RQP 4: Testing for ions Flashcards
What is the trend in the hydroxides of group two elements
They get more soluble as you go down the group
Descirbe the practical method for testing for gorup 2 metal ions and transition metal ions in solution
- Place about 10 drops of the metal ion solution in a test tube
- Add ten drops of sodium hydroxide solution, mixing well.
- Continue to add sodium hydroxide solution dropwise with gentle shaking until in excess
Describe how to test for ammonium ions NH4+
- Place 10 drops of the solution in a test tube and add an equal amount of sodium hydroxide solution.
- Shake the mixture and warm gently using a water bath.
- Test the fumes by holding a piece of damp red litmus paper in the mouth of the test tube- alkaline ammonia gas it released which turns the red litmus paper blue.
What are the results when sodium hydroxide is added to test for group two elements
- Magnesium hydroxide is insoluble in water so produces a white precipitate,. The pH is slightly alkaline (around pH 9)
- Calcium hydroxide is sparingly soluble so appears as a suspended white precipitate. The solution in alkaline so has a pH of around 11.
- Strontium and barium salts will not form a hydroxide precipitate due to their high solubility. Their solutions with sodium hydroxide are highly alkaline.
What is the trend in the sulfates of group two elements
They become less soluble down the group
What are the results when sodium hydroxide is aded to test for group 3 transition metal ions
- Aluminium saltsa will form a white precipitate of aluminium hydroxide (complex ion) which will dissolve in excess sodium hydroxide to form a colourless solution
- Copper solutions ofrm a blue ppt
- Iron (II) solutions form a green precipitate
- Iron (III) solutions form a brown precipitate
What is the alternative test to adding sodium hydroxide to test for group 2 metal ions and what are the results
- Add sulfuric acid (or another soluble sulfate solution)
- Strontium and barium solutions form white precipitates of their sulfates
- The other elements will not form a precpitate due to their high solubility
Describe how to test for the presence of a sulfate ion
- Add BaCl2 solution acidified with hydrochloric acid to the sample to test for sulfate ions
- If the sample contains sulfate ions, a white precipitate of barium sulfate forms .
Why is the barium chloride acidified with hydrochloric acid when testing for sulfate ions
The hydrochloric acid is needed to react with carbonate impurities that are often found in salts which would form a white barium carbonate precipitate and so give a false result.
Describe how to test for halide ions
- Acidify with nitric acid then add silver nitrate.
- Chloride ions form a white precipirate
- Bromide ions form a cream precipitate
- Iodide ions form a yellow precipitate
- Then add dilute ammonia, followed by concentrated ammonia
- Silver chloride redissolves in dilute ammonia
- Silver bromide redissolves in concentrated ammonia
- Silver iodide does not react further
How do you test for the presence of carbonate ions
- Add any dilute acid- effervescence is observed
- If this gas is bubbled through limewater is turns it cloudy
How do you test for the presence of hydroxide ions
Alkaline hydroxides turn red litmus paper blue
What would happen after an insoluble salt has been made in solution
- it would be removed by filtration
- then it would be washed with distilled water to remove soluble impurities
- it would then be dried on filter paper
What are the two types of filtraiton apparatus you can use and when do you use them
- Gravitational filtration: filter paper and funnel. Used when small amounts of solid are formed.
- Vaccum filtration: a buchner funnel woth filter paper is connected to a flask that has a water pump producing a vaccum. Used if larger amounts of solid are formed.
