Chemical tests help identify the presence of different cations.🤓
Take a look at this example🙃
2.37 Describe tests for the cations
i) Li+,Na+, K+, Ca2+ using flame tests
ii) NH4+, using sodium hydroxide solution and identifying the ammonia gas evolved
iii) Cu 2+, Fe2+ and Fe3+ , using sodium hydroxide solution
Part I: Flame Test
Procedure: First, dip a clean nichrome wire into concentrated hydrochloric acid. Then, dip the nichrome wire into each of the cations. Finally, put the end of the wire in the Bunsen flame and observe the color of the flame.
Li+ : Deep Red
Na+ : Golden-yellow
K+ : Lilac
Ca2+: Brick-red
Part II
NH4+(aq) + OH- (aq) →NH3 (g) + H2O (l)
A solution containing NH4+not give any precipitate with sodium hydroxide. Instead, a solution containing NH4+ gives off ammonia gas when warmed with sodium hydroxide.
🧐Further Details:
NH3 (g) turns moist red litmus paper blue
Part III
Procedure: Prepare solutions containing Cu 2+ , Fe2+ and Fe3+ ions and add sodium hydroxide to each of the sample solutions. The cations can be distinguished based on the observations
1) Cu2+ (aq) + 2OH- (aq) →Cu(OH)2 (s)
🧐Formation of a pale blue precipitate
2) Fe2+ (aq) + 2OH- (aq) → Fe(OH)2 (s)
🧐Formation of a green precipitate.
Further Details:
The green Fe(OH)2 turns brown on prolonged standing in air due to the formation of Iron (III) hydroxide :
4Fe(OH)2 (s) + 2H2O (l) + O2 (g) → 4Fe(OH)3 (s)
3)Fe3+ (aq) + 3OH- (aq) →Fe(OH)3 (s)
🧐Formation of a reddish brown precipitate
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