Flame Test Colours: The Chemistry Of Fireworks

Fireworks have been in existence for thousands of years, starting in China over 2,000 years ago.

Since then, they have been developed for use in weaponry before being used in a far more thrilling and enjoyable way as entertainment.

The History Of Fireworks Night

The history of Fireworks Night is a peculiarly English one, dating back to the Gunpowder Plot of 1605, when an attempt was made to blow up Parliament using kegs of gunpowder and assassinate King James I (James VI of Scotland). The plot was discovered and the plotters were executed for treason.

Fireworks, bonfires and the burning of a 'guy' have been an English tradition ever since. The occasion was first known as 'Gunpowder Treason Day', with the original 'guy' relating to the plotters.

Nowadays, fireworks are used around the world to celebrate many events, from New Year's Day to Diwali, the festival of lights, and numerous birthdays and celebrations in between.

What Makes The Colours In Fireworks?

Metal salts give fireworks their individual colours. As a metal salt heats up, electrons around the nucleus of its atoms gain enough energy to 'jump' to a higher energy level. As the electrons then lose energy, they fall back to their original level, and that lost energy is given out as light. The wavelength of the light, and so its colour, depends on the metal used:

• Copper: Blue
• Potassium: Lilac
• Sodium: Yellow
• Barium: Green
• Strontium: Red

Adding magnesium powder to a flame will also provide sparks.

In the lab, we can show this energy release with flame test demonstrations, where a little of the solid or dissolved salt is heated to produce coloured flames.

We have put together a flame test demonstration protocol below, along with a sparkly highlight for your next open evening.

You can also download our free Flame Test Sheets to use alongside your demonstrations.

Flame Tests Using Dissolved Metal Salts

Remember: always take appropriate safety precautions when handling chemicals. A risk assessment should be carried out before each of these demonstrations.

You will need:

• Safety screen
• Goggles
• Bunsen burner and heatproof mats
• Spray bottles (ones with a trigger spray are best), one per metal salt
• Water
• Alcohol (ethanol or IMS)
• Metal salts, which can include calcium chloride, lithium chloride, sodium chloride, strontium chloride, copper chloride and potassium chloride (check with CLEAPSS for details and risks)

Part 1: Prepare The Spray Bottles

1. Make a 50/50 mix of alcohol and water and add enough to each bottle to cover the bottom of the spray tube.
2. Add a few mg of a metal salt to one of the bottles and shake to dissolve (alternatively, make a 50:50 mix of 1M metal salt solution and ethanol). Label the bottle appropriately.
3. Repeat for the other bottles and metal salts.
4. Test the solutions by spraying each one through a blue Bunsen flame, pointing away from you. The metal in the solution will colour the flame as it passes through.

Part 2: In The Classroom

1. Set up a safety screen between the Bunsen burner and the students, and ensure everyone wears safety goggles.
2. Place several heatproof tiles around and under the Bunsen burner in case of spillage.
3. Set the Bunsen burner to a blue flame.
4. Shake each bottle before use and gently spray each one, in turn, through the flame, pointing away from you and away from the students (parallel to the safety screen is best). A hard spray may blow the flame out.

After the demonstration, decant each metal solution into a labelled, screw-topped bottle with the appropriate hazard symbols, ready for next time, and store it in the flammables cabinet. If you leave the solutions in the spray bottles, the liquid will evaporate and the metal compound will clog the spray mechanism.

Open Evening Demo: Cannon Fire

You will need:

• Safety screen
• Goggles
• Fire blanket
• Evaporating dish
• Heatproof mat
• 100 ml beaker
• Spatula
• Potassium manganate(VII), KMnO₄ (approx. 0.5 g)
• Hydrogen peroxide, H₂O₂ (20 vol)
• Ethanol (or IMS)

Method

1. Pre-mix 20 ml ethanol and 30 ml H₂O₂ in the beaker.
2. Set up the demonstration station: evaporating dish on a heatproof mat, with a safety screen between the demonstration and the students. Ensure safety goggles are worn by the demonstrator and students.
3. Add the contents of the beaker to the evaporating dish and set it alight.
4. Sprinkle the KMnO₄ crystals into the burning mixture.
5. The reaction will stop once the KMnO₄ has run out. You can smother the flames with the fire blanket if necessary.

Along with the purple flame, you will hear the crackles and bangs often associated with fireworks, caused by pockets of high and low energy release as the potassium manganate(VII) burns.

Stock Up For Your Demonstrations

Find everything you need for your Fireworks Night demonstrations, from chemicals and flame testing equipment to PPE, on our website. Don't forget to download our free Flame Test Sheets, or contact our team for advice tailored to your department.

Please be advised that this guide is intended for informational purposes only and Breckland Scientific cannot accept any responsibility for the outcome of any usage based on this guide.