Light and Chemistry

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Jelly fish

A short demonstration about the interaction of light and chemistry can be appreciated by all age groups. From primary school pupils through to adults, this demo lecture relates to everyday examples of light and chemistry, including fireworks, colour-changing glasses, glow sticks and security markings on bank notes!

 

For this lecture you will need the following:

  • UV torch (eg. inexpensive UV LED torch from Moobom, online retailer)
  • transition lens glasses
  • Bunsen burner (you can buy burners that attach to camping gas canisters, for venues without gas taps!)
  • lighter
  • metal salts (NaCl, CaCl2, CuSO4, K2CO3, FeCl2)
  • nichrome wire
  • beaker of water
  • luminol
  • NaOH
  • household bleach
  • glow sticks
  • highlighter pens
  • tonic water
  • turmeric
  • bank notes
  • driving license / passport
  • cheque
  • stamps

Please download and read the Safety Card before trying this activity.

Health and Safety

It is advised that the demonstrator wears goggles for these demonstrations, and that the experiments are conducted at a distance from the front row. Do not look directly into the UV-torch and be careful not to shine it directly into the eyes of the audience.

The Lecture

In this demonstration, there are 4 different concepts to introduce:

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transition lenses

1) Using light to drive a chemical reaction

Can you think of an example where we use light to drive a chemical reaction? This is called photochromism, and is used in colour-changing lenses. When we go outside and UV light from the sun irradiates on these lenses, a reversible chemical reaction happens, which changes colour to make the lenses go dark.

If you have a pair of transition lens glasses, this can be demonstrated with a UV torch.

2) Using heat to produce light from metal ions

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fireworks containing metal salts

Have you ever seen a saucepan boil over on a gas hob? If there is salt in the water, you see the colour of the flame change colour. When metal salts are heated, they can emit light; if you excite an atom or an ion by very strong heating, electrons can be promoted from their normal unexcited state into higher orbitals. As they fall back down to lower levels (either in one go or in several steps), energy is released as light. Fireworks contain metal salts, to create the brilliant colours that we see in the sky.

In chemistry we sometimes use ‘flame tests‘ to identify metal ions from their colour. This can be demonstrated easily to a class with a Bunsen burner, a nichrome wire, and metal salts (i.e. NaCl (orange/yellow), CuSO4 (green), CaCl2 (red) , K2CO3 (lilac), FeCl2 (gold). To do this in a lecture theatre or classroom without gas taps, it is possible to get a burner that screws onto an lpg canister (for camping). Light the Bunsen burner, and close the hole to produce a hot, blue/colourless flame. Wear goggles, and have a beaker of water next to the burner. Dim the lights, and dip the wire in the water, and then into a metal salt, and hold the metal salt high up in the flame, to see the colour. Dip the wire in the water to wash off any remaining salt and try another.

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glow sticks

3) Using a chemical reaction to produce light

When light is produced from a chemical reaction, this is called chemiluminescence. Chemiluminescence involves the production of an electronically excited species from a number of reactants, which then goes on to release visible light in order to revert to its ground state energy.

Luminol is sometimes used in forensic science to identify blood; it  reacts with haemoglobin in blood stains which have been washed away, therefore forensic teams sometimes use a luminol-containing spray all over a crime scene to detect suspected blood traces in a darkened room.

Chemiluminescence also occurs in nature, and some animals can produce light due to chemical reactions in their bodies. This is often refered to as ‘bioluminescence’. Fireflies contain a chemical called luciferin, which chemiluminesces when it reacts in the animal’s body with ATP, in the presence of the luciferase enzyme. Many deep-sea animals also bioluminesce.

Recipe for luminol chemiluminescence demonstration:

  • 100 mL (0.5 % w/v) NaOH (aq) solution
  • 100 mL (1 % v/v) Household bleach solution
  • ca. 50 mg luminol (s)

Just before the demonstration, add the luminol (just a small amount off a spatula, no need to weigh this particularly), to the NaOH (aq) solution, and stir until dissolved.

Turn the lights off in the classroom for the best effect, and pour the two solutions together to see a beautiful blue luminescence from the mixture.

Chemiluminescence is also used in glow sticks to produce the ‘glow’; as the sticks are snapped, an internal tube is broken and the two reactants are then mixed together, react, and emit light. Handing out glow sticks to the audience usually goes down well!

4) Shining light on fluorescence compounds to produce light of a different colour

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fluorescence

To demonstrate fluoresence to an audience – shine a UV torch on a bottle of tonic water; it should brightly fluoresce blue under UV light. Sprinkle some turmeric into ethanol (or vodka), under a UV torch, and see the bright yellow fluorescence.

Shine a UV light on a pack of fluorescent highlighter pens – most people will know the word ‘fluorescent’ in this context, but may not understand that these inks actually fluorescence.

Fluorescence is widely used in security markings, to make things difficult to copy. Show the audience a £5 note, a driving license, a cheque under the UV torch. Postal stamps are read under UV light to determine whether they are first class or second class – take a look at the fluorescent stripes on stamps under UV light. Fluorescent ink is also used in crime prevention, in anti-theft traps on high-value items such as ‘SmartWater‘.

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Luminescent security marks on the 5 Euro note, to prevent fraud

Page written by Dr Nicola Rogers
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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