Testing for Cations

Edited by Jamie (ScienceAid Editor), SpellBot, Anonymous


Flame Test

A flame test can be used where a a compound is put under a flame. The procedure is as follows:

  1. 1
    Heat the nichrome wire
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  2. 2
    Dip in the hydrochloric acid
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  3. 3
    Dip wire into compound so a small blob is collected on the wire
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  4. 4
    Put under a flame and see what color it turns.
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how to carry put the flame test

These are the colours you will see for different ions:

Ion Color
Sodium (Na+) Orange-yellow
Potassium (K+) Lilac
Calcium (Ca2+) Brick-red
Copper (Cu2+) Green

Sodium Hydroxide Test for Cations

Add several drops of sodium hydroxide (NaOH) solution to the solution being tested. If a coloured precipitate is formed then stop and find out what the cation is. If a white precipitate forms then continue to add NaOH to it and observe whether the precipitate dissolves.

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Cation Precipitate color Further results
Aluminium (Al3+) White Precipitate dissolves as more NaOH is added to the solution
Calcium (Ca2+) White Precipitate will not dissolve in the NaOH solution
Copper (Cu2+) Pale blue none
Iron(II) (Fe2+) Pale green none
Iron(III) (Fe3+) Red-brown none

The ionic equation for these reactions are all very similar, here is an example it with Aluminium:

Al3+(aq) + 3OH- ==>> Al(OH)3(s)

All you have to do for any other ionic equations for this test is to change the number of OH- ions so that it balances with the oxidation state of the metal anion. E.g. Iron (II) would need two OH- whereas Iron(III) needs 3.

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Article Info

Categories : Applied

Recent edits by: SpellBot, Jamie (ScienceAid Editor)

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