Some cations have different charges, e.g. Fe(II) (Fe2+) and Fe (III) (Fe3+). So you need to do a NaOH precipitate test to differentiate the charges
1 of 16
Method:
Add a few drops of NaOH to the unknown substance. This will form a coloured precipitate
2 of 16
Copper (II), Cu2+
Light blue precipitate. Cu2+ + 2OH- ==>Cu(OH)2
3 of 16
Iron (II), Fe2+
Sludgy green precipitate. Fe2+ +OH- ==> Fe(OH)2
4 of 16
Iron (III), Fe3+
Reddish-brown precipitate. Fe3+ +OH- ==> Fe(OH)3
5 of 16
2) Flame tests
Compounds of some metals burn with a characteristic colour. You can test for various metal ions by heating your substance and seeing if it burns with a distinctive coloured flame
6 of 16
Method:
Dip a platinum wire loop into HCl acid then hold it in a flame. Place the wire loop in the sample then into the flame again and observe the colour
7 of 16
Why is HCl used?
1) To clean the wire loop and avoid contamination. 2) HCl has a negative charge and we are only testing for positives (cations), therefore it won't interfere with the flame test
8 of 16
Lithium (Li+)
Bright red
9 of 16
Sodium (Na+)
Golden yellow
10 of 16
Potassium (K+)
Lilac
11 of 16
Calcium (Ca2+)
Brick red
12 of 16
Copper (Cu2+)
Blue-green
13 of 16
3) Test for ammonium ions
NH4+ + OH- ==> NH3 + H2O
14 of 16
Method:
Add a few drops of NaOH to a sample and gently warm it. If the solution contains ammonium ions, ammonia will be given off as well as water
15 of 16
Test for ammonia
1) Turns damp RED litmus paper BLUE. 2) Forms a white smoke of ammonium chloride when hydrogen chloride gas, from concentrated hydrochloric acid, is held near it. 3) Begins to smell
16 of 16
Other cards in this set
Card 2
Front
Method:
Back
Add a few drops of NaOH to the unknown substance. This will form a coloured precipitate
Comments
No comments have yet been made