hydroxide, similar to reaction with NaOH.

(h) Zinc, Zn^2+:
With aqueous sodium hydroxide: Initially forms white precipitate of zinc hydroxide (Zn(OH)2), then dissolves in excess NaOH due to formation of soluble zincate ion, [Zn(OH)4]^2−.

With aqueous ammonia: Forms white precipitate of zinc hydroxide, similar to reaction with NaOH.

Question 3

Tests To Identify The Gases
- Ammonia
- Carbon Dioxide
- Chlorine
- Hydrogen
- Oxygen
- Sulfur Dioxide

Answer 3

(a) Ammonia, NH3, using damp red litmus paper:

Ammonia turns damp red litmus paper blue, Ammonia is Basic

(b) Carbon dioxide, CO2, using limewater:

Carbon dioxide reacts with limewater (aqueous calcium hydroxide, Ca(OH)2), forming a white precipitate of calcium carbonate (CaCO3).
This reaction causes the limewater to turn milky.

(c) Chlorine, Cl2, using damp litmus paper:

Chlorine gas does not directly react with litmus paper.
However, it bleaches the paper, turning red litmus paper white and destroying the color of blue litmus paper.

(d) Hydrogen, H2, using a lighted splint:

Hydrogen gas ignites with a ‘pop’ sound when ignited with a lighted splint. This is due to its highly flammable nature, rapidly undergoing combustion in the presence of oxygen to form water (H2O).

(e) Oxygen, O2, using a glowing splint:

Oxygen supports combustion.
When introduced to a glowing splint, oxygen reignites it with a bright flame, enhancing combustion.

(f) Sulfur dioxide, SO2, using acidified aqueous potassium permanganate(VII) (KMnO4):

Sulfur dioxide gas reduces acidified aqueous potassium permanganate(VII) (KMnO4) to manganese(II) ions (Mn^2+).

This reaction results in the purple color of the solution turning colorless or pale pink, indicating the reduction of manganese(VII) to manganese(II).

Question 4

Flame Test To Identify The Cations

• Li +
• Na +
• K +
• Ca 2+
• Ba 2+
• Cu 2+

Answer 4

(a) Lithium, Li +:

• it gives crimson or red color to the flame. This color is often faint and may require careful observation.

(b) Sodium, Na^+:

• it gives a bright yellow flame when introduced into a flame. The yellow color is intense and easily visible.

(c) Potassium, K^+:

• it gives a lilac or violet color to the flame. This color is often quite distinct and noticeable.

(d) Calcium, Ca^2+:

• Calcium ions produce an orange-red flame when introduced into a flame. it tends to be more intense than lithium.

(e) Barium, Ba^2+:

• it gives a pale green flame when introduced into a flame. The color can be somewhat faint, so careful observation is necessary.

(f) Copper(II), Cu^2+:

• it produces a blue-green flame when introduced into a flame. This color is often quite striking and distinct from the colors produced by other cations.