Mandatory Experiment 1.1
Flame tests (Li, Na, K, Ba, Sr, Cu)
Student Material
Theory
The characteristic colours given off when salts of certain metals are heated in the flame
of a Bunsen burner are noted. The colours obtained can then be used to identify the
metals present in unknown compounds.
Method 1 – using a platinum wire
Chemicals and Apparatus
Lithium chloride
Sodium chloride
Potassium chloride
Barium chloride
Strontium chloride
n
n
Copper(II) chloride
Concentrated hydrochloric acid
Platinum (or nichrome) wire held in glass rod
Bunsen burner
Six small beakers
Test tubes
Pestle and mortar
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Procedure
NB: Wear your safety glasses.
1. Using the fume cupboard, place some concentrated hydrochloric acid in a test
tube. Dip the platinum wire into the concentrated hydrochloric acid in the test
tube. This helps to clean the wire.
2. Hold the platinum wire at the tip of the blue cone of a Bunsen flame. If the wire is
clean there should be no colour from it when it is placed in the Bunsen flame. If
there is a colour, repeat the cleaning process.
Fig. 1
3. Crush a little of the salt to be tested with a pestle and mortar.
4. Again dip the platinum wire in concentrated hydrochloric acid and then in the salt
to be tested. Some of the salt should stick to the wire.
5. Hold the platinum wire in the flame of the Bunsen burner (Fig. 1) and note the
colour given off.
6. Repeat the experiment for each of the other salts given to you by your teacher.
(To avoid cross contamination, it is best to have a separate platinum wire for each
sample of salt. If this is not possible, leave the sodium salt until last, as its colour
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is very persistent.) Again, note the colour in each case. If you are given an
unknown salt, you should be able to identify the metal in the salt from the results
of your experiment.
Method 2 – using a soaked wooden splint
As platinum wire is expensive, many schools may not possess it in sufficient quantities
for this experiment. Wooden splints (e.g. lollipop sticks) soaked overnight in water work
quite well in place of the platinum. You will need a wooden splint for each sample to
avoid cross contamination.
Chemicals and Apparatus
Lithium chloride
Sodium chloride
Potassium chloride
Barium chloride
Strontium chloride
n
n
Copper(II) chloride
Bunsen burner
Six small beakers
Test tubes
Pestle and mortar
Wooden splints (e.g. lollipop sticks)
Procedure
NB: Wear your safety glasses.
1. Crush a little of the salt to be tested with a pestle and mortar.
2. Label 6 beakers, and place a small quantity of the corresponding salt in each.
3. Dip the soaked splint in the salt to be tested. Some of the salt should stick to the
splint.
4. Gently wave the splint in the flame of the Bunsen burner and note the colour
given off. Be careful not to hold the splint too long in the flame, as charring may
occur and give a false colour.
5. Repeat the experiment for each of the other salts. To avoid cross contamination,
use a separate splint for each salt. Again, note the colour in each case.
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6. If you are given an unknown salt, you should be able to identify the metal in the
salt from the results of your experiment.
Questions relating to the experiments
1. What colour is observed in each case when the following salts are heated in a
flame: copper sulfate, lithium carbonate, sodium sulfate, strontium nitrate, barium
nitrate, potassium sulfate?
2. Why do metals give off a characteristic colour in a flame?
3. What procedure is used to avoid cross contamination?
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Teacher Material

Potassium salts are often contaminated with sodium. For this reason the yellow
colour of sodium may interfere with the lilac colour of the potassium.

Each metal gives off a characteristic colour. These colours are lithium (deep red),
sodium (yellow), potassium (lilac), barium (yellow-green), strontium (red) and
copper (blue-green).

If wooden splints are being used, do not leave them in the flame for too long.
Gently pass through the flame and observe the colour.

If too much salt adheres to the splint, there is a danger of some falling into the
Bunsen burner. This is likely to cause a prolonged colour tinge in the flame,
making the Bunsen burner unsuitable for further use in this experiment.

An alternative to soaking wooden splints in water is to soak them overnight in
solutions of the salts to be tested. This eliminates the potential problem of
contamination of the Bunsen burner by solid material.

Nichrome wire may be used instead of platinum wire in Method 1.

If a platinum or nichrome wire not held in a glass rod is used, then a tongs
should be used to hold the wire in the flame.

The wooden splint method has two advantages: The problem of cross
contamination is eliminated, and there is no need to use concentrated
hydrochloric acid for cleaning.
Safety considerations
Safety glasses must be worn.
Chemical hazard notes
Concentrated hydrochloric acid
very irritating to lungs.
Barium chloride
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n
: Very corrosive to eyes and skin, and its vapour is
is harmful by ingestion and inhalation.
Copper(II) chloride
Lithium chloride
is toxic if swallowed, and is an eye and skin irritant.
n is
harmful by ingestion, and is a severe eye and skin irritant.
Potassium chloride is an eye irritant.
Disposal of wastes
Dilute with excess water, neutralise (if necessary) with anhydrous sodium carbonate, and
flush to the foul water drain.
Solutions to student questions
1. What colour is observed in each case when the following salts are heated in a
flame: copper sulfate, lithium carbonate, sodium sulfate, strontium nitrate, barium
nitrate, potassium sulfate?
Copper sulfate: Blue-green
Lithium carbonate: Deep red
Sodium sulfate: Yellow
Strontium nitrate: Red
Barium nitrate: Yellow-green
Potassium sulfate: Lilac
2. Why do metals give off a characteristic colour in a flame?
The energy differences between energy levels in metal atoms vary from metal to
metal. Using energy from the flame, electrons in the metal atoms move to higher
energy levels, and then return to lower energy levels, emitting light whose
energies in each case is equal to the energy difference between the higher energy
level and the lower energy level.
3. What procedure is used to avoid cross contamination?
If platinum wire is used, either a separate platinum wire is used for each test, or
the wire is thoroughly cleaned with concentrated hydrochloric acid after each test.
If wooden splints are used, a separate splint is used for each test.
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