Qualitative analysis- tests for ions Flashcards
What is an ion?
A charged atom, molecule or particle.
Why must a test for a specific ion be unique?
Tests are used to identify ions and must
be unique so that the test gives an easily
observed result which is specific to the
ion that is present.
Describe how you would carry out a flame test to test
for ions
- Clean a nichrome wire using hydrochloric acid.
- Turn the Bunsen burner onto the blue flame.
- Dip the wire in a solution of the substance being
tested. - Place the wire in the flame and record the
colour
What is the result of the flame test on lithium ions?
Crimson flame
What is the result of the flame test on sodium ions?
Yellow flame
What is the result of the flame test on potassium
ions?
Lilac flame
What is the result of the flame test on calcium ions?
Orange flame
What is the result of the flame test on copper ions?
Blue-green flame
Why must the wire be cleaned before carrying out a
flame test?
To remove any unwanted ions that might
obscure the colour of the flame
Why can a flame test not be used when a compound
contains a mixture of metal ions?
The flame colours of the ions will blend
together so the individual flame colours
won’t be seen meaning the ions can’t be
identified.
What is a precipitate?
An insoluble solid suspended in a liquid.
Without using the flame test, how can you test for
aqueous metal ions?
Add sodium hydroxide solution to the
metal ions. Observe the colour of the
precipitate.
What colour precipitate forms when sodium
hydroxide reacts with calcium ions?
White
What colour precipitate forms when sodium
hydroxide reacts with copper(II) ions?
Blue
What colour precipitate forms when sodium
hydroxide reacts with iron(II) ions?
Green
What colour precipitate forms when sodium
hydroxide reacts with iron(III) ions?
Brown
What colour precipitate forms when sodium
hydroxide reacts with aluminium ions?
White (initially)
With excess NaOH, the precipitate
re-dissolves to form a colourless solution.
What is observed when sodium hydroxide reacts
with ammonium ions?
A pungent smelling gas is produced
(NH3) which will turn damp red litmus
paper blue.
How could you distinguish between two solutions
containing aluminium ions and calcium ions?
Add excess sodium hydroxide.
Both will form white precipitates initially. The
precipitate formed from aluminium ions will
re-dissolve to form a colourless solution.
How can you test for carbonate ions?
- Add a few drops of HCl to the sample in a test tube.
- Connect this test tube to a test tube of limewater.
- If carbonate ions are present, they will react with the
acid to produce carbon dioxide which will turn the
limewater cloudy when it is bubbled through
limewater.
Write the chemical equation for the reaction between
HCl and Na2CO3
Na2CO3 + 2HCl → CO2 + 2NaCl + H2O (check google if unsure)
How can you test for sulfate ions?
- Add HCl to remove any CO3 ^2- ions as these will obscure the results. - Add a couple of drops of barium chloride. - If sulfate ions are present a white precipitate of barium sulfate will form.
Write the chemical equation for the reaction between
BaCl2
and MgSO4
BaCl2 + MgSO4 → BaSO4 + MgCl2
BaSO4 is a white precipitate
(check google if unsure)
How do you carry out a test for halide ions?
- Add a few drops of nitric acid to react with
any carbonate ions which might obscure
the result. - Add a couple of drops of silver nitrate.
- Observe the colour of the precipitate.
What colour precipitate is formed when silver nitrate
is added to a chloride solution?
White precipitate of silver chloride
What colour precipitate is formed when silver nitrate
is added to a bromide solution?
Cream precipitate of silver bromide
What colour precipitate is formed when silver nitrate
is added to an iodide solution?
Yellow precipitate of silver iodide
What is meant by the phrase instrumental methods
of analysis? Give an example of an instrumental
method used for the analysis of elements or
compounds
Methods of analysis that use machinery
such as mass spectrometry and gas
chromatography.
What are the benefits of using instrumental methods
for analysis?
- Accurate
- Sensitive
- Quick to carry out
What is a flame photometer?
A device used in inorganic analysis. It
can be used to identify or determine the concentration of metal ions.
How can a flame photometry be used to identify metal ions?
An emission spectrum is produced by the flame photometer. Each metal ion produces a unique spectrum so comparing the unknown spectrum to reference spectra can identify the ion
How can a flame photometry be used to determine
the concentration of metal ions?
Take readings using a flame photometer of the metal ions at different concentrations. Plot a calibration curve. Take a reading of the unknown sample and compare to the curve.
What is the major advantage of flame photometry compared to simple flame testing?
Flame photometry can be used to analyse a mixture of ions whereas flame tests can only be used to identify one ion at a time.
ROYGBIV =__________
LiCaNaCuK
R=Li O=Ca Y=Na GB=Cu IV=K
