BSI Education

Practice Assignment 06

How do they put the colours into fireworks?

The Chinese made the first fireworks thousands of years ago, but the use of different colours in pyrotechnic displays is a more recent discovery. Before the 19th century, only various yellow and orange colours could be produced using steel and charcoal.

When a rocket is ignited, burning gunpowder fires it into the sky. Another charge then explodes, scattering thousands of hot tiny particles into the air. Powdered metals, such as iron or aluminium, burn brightly. Cations in metal compounds glow with their characteristic colours, just as they do in flame tests. Anions, such as chloride or nitrate, give no colour.

Your task is to investigate which compounds give the best colours.

What you have to do

  1. Get a copy of the Standard Procedure SP 0006-2: 2005. It tells you how to perform flame tests to identify the metal in an unknown compound. However, in this assignment you will use flame tests on known compounds. The purpose is to compare the flame colours they produce.
  2. Before starting you need to:
  • read through the procedure carefully
  • check all the safety aspects and carry out a risk assessment on the process;
  • collect all the necessary equipment.
  1. Follow section 6 in the Procedure, but in the 4th step moisten your sample with water instead of hydrochloric acid.
  2. Find out which of your test samples produce the colours yellow, red, reddish-orange, green and lilac. Record your results in a suitable table.
  3. Watch carefully. Try to compare the brightness of the flames for different samples containing the same metal.

Some hints

  • Flame tests work best with powdered compounds, so be prepared to use a pestle and mortar to grind up crystals or lumps.
  • Note the colour of each sample, to see whether it gives a clue about what flame colours it will produce.
  • It is very important to burn off any traces of sodium on the nichrome wire. The bright yellow sodium flame will mask any other colour you want to see. You can also view the flame through blue glass as suggested in the procedure.
  • Make sure you thoroughly wash and dry the watch glass in between tests.

Some questions

Don’t forget to write up your work and the results in your Laboratory Notebook.

  1. Why is it important to thoroughly clean the wire and watch glass in between tests?
  2. If you get a yellow flame with most of the tests, how can you be sure that the test is accurate, and impurities are not masking the true colour?
  3. When you have carried out the test, say what you found difficult, and what you did to overcome it.
  4. Looking back, are there any parts of the process that you could have done better?
  5. Look at the table of results for all the samples. What do samples that give the same flame colour have in common?
  6. Which compound of each metal gave the best (brightest) flame colour?
  7. Flame tests are normally used for analysis, to identify the metal cation in a compound. The Procedure requires you to moisten the sample with concentrated hydrochloric acid, rather than water. Suggest why. (The answer to Q6 should give you a hint.).
  8. Find out if it’s possible to produce blue coloured firework displays. If so, how? If not, why not?

To help you find out more

  • “Contemporary chemistry for schools and colleges” Vanessa Kind, Royal Society of Chemistry, 2004. (A copy should be in school.)
  • Kaboom
  • Kimbolton Fireworks (a firework company started by a teacher, who was also a Reverend!)

Try finding out about Robert Bunsen (the same one that developed the burner you use in the laboratory) and Gustav Kirchoff. They were interested in finding new elements, which they discovered using flame tests.

You can find out more by using these names in your favourite web search engine.

Which elements did they discover, and what are their names?

Would any of these elements be useful in fireworks? Explain your answer.

Related links