Year 9 Term 2 - Flame Test

Experimental Assessment

Due date:  to be announced separately by teachers in various classes, since experiments will be performed at different times.

Marks: 5 marks for practical procedure in class + 10 marks for formal report. (See your booklet for marking criteria)

The Flame Test

Aim:   To determine the characteristic flame colour for a range of known elements in solution; to use these results to identify the elements present in unknown solutions.

Information about flame tests

Materials:

Labelled diagram: this should also be included in your report.

Procedure: (When writing your report, this should be in past tense and in your own words.)

1. Collect goggles, aprons and safety gloves.
2. Set up and light the Bunsen burner, leave on safety flame. Adjust to blue flame when you are ready to use the Bunsen. (Remember please return to safety flame between tests.)
3. Collect an icy-pole stick, which has been soaking in sodium nitrate, from the front bench and return to your desk.
4. Briefly hold the iceblock stick in the flame. Observe and record the colour of the flame. (Note: do not continue until the stick itself catches alight.)
5. Remove the stick from the flame and allow it to cool down before placing it in the bin.
6. Repeat these steps with the remainder of the known and unknown solutions.
7. Place all the sticks into the bin and return all other equipment.

Results:

Discussion (Questions)

1. Which element produced a - a) yellow flame b) bright green flame c) bright red flame d) purple flame e) pale yellow/green flame
2. How can you be sure that the metal element rather than the nitrate in the solution caused the various colours of the flames?
3. Chemists say that the colour emitted by an element is its ‘fingerprint’. Explain what this means.
4. Is this test a quantitative test or a qualitative test?
5. Colourful light emissions are applicable to everyday life. Suggest some ways in which this ability of many substances to produce different coloured flames could be used.
6. Sodium chloride is an impurity present in many different chemicals. How could the flame test be used to detect the presence of the sodium chloride impurity?
7. Why is it important not to allow any of the solutions to drip onto the Bunsen burner near the flame?
8. Sodium lights (in use in many Brisbane street lamps and also some in the Campbell Centre) glow with a yellow light. Find out what causes this coloured glow.
9. If lithium lights were used, what colour might you expect these lights to be?
10. Refer to your textbook or other information sources and explain why many substances emit light when heated.
11. Explain why different elements emit different coloured light?
12. Read the statement below. What is meant by the term ‘ambiguities’?
13. What are the two main limitations of the flame test mentioned above?
14. What is another factor that may contribute to error in this experiment?

What are the limitations of this test?
The value of the flame test is limited by interference from other brighter colours and by ambiguities where certain different metals cause the same flame colour. Sodium, in particular, is present in most compounds and will colour the flame. Sometimes a coloured glass is used to filter out light from one metal. Cobalt glass is often used to filter out the yellow of sodium .

Conclusion - Do not forget to include a conclusion in your report!

Refer to the Practical report format or refer to your CD for the method to write up your formal report.