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Flame test dragon with sodium

 

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Flame Tests

What's it all about?

When solutions of metals are heated in a Bunsen burner flame, they give off characteristic colours. For example, sodium makes the flame turn bright orange – this is the same orange colour made by sodium street lamps and many fireworks. You can be an analytical chemist or forensic scientist in this activity, by finding out what colour flames different metals make, and working out the identity of some unknown metal solutions.
 

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Student notes

First make sure that you have a clean flame test wire. Do this by holding the metal loop in the hottest part of the Bunsen burner flame. If it is clean, there should be no change in the colour of the flame when the metal loop is put in it. If it is not clean, clean it by dipping it into the concentrated acid provided, then holding the loop in the Bunsen burner flame. Repeat this cleaning until there is no more change in the colour of the flame.

The next job is to do your flame tests. Dip the flame test loop into one of the known test solutions, then hold the metal loop in the hottest part of the Bunsen burner flame. Make a note of the colour of the flame on your Flame Test Chart (like the one below).
 

barium

calcium

copper

lead

potassium

sodium

 

 

 

 

 

 

Clean the flame test wire, then test another known test solution. Keep going until you have recorded the colour of all of the known solutions.

Get your results checked, then flame test the four unknown solutions and make a note of their flame colours on your Flame Test Chart.
 

 

sample ...

sample ...

sample ...

sample ...

flame colour

 

 

 

 

metal

 

 

 

 

Can you work out which metals are in each of them?

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Teacher notes

We investigate barium, calcium, copper, lead, potassium and sodium, as they give readily identifiable colours. The flame test wires should be cleaned between each test by dipping in nitric acid and heating, but it works best if each solution has its own labelled flame test wire. Sodium in particular is difficult to remove, and students will end up thinking everything contains sodium or makes an orange flame! The expected colours are shown in the table below:
 

metal

flame colour

barium

light green

calcium

brick red

copper

blue/green

lead

blue/white

potassium

lilac

sodium

bright orange

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Technician notes

In the lab:

Test tube racks
Bench mats
5 x Bunsen burners
Minimum of 10 flame test wires (cleaned)
5M hydrochloric acid in labelled test tubes

In tubes labelled with the name of the metal:
Approx. 0.5M solutions of:

barium chloride
calcium chloride
copper(II) sulphate
lead(II) nitrate
potassium nitrate
sodium chloride

Four of the six solutions as unknowns, labelled 1, 2, 3 or 4

Maintain stocks of these solutions for replenishment

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