Tests for Cations (+)

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1) NaOH precipitate test
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
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Method:
Add a few drops of NaOH to the unknown substance. This will form a coloured precipitate
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Copper (II), Cu2+
Light blue precipitate. Cu2+ + 2OH- ==>Cu(OH)2
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Iron (II), Fe2+
Sludgy green precipitate. Fe2+ +OH- ==> Fe(OH)2
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Iron (III), Fe3+
Reddish-brown precipitate. Fe3+ +OH- ==> Fe(OH)3
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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
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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
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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
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Lithium (Li+)
Bright red
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Sodium (Na+)
Golden yellow
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Potassium (K+)
Lilac
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Calcium (Ca2+)
Brick red
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Copper (Cu2+)
Blue-green
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3) Test for ammonium ions
NH4+ + OH- ==> NH3 + H2O
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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
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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
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Card 2

Front

Method:

Back

Add a few drops of NaOH to the unknown substance. This will form a coloured precipitate

Card 3

Front

Copper (II), Cu2+

Back

Preview of the front of card 3

Card 4

Front

Iron (II), Fe2+

Back

Preview of the front of card 4

Card 5

Front

Iron (III), Fe3+

Back

Preview of the front of card 5
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