Identification of Cations

Question 1

What are the reactions of group 1 metal cations (Na+,Li+,K+) and how are they identified?

Answer 1

No ppt with NaOH(aq)
No ppt with Na2CO3(aq)
must use flame test for identification,
Na+ -> yellow flame
Li+ -> red flame
K+ -> lilac flame

Question 2

What are the reactions of group 2 metal cations except for Mg2+,(Ca2+, Sr2+, Ba2+) and how are they identified?

Answer 2

No ppt with NaOH(aq)
White ppt with Na2CO3(aq)
must use flame test for identification,
Ca2+ -> brick red
Sr2+ -> crimson
Ba2+ -> yellow-green

Question 3

How is Mg2+ identified?

Answer 3

Forms a white ppt with NaOH(aq), this white ppt is insoluble in excess NaOH as Mg(OH)2 is NOT AMPHOTERIC.

Question 4

How is Zn2+ identified?

Answer 4

White ppt with NaOH(aq) which dissolves in excess NaOH as the Zn(OH)2 formed is amphoteric.
Zn2+ does react with NH4OH(aq) to form a white ppt which dissolves in excess NH4OH(aq) due to the formation of [Zn(NH304]2+ complex

Question 5

How is Al3+ identified?

Answer 5

White ppt with NaOH(aq) that dissolves in excess NaOH as the Al(OH)3 is amphoteric .
It does form a white ppt with NH4OH(aq) but this does not dissolve in excess NH4OH(aq)
Forms no ppt with HCl
does not reduce KMnO4 to create a purple to pink colour change.

Question 6

How is Pb2+ identified?

Answer 6

forms a white ppt with NaOH(aq) which dissolves in excess as Pb(OH)2 is amphoteric.
Also forms a white ppt with HCl which dissolves when heated.

Question 7

How is Sn2+ identified?

Answer 7

Forms a white ppt with NaOh(aq) that dissolves in excess as Sn(OH)2 is amphoteric.
Sn2+ ions are reducing agents and thus reduces KMnO4 which has a colour change from purple to pink.