A chemist is given a dry solid mixture containing nickel powder, ammonium chloride (NH_4Cl… — JAMB UTME

A chemist is given a dry solid mixture containing nickel powder, ammonium chloride ( $NH_4Cl$ ), fine barium sulfate ( $BaSO_4$ ) powder, and sodium chloride ( $NaCl$ ). What is the correct chronological sequence of laboratory procedures required to isolate each pure component from the mixture?

1Pass a magnet repeatedly over the dry solid mixture to isolate the nickel powder.
2Heat the remaining dry residue in an evaporating dish covered with an inverted funnel to sublime ammonium chloride.
3Add distilled water to the solid residue and stir thoroughly to form a mixture of dissolved solute and suspended solid.
4Centrifuge the mixture to rapidly spin down fine insoluble barium sulfate particles at the bottom of the tube.
5Decant the clear supernatant liquid and evaporate it to dryness to recover sodium chloride crystals.

Answer

The correct sequence of separation steps is: 1. Pass a magnet over the dry solid mixture to isolate nickel powder. 2. Heat the dry residue to sublime ammonium chloride. 3. Add distilled water to dissolve sodium chloride. 4. Centrifuge the suspension to spin down fine barium sulfate powder. 5. Decant the clear supernatant liquid and evaporate to dryness to recover sodium chloride crystals.

The correct sequence begins with magnetization on the dry solid mixture to remove ferromagnetic nickel. Next, sublimation is performed on the dry residue to separate ammonium chloride, which must occur before adding water because both ammonium chloride and sodium chloride are water-soluble. Distilled water is then added to selectively dissolve sodium chloride while leaving barium sulfate in suspension. Centrifugation rapidly spins down the fine barium sulfate precipitate. Finally, decanting and evaporating the clear supernatant liquid yields pure sodium chloride crystals.

Step-by-Step Solution

Extract ferromagnetic nickel powder using a bar magnet.

Dry nickel powder is separated cleanly from the solid mixture.

Nickel is ferromagnetic and is attracted by a magnetic field without requiring chemical reagents or solvent addition.

Apply controlled heat to the dry residue under an inverted funnel to sublime ammonium chloride (

NH_4Cl

Ammonium chloride sublimates directly into gas upon heating and deposits as pure solid crystals on the cool funnel surface.

Sublimation must precede water dissolution because both

NH_4Cl

and

NaCl

are soluble in water; dissolving first would make thermal separation of the two salts impossible.

Add distilled water to the remaining mixture of

NaCl

and

BaSO_4

and stir.

Sodium chloride dissolves into aqueous solution (

NaCl_{(aq)}

) while insoluble

BaSO_4

forms a fine suspension.

Water acts as a selective solvent based on solubility differences between soluble

NaCl

and insoluble

BaSO_4

Spin the suspension in a laboratory centrifuge.

Dense, fine particles of

BaSO_4

compact tightly at the bottom of the tube, separating clearly from the aqueous liquid.

Centrifugation rapidly forces fine suspended solids out of liquid phase much faster and more completely than gravity filtration.

Decant the supernatant solution and evaporate the solvent.

Water evaporates off, yielding pure crystalline sodium chloride.

Evaporation separates a non-volatile soluble solid solute from its volatile liquid solvent.

Key Concept

Sequential Separation of Multi-Component Mixtures

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