3.1  Practical 1 Flame tests

In this section you will investigate the presence of metal ions by recording the colour observed when heating a metal salt in a flame. You will be using solid samples of different metal salts. The metal chlorides you will be analysing are those of copper, lithium, potassium and strontium.

You will perform the experiment in the virtual OpenScience Laboratory (the OU online laboratory for practical science).

What you need to do:

• 1. Turn on the gas, pick up the lighter and light the Bunsen burner, which should give a low yellow flame. Rotate the barrel of the burner so the air hole is open and the flame is blue.
• 2. One end of the nichrome wire is embedded in a cork for safe handling, and the other end has a small loop. Clean the loop of wire by dipping it into the small beaker containing a solution of hydrochloric acid.
• 3. Place the loop into the side of the blue flame, as shown in Figure 10. If the wire is clean it should make no difference to the colour of the flame. If the colour of the flame does change there is an impurity on the wire. Dip it again in the acid and return it to the side of the flame.
• 4. Dip the loop of the wire into the acid and then use it to pick up a few grains of a metal salt.
• 5. Place the loop in the side of the flame and note down the colour of the flame in your copy of Table 2.
• 6. Use the hand spectroscope provided to separate the constituent colours present in the light and look at the emission line spectrum of the flames. Note down the description of spectra in your copy of Table 2. The hand spectroscope is a simple piece of equipment that houses a prism system in order to provide spectra from visible light (Figure 11).
• 7. Repeat this procedure and observe the flame colour given by the other metal salts. Record your observations in Table 2.

Figure 10  Components of Bunsen burner and the position of nichrome wire for a flame test.

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Figure 10 shows an illustration of a Bunsen burner in which its components are labelled and the position of nichrome wire for a flame test is shown. The Bunsen burner is a metal thin hollow vertical tube on a flat metal stand. Gas passes into this via a flexible rubber tube and is controlled by the barrel, which is at the base of the metal tube. This can be rotated to allow gas to pass up the main tube. In the diagram this is open, so gas is passing through and the flame at the top of the metal tube is blue. A thin long nichrome wire is shown being held in the side of the flame. This has a loop at the flame end and a cork at the other. The cork is used to hold the wire.

Figure 10  Components of Bunsen burner and the position of nichrome wire for a flame test.

Figure 11  Main components of the hand spectroscope.

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Figure 11 shows an illustration of the main components of the hand spectroscope. The figure is drawn as two thin rectangles in landscape format so that they appear as square tubes, open at facing ends. The right tube (the barrel) is inserted into the left tube to make one long tube. At the left hand end of the long tube is an eyepiece lens, so that you can look into the tube. At the left hand end of the inserted tube is another lens and at the right hand end is a glass window. Inside the right hand part of the inserted tube is a prism. This is shown as three triangles, fitted together to make a trapezium.

Figure 11  Main components of the hand spectroscope.

• 8. Now try looking at the colour of the flame obtained with the unknown mixtures of metals. Write down what you observed.

Follow the link to access the experiment. Instructions are also provided within the experiment, under ‘Help’.

Practical 1 Flame tests in the OpenScience Laboratory [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]