Qualitative Analysis – Identification of Ions & Gases

Question–answer flashcards covering qualitative analysis fundamentals, anion and cation identification tests, amphoteric behaviour, and common laboratory gas tests.

What is meant by qualitative analysis in chemistry?

It is the use of observational evidence (e.g., colour changes, precipitates, gases) to identify the ions or chemical properties present in an unknown substance.

In qualitative analysis, what is a confirmatory test?

The final test that produces a characteristic reaction (e.g., precipitate, colour change) proving the presence of a specific ion.

Why are negative test results (no observable change) still important?

They help rule out specific ions or properties, narrowing down the identity of the unknown.

Why is dilute nitric acid often added before silver-nitrate or barium-nitrate tests?

To remove interfering ions such as carbonates (all nitrates are soluble), ensuring only the target anion reacts.

Effervescence on adding dilute acid to a sample indicates which anion and how is it confirmed?

Carbonate (CO₃²⁻); the CO₂ evolved turns limewater milky (forms CaCO₃).

Which reagent is used to test for carbonate ions and what is the positive result?

Add dilute acid → effervescence of CO₂ gas that makes limewater form a white precipitate.

Describe the chloride ion test (reagents, observation, equation).

Acidify with dilute HNO₃ then add AgNO₃ → white precipitate of AgCl; Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

Describe the iodide ion test (reagents, observation, equation).

Acidify with dilute HNO₃ then add AgNO₃ → yellow precipitate of AgI; Ag⁺(aq) + I⁻(aq) → AgI(s).

Describe the sulfate ion test (reagents, observation, equation).

Acidify with dilute HNO₃ then add Ba(NO₃)₂ → white precipitate of BaSO₄; Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

How is the nitrate ion confirmed?

Add NaOH and aluminium foil, warm → effervescence of ammonia gas (turns damp red litmus blue); nitrate is reduced to NH₃.

What happens when NaOH is added to a solution containing ammonium ions?

No precipitate; on warming, ammonia gas is evolved (colourless, pungent, turns damp red litmus blue).

State the observation when Ca²⁺ ions are treated with (a) NaOH and (b) NH₃.

(a) NaOH: white precipitate insoluble in excess. (b) NH₃: no precipitate.

Give the reactions of Zn²⁺ with NaOH and with NH₃.

Both produce a white precipitate of Zn(OH)₂ which dissolves in excess of either alkali, giving a colourless solution (amphoteric behaviour).

How does Al³⁺ behave with NaOH versus NH₃?

NaOH: white ppt of Al(OH)₃, soluble in excess. NH₃: white ppt, insoluble in excess.

Lead(II) ions show what behaviour with NaOH and NH₃, and how are they distinguished from Al³⁺?

Pb²⁺: white ppt with both alkalis; soluble in excess NaOH but insoluble in excess NH₃. Distinction: PbCl₂ / PbI₂ are insoluble whereas Al³⁺ forms no such precipitate.

Describe the observations when Cu²⁺ reacts with (a) NaOH and (b) excess NH₃.

(a) Light blue precipitate of Cu(OH)₂, insoluble in excess NaOH. (b) Precipitate dissolves in excess NH₃ forming a deep-blue solution of [Cu(NH₃)₄]²⁺.

State the precipitate colours for Fe²⁺ and Fe³⁺ with alkalis.

Fe²⁺ → green precipitate of Fe(OH)₂ (insoluble in excess). Fe³⁺ → red-brown precipitate of Fe(OH)₃ (insoluble in excess).

Which metal hydroxides are amphoteric and therefore dissolve in excess alkali?

Zn(OH)₂, Al(OH)₃ and Pb(OH)₂ (remember the mnemonic ZAP).

Why does NH₃ act as a weaker alkali than NaOH in precipitate tests?

NH₃ is only partially protonated in water (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻), giving a lower OH⁻ concentration than fully dissociated NaOH.

What is the test for ammonia gas?

It turns damp red litmus paper blue (due to its alkaline nature).

How is carbon dioxide gas identified?

It forms a white precipitate in limewater (CaCO₃) which dissolves with excess CO₂ to give Ca(HCO₃)₂.

State the test and result for chlorine gas.

Turns damp blue litmus red then bleaches it white (acidic, oxidising and bleaching gas).

How do you test for hydrogen gas?

A lighted splint is extinguished with a ‘pop’ sound (explosive combustion with oxygen).

How is oxygen gas confirmed?

It relights a glowing splint (supports combustion).

Describe the chemical test for sulfur dioxide gas.

It turns purple acidified potassium manganate(VII) (KMnO₄) solution colourless (SO₂ is a reducing agent).