What is a cation?

A cation is a positively charged ion.

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What are the two different tests we can use for identifying metal ions?

  1. The flame test.
  2. The sodium hydroxide precipitate test.

(you could also use flame emission spectroscopy / flame photometry)

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What is a flame test?

A method of identifying metal ions by seeing what colour flame they produce when they burn.

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Outline the steps required to perform a flame test.

  1. Take a platinum (or nichrome) wire loop, and clean it by dipping it in some dilute hydrochloric acid, rinsing it in distilled water, and then heating it over a Bunsen burner flame.
  2. Dip the wire loop into the compound you want to test.
  3. Hold the wire loop in the clear blue part of the Bunsen burner flame (this is the hottest part).
  4. See what colour the flame turns as the compound burns.

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What colour will lithium ions burn with? 

Lithium burns with a crimson flame.

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What colour will sodium ions burn with? 

Sodium ions burn with a yellow flame.

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What colour will potassium ions burn with? 

Potassium ions burn with a lilac flame.

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What colour will calcium ions burn with? 

Calcium ions burn with an orange-red flame.

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What colour will copper ions burn with? 

Copper ions burn with a green flame.

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Why does the flame test not work very well if the sample contains multiple metals?

The different flame colours will mask each other.

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Why should the wire be clean for a flame test?

So there are no impurities which could alter the colour of the flame.

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How do you test for metals using sodium hydroxide?

Add sodium hydroxide to the mystery compound.


The colour change will indicate which metal is in the compound.

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When performing the sodium hydroxide test for metal ions, what is the colour of the calcium precipitate?

White

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When performing the sodium hydroxide test for metal ions, what is the colour of the copper precipitate?

Blue

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When performing the sodium hydroxide test for metal ions, what is the colour of the Fe2+ (iron 2+) precipitate?

Green

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When performing the sodium hydroxide test for metal ions, what is the colour of the Fe3+ (iron 3+) precipitate?

Brown

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When performing the sodium hydroxide test for metal ions, what is the colour of the aluminium precipitate?

White at first.


Then redissolves in excess sodium hydroxide to form a colourless solution

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When performing the sodium hydroxide test for metal ions, what is the colour of the magnesium precipitate?

White

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