ROBERT BUNSEN SCIENCE SHEET
Using flame tests to identify the metal part of a salt
In this experiment you will test a series of salts to find the characteristic flame colours
which enable identification of the metal part of the salt. You will also use an
instrument called a spectroscope to see more precisely which colours of visible light
are emitted by the metals in the flame.
What is a salt?
Salts are compounds which contain a metal part and a non-metal part
E.g. Ordinary table salt is sodium chloride – this is a compound of sodium and
chlorine. The metal part is sodium and the non-metal part is chloride.
Note that in sodium chloride the sodium is present as sodium ions, formula Na+, and
the chlorine is present as chloride ions, formula Cl−. Whereas sodium metal is made
up of sodium atoms and has the formula Na and chlorine gas is made up of chlorine
molecules (two chlorine atoms joined together) and has the formula Cl2.
We will use chloride salts and sulphate salts in our experiment.
Robert Bunsen and Flame Emission Spectroscopy
Robert Bunsen developed and improved the design of the Bunsen burner; the
original design was the work of Bunsen’s colleague Peter Desaga. What was the
improvement Bunsen introduced? How does this improvement alter (a) the
temperature and (b) the appearance of the Bunsen flame? Based on your experience
of flame tests why was this such a useful improvement?
In this experiment you will also use a spectroscope. Who invented the spectroscope
and what does it do?
Robert Bunsen made many important discoveries. His work on flame emission
spectroscopy resulted in the discovery of two new elements. What were these
elements and how did he discover them?
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Today flame emission spectroscopy (also sometimes known as flame photometry)
has many uses including an important application in medicine. Can you find out what
this medical application is?
The Spectroscope
Using the spectroscope you can see the line emission spectrum produced by a
particular metal. Can you explain why the spectrum is made up of distinct fine lines of
certain colours and not just all the colours (ie white light)? The explanation requires
an understanding of how electrons are arranged inside atoms – do some research on
this. Each line corresponds to a particular wavelength of visible light and each
wavelength has a particular amount of energy associated with it given by the
relationship
E = hf = hc
λ
Where f is the frequency and λ is the wavelength of the light.
h and c are constants: h is called ‘Plank’s constant’ and c is the speed of light.
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Carrying out the flame tests
Instructions
1. Adjust the air hole of the Bunsen burner to produce an almost
colourless flame.
2. Dip the wire loop into the solution of salt provided then put the loop into
the flame. Only use the wire loop provided for one solution - to prevent
cross contamination a different solution is provided at each
workstation.
3. Now put a wooden splint which has been soaked in the solution into the
flame. If the wood starts to burn you should stop as this will mask the
colour of the flame. Which method do you think is more effective – wire
loop or splint?
4. Write down the flame colour you observe for each solution.
5. At two of the workstations you will find a spectroscope. Look at the
flame through the spectroscope and note down how many lines you can
see and what their colours are.
Results of flame tests
Solution under
test
Flame colour
Lithium chloride
Sodium sulphate
Potassium
chloride
Strontium
chloride
Barium chloride
Calcium chloride
Copper sulphate
Solution X
Can you identify which metal solution X contains?
The metal present in Solution X is ________________
© Crown copyright 2011, Department for Education. This resource has been created through the LinkedUp Award Scheme.
Observations using a spectroscope
Solution tested
Number
of lines
in
spectrum
Colour of lines
What would be the advantage of using a spectroscope to identify the metal?
© Crown copyright 2011, Department for Education. This resource has been created through the LinkedUp Award Scheme.