5. TESTS TO IDENTIFY METAL IONS

test tube reactions

a qualitative method of working out the identity of unknown metal ions in solution

adding different reagents like sodium hydroxide, ammonia and sodium carbonate

and recording observations help you identify what metal ion is present

what to do:

• measure out samples of unknown metal solutions into 3 seperate test tube

• for tt1 add sodium hydroxide dropwise using pipette and record changes, then add more in excess n record changes

• to tt2 add ammonia solution dropwise using pipette and record changes then add in excess n record changes

• for tt3 add sodium carbonate solution dropwise n record observations

you need to know the reactions for

• Cu²⁺

• Fe²⁺

• Fe³⁺

• Al³⁺

aqua ions

AQUA IONS + NaOH

• all form precipitates

• only aluminium hydroxide WHITE precipitate will dissolve in excess NaOH

• because it is amphoteric

AQUA IONS + AMMONIA

• all form precipitates

• only copper hydroxide precipitate will dissolve in excess ammonia leaving a deep blue solution

• because it undergoes ligand substitution

AQUA IONS + Na2CO3

• all form precipitates

• Al³⁺ and Fe³⁺ form CO2 gas, so you will observe bubbling

• this test differentiates between green Fe²⁺→ green ppt and yellow Fe³⁺→ brown ppt + bubbles

Cu²⁺ metal aqua ion solution

BLUE

Fe²⁺ metal aqua ion solution

GREEN

Al³⁺ metla aqua ion solution

COLOURLESS

Fe³⁺ metal aqua ion solution

YELLOW