Some notes on Cation testing. 🙂
Some students have problem remember these coloured precipitates so i came up with some funny statements for students to remember these chemistry facts easier. 🙂 🙂
There are 3 coloured precipitates you have to know. Cu²+,Fe²+,Fe³+.
I have come up with a funny statement below to remember the colours associated with the ions. Hope its blesses you! 🙂
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Look at all the Caps Letters. 🙂
Cu²+ Blue ppt
Fe²+ Green (dirty green actually) ppt
Fe³+ Brown (reddish brown actually)ppt
For the white precipitates (Zn²+, Ca²+, Al³+, Pb²+), recall that ZAP are actually the amphoteric metal oxides ions. 🙂
Ca²+ forms a ppt that is insoluble in sodium hydroxide and yet forms no ppt with aqueous ammonia, hence pls test for this first. ?
If the ppt dissolves in excess sodium hydroxide, test with aq ammonia and see if the ppt dissolves in excess too. If it does, its Zn²+. (FUNNY MEMORY TIP : Z–>ZEBRA, A VERY MEEK ANIMAL SO IT KENNA ‘EATEN UP’ BY BOTH SOLUTIONS) 🙂
Both Pb²+ and Al³+ ions have the SAME reactions with sodium hydroxide and aq ammonia. Their ppt dissolves in excess sodium hydroxide but remains insoluble in excess aq ammonia.
Irritating max right?? Keke. Nevermind, just means 1 more point to exercise our memorising muscle.
lead cation can be further distinguished from aluminium cation by adding aqueous potassium iodide. If lead cation is present, a yellow ppt. of lead (II) iodide is formed.
I hope you will be thinking WHY aluminium iodide is not formed by adding potassium iodide meh?
Yes!! No reaction! No ppt observed!
In sec 3/4 syllabus, just note aluminium iodide is obtained from a reaction between metallic aluminum or aluminum hydroxide with hydrogen iodide or hydroiodic acid. OR aluminium iodide may be prepared by heating aluminium with iodine in a sealed tube or by passing iodine vapour over heated aluminium.
Enjoy! Next week i will test the sec 3/4 chemistry students. If you receive my WA msg on this blog post link, its You! Keke.