CP 07 & CP 15 - Analysis of unknown compounds Flashcards
How can a flame test be used to identify
metal ions?
● Clean wire loop with H2SO4
or HCl
● Dip the loop into the sample.
● Place the loop into a blue flame (using a bunsen
burner).
● Record the colour of the flame
What are some group 1 metal ion flame
colours?
What are some group 2 metal ion flame
colours?
How can you use sodium hydroxide to
identify cations?
● Dissolve the unknown substance in water.
● Add aqueous NaOH dropwise, until in
excess and no further change occurs.
How do you test for a saturated
hydrocarbon?
Add bromine water to the sample. If the
solution turns from orange to colourless
then a unsaturated hydrocarbon was
present.
what are the 3 ways to test for an aldehyde and how are they performed?
● Add Tollen’s reagent to a sample of the suspected aldehyde.
Warm → The presence of a silver mirror confirms an
aldehyde was present.
● Add Fehling’s/Benedict’s solution → aldehyde turns clear
blue solution to brick red ppt
● Add dichromate ions (eg. K2Cr2O7 / Na2Cr2O7) acidified
eg. with H2SO4 & heat → turns orange solution green
How do you test for a carboxylic acid?
● Add an alcohol & acid (eg. H2SO4) → glue or
pear-drop smell (formation of an ester).
● Add a metal carbonate → effervescence as CO2 is
released (test with limewater)
● Add a reactive metal → effervescence as H2 is
released
● Add an acyl chloride → misty fumes (HCl
How do you test for ammonium (NH4+) ions?
● Add an equal volume of NaOH to the sample.
Shake.
● Warm the solution in the test tube.
● Test the gas released with damp red litmus paper.
● If it goes blue, ammonium ions are present.
How do you test for group 7 / halide
ions extensively?
● To the compound being tested, add
nitric acid and silver nitrate. Record
observations.
● To samples of this solution, add dilute
and then concentrated ammonia.
draw the table forthe results from the reactions
of halide ions with silver nitrate and
ammonia?
How do you test for carbonate (CO32-)
and hydrogencarbonate (HCO3-) ions?
● Add dilute acid.
● If carbonate/hydrogencarbonate ions are present, the
solution will effervesce.
● You can bubble the gas given off through limewater and it
should go cloudy, confirming the presence of carbonate /
hydrogencarbonate ions.
How do you test for sulfate (SO42-) ions?
● Add HCl and BaCl2 to the suspected sulfate solution.
● If sulfate ions are present, a white
precipitate of BaSO4
will form.
What is the order of testing ions?
(Carbonate, halide and sulfate)
Carbonate → Sulfate → Halide
This prevents false +ve results form occurring, e.g. unexpected insoluble precipitates like
Ag2SO4, Ag2CO3 and BaCO3
