Unite 2 chemical tests

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Group 7 - silver nitrate

Used to identify the chloring bromine and iodine IONS.

Doesn't react with flourine as flourine has weak F-F bonds. Therefore it froms hydrogen bonds with water molecules and becomes soluble in water. So it cannot form a metal precipitate with silver.

1) Add nitric acid to the soloution to remove any carbonate or hydroxide ions. These could form silver carbonate or hydroxide precipitate.

2)Add silver nitrate to the soloution

Flourine- no precipitate

Chlorine- white precipitate

Bromine- cream precipitate

Iodine- yellow precipitate

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Group 7 - further test (ammonia)

Add ammonia to the chlorine, bromine and iodine to diffrentiate them

Chlorine - dissolves in dilute ammonia

Bromine - dissolves in concentrated ammonia

Iodine- insoluble in concentrated ammonia

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Test For sulphate ions

To test for sulphate ions in a soloution you can use barium sulphate as it is virtually insoluble.

- Add nitric or hydchloric acid to acidify the soloution- this removes carbonate ions as carbon dioxide

-Add barium chloride to the soloution

-If a sulphate is present the chlorine will be displaced and a white precipitate of barium sulphate will form

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Distinguishing between aldehydes and ketones

  • The Tollens Test-

Add tollens reagant to the soloution (it is a mixture of ammonia and silver nitrate)

This will oxsidise the aldehydes but will have no affect on the ketones

On warming the aldehyde with tollens reagant a deposit of metallic silver will form on the inside of the test tube. Will create a silver mirror

  • The Fehlings/Benedicts Test-

Both the Fehlings or the Benedicts solution will have the same effect. They are both gentle oxsidising reagants. They will oxsidise aldehydea but not ketones. 

On heating the reagant with the aldehyde the soloution will change colour from blue to a brick red precipitate

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