Activity: Coloured flame

Last weekend, some friends and I went and saw a fireworks display over Lake Burley Griffin, here in Canberra. 200 years ago, fireworks were all orange and yellow, but the fireworks we saw included blue, red, orange, purple and white. This week, we will see how these colours are produced.

You will need

• Paper clips, the larger the better.
• Water
• A source of blue flame. You will need an adult to help you use it safely. You could use:
  • A gas stovetop or burner.
  • A small blowtorch
  • A bunsen burner (if you are at school)
• If you can't find a blue flame, you could try a candle, but it won't work as well.
• Table salt (Sodium Chloride, NaCl)
• Cream of Tartar or Tartaric Acid (potassium hydrogen tartrate, KC4H5O6)
• Boric Acid also called Boracic acid ( B(OH)(3) ), available from chemists.
• A plate

Caution: This activity involves fire. For any activity involving fire you must have an adult present, plus water, a fire extinguisher and any other safety equipment you or the adult feel is needed. Take care to only use fire in a safe place. Be particularly careful of the blue flame, as it is hotter and harder to see than a yellow candle flame.

What to do

1. Straighten the paper clip.
2. Make a small pile of table salt on the plate.
3. Dip one end of the paper clip into the water, then into the salt. The crystals should stick to the end.
4. Holding the paper clip at the other end, place the salt crystals into the flame. You should see them turn yellow.
5. Repeat steps 2-4 with the other chemicals. You will see purple and green. Use a separate paper clip for each chemical.

What's happening?

You should find the different chemicals produce different colours in the flame.

The colours are caused by the elements in the salts. When the atoms are heated enough, they start to produce flashes of colour. The exact colours they make depend on the atom. The colours you are seeing are mostly due to the metal atoms.

Up until a few centuries ago, fireworks produced their colours through incandescence. This is when an object is heated up so much that is starts to glow. If a piece of iron is heated enough, it will glows red, then orange, yellow and white. In old fireworks, they used small pieces of iron that would heat up when the firework exploded, so they glowed. The Sun, stars and light-bulbs also produce light from incandescence.

Modern fireworks produced their colours using another effect, called luminescence. Inside an atom, there is a nucleus in the middle with electrons orbiting around it. Luminescence is caused by the electrons changing their orbits. If an atom is heated enough, one of the electrons orbiting around the nucleus can absorb some of the heat energy and move into a different orbit. After a little while, the electron will move back to its original orbit. When it does, it emits its extra energy as a flash of light.

Different types of atoms will luminesce at different temperatures. When electrons change orbits, they always release the same amount of energy, so they always produce the same colours. When you heated up the chemicals, the Sodium in the table salt produced yellow light, Potassium in the Cream of Tartare produced purple and the Boron in the Boranic Acid produced green.

Applications

• A basic firework that explodes in the air contains explosives in the middle, surrounded by little balls of flammable powder, called "stars". When it explodes, the stars catch fire and are blasted out to produce the fireworks we see in the air. The colours of the fireworks comes from chemicals mixed into the stars luminescing. Some chemicals used to colour fireworks include:
  • Strontium Chloride for red
  • Calcium Chloride for orange.
  • Copper Chloride for blue.
  • Barium Chloride for green
  • Sodium Chloride for yellow

These fireworks used different chemicals to produce different colours.

• In some fireworks, the stars have layers containing different chemicals. As they burn inwards, the colour they produce changes. This is how they make fireworks that change colour in mid-air.
• Different atoms produce different colours when they luminesce. This information can be used to work out what an unknown chemical is made of. This technique is called spectroscopy. It even lets astronomers analyse the chemicals in glowing clouds of gas in space.