Demonstrations designed to capture the student's imagination

Demonstrations to capture the student's imagination by Adrian Guy of Blundell's School.  In this issue: Flame colours in burning hydrogen

This demonstration is based on an idea of Georgina Batting, head of science at Blundell's School. Igniting balloons filled with hydrogen gas is an exciting, well-known and much loved demonstration. But, by adding some metal salts to the balloons, it becomes a demonstration of flame colours. 

Colourful balloons

The thermal energy produced by the combustion of hydrogen gas in air is sufficient to excite electrons within the metal ions to higher energy levels. On 'falling' back to their 'ground state' the absorbed energy is released as photons of light. According to the relationship between energy (E) and frequency (λ),  ie  = hλ, where h is Planck's constant, the higher the energy of the photon, the higher the frequency.  

With many different electron transitions possible, photons of different energies (and thus distinct frequencies) are emitted, each of which corresponds to a particular electron transition. If the energy of the transition corresponds to a frequency within the visible region of the electromagnetic spectrum, colours are observed. Flame colours of metal salts passed through a prism or diffraction grating can be seen as distinct lines of colour, (line emission spectra), where each line is produced by a unique electron transition. 

image - Exhibition chemistry - image 1

Source: Adrian Guy

Kit

  • balloons - standard party size
  • hydrogen cylinder
  • distilled water
  • 5ml syringe (without a needle) or small measuring cylinder
  • metal salts, eg strontium nitrate, barium chloride, potassium chloride, sodium chloride, lithium chloride; pestle and mortar
  • cotton thread
  • metre rule and splints
  • goggles and nitrile gloves 

Procedure 

image - Exhibition chemistry - image 2

Source: Adrian Guy

Using a clean, dry pestle and mortar grind 0.1 g of your metal salt into a fine powder resembling icing sugar. Pour the powder into a balloon using a piece of folded paper as a funnel. Using a small syringe, add 1 ml of water to the balloon. Fill the balloon with hydrogen gas and tie a knot to seal. Rotate the balloon to coat evenly the inside surface with the metal salt. Attach a 2 m length of cotton thread to the knot and tie the balloon to a secure object, allowing it to float ready for ignition. 

To ignite the hydrogen, tape a splint to one end of a metre rule, ignite the splint and, at arm's length, set fire to the balloon. The ensuing combustion and expansion of gases will create a loud 'thud' and a coloured flame. 

Special tips

For the best effect do the demonstration in a darkened room.

Teaching goals

Exploding balloons of hydrogen gas laced with metal salts serves as a useful revision lesson on metal ions and their corresponding flame colours covered at GCSE. At A-level, flame colours form part of the evidence used to postulate the electronic structure of atoms. This is also a nice demonstration to save for end-of-term lessons.

Examining flame colours is a common activity at KS3. The standard method is to dip a Nichrome or platinum wire in hydrochloric acid followed by a metal salt and then hold the wire tip in a Bunsen flame. Alternatively, with younger pupils, you might want to try using damp splints to dip into samples of metal salts instead of using Nichrome wire - this gives excellent results.

Safety

Barium chloride is toxic if swallowed and harmful by inhalation. Lithium chloride is harmful if swallowed and irritating to the eyes and skin. Hydrogen readily forms explosive mixtures with air. Keep the hydrogen supply away from naked flames while filling the balloons. Also, be cautious to keep the filled balloons well away from naked flames. Make sure the audience is more than 50 m away from the balloons because considerable heat and balloon debris are produced. Wear goggles and ear defenders when igniting the balloon.