Tests For Cations

Chemistry Unit 3 revision on testing for Cations. Remember, a cation is a positive metal ion. These tests below assume your sustance is ionic, however, yours might not be. Ionic substances tend to be crystalline solids with a high melting point. Therefore, if your substance is a gas, a volitile liquid or a soft solid you do not need these tests! 

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Tests for Cations - Flame Tests

Flame Tests identify some Metal Ions.

The compounds of some metals burn with a certain colour which means that you can identify them by their colour.

You just have to remember the colours!  

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Flame Test - Lithium

Lithium (Li+)

Burns with a crimson-red flame.

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Flame Test - Sodium

Sodium (Na+)

Burns with a yellow-orange flame.

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Flame Test - Potassium

Potassium (K+)

Burns with a lilac flame.

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Flame Test - Calcium

Calcium (Ca2+)

Burns with a brick-red flame.

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Flame Test - Barium

Barium (Ba2+)

Burns with a green flame.

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Hydroxide Test

Hydroxide Tests identify some Metal Ions.

Some metals form a coloured precipitate with NaOH.

1. Many metal hydroxides are insoluable and predipitate out of a solution when formed. Some of these hydroxides have a characteristic colour.

2. In this test you add a few drops of sodium hydroxide solution to a solution of your mystery compound in the hope of forming an insoluable hydroxide.

3. If you get a coloured insoluable hydroxide you can then tell which metal was in the compound.

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Hydroxide Test - Calcium

Calcium (Ca2+)

Colour of Precipitate: White

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Hydroxide Test - Copper(II)

Copper(II) (Cu2+)

Colour of Precipitate: Blue

 

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Hydroxide Test - Iron(II)

Iron(II) (Fe2+)

Colour of Precipitate: Sludgy green

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Hydroxide Test - Iron(III)

Iron(III) (Fe3+)

Colour of Precipitate: Reddish brown

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Hydroxide Test - Aluminium

Aluminium (Au3+)

Colour of Precipitate: White at first. But then redissolves in excess NaOH to form a colourless solution.

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Hydroxide Test - Magnesium

Magnesium (Mg2+)

Colour of Precipitate: White

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Test for an Ammonium Compound

Ammonium Compound + NaOH = Stinky Ammonia

You can use this to test for ammonium ions (NH) using sodium hydroxide.

Add sodium hydroxide to a solution of your mystery substance.

Ammonia given off means there are ammonia ions present in the compound.

No nasty ammonia smell means no ammonium ions.

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Warning!

Remember, a cation is a positive metal ion.

These tests below assume your sustance is ionic, however, yours might not be.

Ionic substances tend to be crystalline solids with a high melting point.

Therefore, if your substance is a gas, a volitile liquid or a soft solid you...

DO NOT NEED THESE TESTS!

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