What are the tests for cations?
flame test or precipitation with sodium hydroxide
Describe how you would carry out a
Flame Test.
- wear safety goggles and a lab coat
- work in a fume cupboard
- dip nichrome wire in concentrated hydrochloric acid to clean
- heat in a roaring Bunsen flame
- add a few drops of concentrated hydrochloric acid to the solid salt
- mix until the solid starts to dissolve
- dip a clean nichrome wire into the mixture
- hold the wire in a roaring Bunsen flame
- observe the colour
What happens inside an atom during a
flame test?
- the electrons gain energy as heat
- they release energy straight away as light
- each atom has electrons at different energy levels so will release a specific light wavelength
What is the flame colour of Lithium ion?
(Li+)
bright red
What is the flame colour of Sodium ion?
(Na+)
yellow
What is the flame colour of Potassium ion?
(K+)
lilac
What is the flame colour of Calcium ion?
(Ca2+)
brick red
/orange-red
What is the flame colour of Copper ion?
(Cu2+)
blue-green
(green with blue flecks)
What is the flame colour of Strontium ion?
(Sr2+)
deep red
What is the flame colour of Barium ion?
(Ba2+)
apple green
What is the flame colour of Magnesium ion?
(Mg2+)
bright white
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Aluminium ions?
(Al3+)
A white precipitate is formed
(it does redissolve in excess)
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Calcium ions?
(Ca2+)
A white precipitate is formed.
It does not redissolve in excess.
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Copper ions?
(Cu2+)
A blue precipitate is formed.
(similar to the colour of the solution)
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Iron(II) ions?
(Fe2+)
A green precipitate is formed.
The precipitate would go brown over time due to oxidation with the air.
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing Iron(III) ions?
(Fe3+)
A red-brown precipitate is formed.
The precipitate goes more brown as it oxidises with the air.
What would be the expected observation if aqueous sodium hydroxide was added to a solution containing ammonium.
(NH4+)
No precipitate is formed.
Describe how you would test for
halide ions.
(Cl-, Br-, I-)
- add a few drops of nitric acid (to remove any carbonate or hydroxide impurities which could produce false positives)
- add aqueous silver nitrate solution
- a precipitate indicates the presence of a silver halide, observe the colour of this precipitate to determine what anions are present
What colour precipitates do halides Cl-, Br- and I- form with silver nitrate?
Chlorine forms silver chloride (AgCl) which is white
Bromine forms silver bromide (AGBr) which is cream
Iodine forrms silver iodide (AgI) which is pale yellow
The precipitates go darker (and more yellow) as you go down the column.
The precipitates go grey over time afer reacting with UV light.
Describe how you would test for
carbonate ions.
(CO32-)
- add dilute nitric acid
- bubbles indicate the presence of carbonate
Describe how you would test for
sulfate ions.
(SO42-)
- add a few drops of hydrochloric acid
- add aqueous barium chloride or barium nitrate (to remove carbonate impurities)
- a white precipitate indicates the presence of sulfate ions
Why are instrumental methods of analysis better than simple chemical tests?
They are faster, more accurate and more sensitive.
What is a
flame photometer?
A scientific instrument based on flame testing.
What does the flame photometer do?
Identifies metal ions in a sample and determines the concentration of these in solutions.
