Qualitative Analysis: Gas, Cation, and Anion Tests

21 question-and-answer flashcards summarising key gas, cation, and anion tests, along with essential qualitative analysis concepts.

Which gas turns damp red litmus paper blue, and what is its chemical nature?

Ammonia (NH3); it is an alkaline gas.

What observation confirms the presence of carbon dioxide when passed through limewater?

A white precipitate of calcium carbonate forms, which dissolves if excess CO2 is bubbled in.

Which gas first turns damp blue litmus paper red and then bleaches it white?

Chlorine (Cl2), an acidic, bleaching, and strong oxidising gas.

Which gas extinguishes a lighted splint with a characteristic ‘pop’ sound?

Hydrogen (H2); it is neutral and acts as a reducing gas.

Which gas will relight a glowing splint and what does this indicate about the gas?

Oxygen (O2); it supports combustion and is an oxidising gas.

Which gas decolourises purple acidified potassium manganate(VII) solution?

Sulfur dioxide (SO2); an acidic and reducing gas.

What happens when aqueous NaOH is added to a solution containing NH4+ ions and then warmed?

No precipitate forms; effervescence of ammonia gas is produced, which turns damp red litmus paper blue.

Which cation gives a white precipitate with NaOH that is insoluble in excess and behaves the same with NH3?

Calcium ions, Ca2+ (insoluble Ca(OH)2).

Which cation forms a white precipitate with NaOH that dissolves in excess, and also dissolves in excess NH3 to give a colourless solution?

Zinc ions, Zn2+; Zn(OH)2 is amphoteric and soluble in excess alkali or ammonia.

Which cation forms a white precipitate with NaOH that dissolves in excess, but with NH3 gives a white precipitate insoluble in excess?

Aluminium ions, Al3+; Al(OH)3 is amphoteric but insoluble in excess NH3.

Which cation behaves like aluminium with NaOH yet also gives a white precipitate insoluble in excess NH3 and can be distinguished by insoluble chloride/iodide salts?

Lead(II) ions, Pb2+; Pb(OH)2 dissolves in excess NaOH but not in excess NH3.

Which cation produces a light blue precipitate with NaOH (insoluble in excess) and a dark blue solution in excess NH3?

Copper(II) ions, Cu2+; Cu(OH)2 becomes [Cu(NH3)4]2+ in excess ammonia.

Which cation gives a green precipitate with both NaOH and NH3 that is insoluble in excess?

Iron(II) ions, Fe2+; forming insoluble Fe(OH)2.

Which cation gives a red-brown precipitate with NaOH or NH3, insoluble in excess?

Iron(III) ions, Fe3+; forming insoluble Fe(OH)3.

What observation indicates carbonate ions when dilute acid is added to the sample?

Effervescence of carbon dioxide is produced, which forms a white precipitate in limewater.

Which anion gives a white precipitate with acidified silver nitrate solution?

Chloride ion, Cl–, forming insoluble silver chloride (AgCl).

Which anion forms a yellow precipitate with acidified silver nitrate?

Iodide ion, I–, forming insoluble silver iodide (AgI).

Which anion produces a white precipitate with acidified barium nitrate solution?

Sulfate ion, SO4 2–, forming insoluble barium sulfate (BaSO4).

Which anion is reduced to ammonia when a solution is warmed with NaOH and aluminium foil?

Nitrate ion, NO3–; ammonia gas is evolved and turns damp red litmus paper blue.

Why are reagents acidified with dilute nitric acid before adding AgNO3 or Ba(NO3)2 in anion tests?

To remove interfering ions (e.g., carbonates) that might also form precipitates, ensuring only the target anion reacts—nitrates remain soluble.

In qualitative analysis, what is the importance of a negative test result?

A negative result rules out specific ions or chemical properties, helping narrow down the identity of the unknown substance.