O-Level Science (Chemistry) - Chapter 09: Qualitative Analysis

Comprehensive practice flashcards covering qualitative analysis tests for cations, anions, and gases as per the O-Level Science (Chemistry) syllabus.

Qualitative Analysis (QA)

A process used by chemists to identify cations, anions and gases in a given sample based on appearance (colour, physical state, and odour).

Test for Aqueous Cations

Involves adding aqueous sodium hydroxide, $NaOH(aq)$, and aqueous ammonia, $NH_3(aq)$, dropwise and then in excess to observe precipitate formation and solubility.

Ammonium ($NH_4^+$) Cation Test

No precipitate forms with $NaOH(aq)$. On heating, ammonia gas evolves which turns moist red litmus paper blue.

Calcium ($Ca^{2+}$) Cation Test

Forms a white precipitate with $NaOH(aq)$ that is insoluble in excess. No precipitate forms with aqueous ammonia.

Copper(II) ($Cu^{2+}$) Cation Test

Forms a light blue precipitate. It is insoluble in excess $NaOH(aq)$ but dissolves in excess $NH_3(aq)$ to form a dark blue solution.

Iron(II) ($Fe^{2+}$) Cation Test

Forms a green precipitate that is insoluble in excess of both alkalis. The precipitate turns brown on standing due to oxidation.

Iron(III) ($Fe^{3+}$) Cation Test

Forms a red-brown precipitate that is insoluble in excess of both $NaOH(aq)$ and $NH_3(aq)$.

Zinc ($Zn^{2+}$) Cation Test

Forms a white precipitate that dissolves in both excess $NaOH(aq)$ and excess $NH_3(aq)$ to form a colourless solution.

Aluminium ($Al^{3+}$) Cation Test

Forms a white precipitate that dissolves in excess $NaOH(aq)$ to form a colourless solution, but is insoluble in excess $NH_3(aq)$.

Amphoteric Hydroxides

$Zn(OH)_2$ and $Al(OH)_3$; they react with excess aqueous sodium hydroxide to form soluble salts.

Carbonate ($CO_3^{2-}$) Anion Test

Add dilute acid and bubble the resulting gas through limewater. A positive result is a white precipitate in the limewater from carbon dioxide.

Chloride ($Cl^-$) Anion Test

Add dilute nitric acid then aqueous silver nitrate. A white precipitate of silver chloride ($AgCl$) forms.

Iodide ($I^-$) Anion Test

Add dilute nitric acid then aqueous silver nitrate. A yellow precipitate of silver iodide ($AgI$) forms.

Sulfate ($SO_4^{2-}$) Anion Test

Add dilute nitric acid then aqueous barium nitrate. A white precipitate of barium sulfate ($BaSO_4$) forms.

Nitrate ($NO_3^-$) Anion Test

Add aqueous sodium hydroxide and aluminium (foil or powder) and warm. Ammonia gas is evolved, turning moist red litmus paper blue.

Role of Nitric Acid ($HNO_3$) in Anion Tests

Added to remove carbonate and hydroxide ions, which would otherwise form precipitates and cause false positives.

Hydrogen ($H_2$) Gas Test

Place a burning splint at the mouth of the test tube. The gas extinguishes the splint with a 'pop' sound.

Carbon Dioxide ($CO_2$) Gas Test

Bubble the gas into limewater ($Ca(OH)_2$). The gas produces a white precipitate.

Oxygen ($O_2$) Gas Test

Insert a glowing splint into the test tube. The gas relights the glowing splint.

Ammonia ($NH_3$) Gas Test

Place moist red litmus paper at the mouth of the test tube. The pungent gas turns the paper blue.

Sulfur Dioxide ($SO_2$) Gas Test

Place filter paper soaked with acidified potassium manganate(VII) at the mouth of the test tube. The gas turns the purple solution colourless.

Chlorine ($Cl_2$) Gas Test

Place damp blue litmus paper at the mouth of the test tube. The yellow-green gas turns the paper red and then bleaches it.