Cations and Anions

tests for positive ions ( the flame test and precipitation reactions) and tests for negative ions (testing for carbonates, halides and sulphates)

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Flame Test

1) Make sure the wire loop is clean by dipping it into hydrochloric acid

2) Rinse with distilled water

3) Dip the wire loop into a sample of the compound

4)Put the wire loop into the blue bit of the bunsen flame, this is the hottest part

5)See what colour the flame goes

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The colours that the metal ions are

Lithium - crimson flame

Sodium- yellow flame

Potassium- lilac flame

Calcium- red flame

Barium- green flame

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Precipitation Reaction (Positive Ions)

1) Place your compound on a plate

2) Add a few drops of Sodium Hydroxide to the solution

3) If a coloured precipitate forms then you can identify the metal that was in the solution

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The coloured precipitate

Calcium - white

Copper- blue

Iron (II)- green

Iron(III)- brown

Aluminium- white,  but it redissolves in excess NaOH to form a colourless solution

Magnesium- white

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Testing for Carbonates (Negative Ions)

1) React the carbonate soultion with a dilute acid

2) It  will form carbon dioxide as a by-porduct because:

Acid + Carbonate ----------> Salt + Water+ Carbon Dioxide

3) If the compound is a carbonate then carbon dioxide will be produced which can be bubbled through lime water.

4) The lime water will go cloudy if carbon dioxide is present

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Testing for Halides (Negative Ions)

To test for Chloride, bromide or iodide ions:

1) Add dilute nitric acid to the solution

2) Add silver nitrate solution

3) If a halide ion is present then a precipitate will form

4) The halide ion can be identified depending on the colour of the precipitate.

Chloride forms a white precipitate

Bromide forms a cream precipitate

Iodid forms a yellow precipitate

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Testing for Sulphate Ions (Negative Ions)

1) Have a solution that contains sulphate ions

2) Add dilute hydrochloric acid

3) Then add barium chloride solution

4) A white precipitate will form. This is barium sulphate and it means that the original solution contained sulphate ions.

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