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21 February 2018

Starting the chemistry year off with a bang

As another university year begins, we met with Dr Alice Williamson, a chemist, lecturer and science communicator in the Faculty of Science to give us a sneak peak of just some of the experiments students can look forward to learning about when studying chemistry at Sydney.

The oscillating iodine clock

Three different solutions are mixed together at the start of the experiment to give one colourless mixture. The solution contains (among other things) potassium iodate, starch, hydrogen peroxide and a strong acid.

In this reaction the solution continuously changes colours - it 'oscillates' between colourless, amber and dark blue. These colour changes are the result of several competing chemical reactions occurring within the flask, each resulting in a different colour.

So, what's happening?

The amber colour results from a reaction of the iodate ion to form iodine. The iodine then reacts to form triiodide which sticks to the starch and forms a complex - which results in a blue solution. This blue complex breaks down in another chemical reaction and the sequence of colour changes repeats over and over again until the mixture settles to a permanent inky blue.

The big bang

Science can be used to recreate a similar special effect to that used in one of the most iconic movie moments in history - the Death Star explosion. A simple chemical reaction between hydrogen bubbles and a flame creates the impressive bang noise used in the movie.

Hydrogen gas is bubbled through soapy water to form clouds of bubbles containing hydrogen. Given hydrogen gas is lighter than air, the bubble clouds rise until they come into contact with the flame. The hydrogen ignites causing an explosion as it reacts with oxygen in the air to form water vapour.

All about electrons

Flames are usually orange in colour, so why are these flames turning bright red, green, purple and a range of colours in between? It's all down to some beautiful chemistry facilitated by the contents of a spray bottle.

Each spray bottle contains metal salts that are dissolved in water to create a solution. When the solution is sprayed into the flame, the colour change is virtually instantaneous. This happens because the electrons in each of the metals have fixed energies and some of them can be boosted up to higher energies by the flame.

The electrons can't stay at the higher energy for too long - they fall back down like a ball that has been tossed into the air. And when they fall back down to their usual energy, coloured light is emitted, changing the colour of the flame. Different metals have different energies of electrons and these correspond to different colours of light.

The cold light

This striking reaction demonstrates a phenomenon called chemiluminescence - the emission of light as a result of a chemical reaction.

So, what's happening?

A molecule called luminol (contained within one of the bottles) reacts with hydrogen peroxide (in the other bottle) in the presence of a metal catalyst.

Once again, this reaction is due to the excitement of electrons, this time in an organic molecule called luminol. The electrons hop up to a higher energy level and then fall back down again causing emission of this blue light.

A similar effect, called bioluminescene, the emission of light due to a biological process, is observed in the tails of fireflies and glow worms. Unlike electric lights that also give out heat, this form of light is cold and so it doesn't burn the bodies of these glowing creatures.

Fireworks

Who would have thought that energy released by sugar could produce spectacular fireworks? Small bowls containing sugar, metal salts and potassium chlorate sit under an extractor. When a drop of strong acid is added to the mixture it reacts with potassium chlorate to form oxygen, which then reacts with the sugar to give out lots of energy - hence the light and flame!

The different colours produced in this experiment, once again come down to the use of a variety of metal salts and their electrons. They take in energy from the reaction and their electrons jump up in energy and then fall back down and release the energy lost as different colours of light.

Fireworks pt. 2

Much the same as the above experiment, this chemical reaction can occur on a larger scale if using a long tube instead of smaller bowls.

This tube still contains sugar, metal salts and potassium chlorate, with a strong acid again initiating the reaction.