Smoke and mirrors

6th January 2006 at 00:00
Most people love a bit of whoosh and bang in their lives, so why not turn your pupils on to science with a few clever but safe experiments. Ray Oliver shares his knowledge

Choosing chemistry experiments that intrigue or amaze children can encourage a passion for the subject. The three experiments described here, one for each secondary key stage, should all make an impression. Like magicians and card tricks, it is essential to try out the experiments yourself first. You will need to make a risk assessment for each.

Key stage 5post-16: The Silver Mirror

In the 19th century, there were large audiences for public lectures on scientific themes. One of the most popular performers was Michael Faraday at the Royal Institution. (Today's Christmas lectures continue the tradition.) Faraday stood behind a large narrow glass tank, filled with a colourless liquid. As the lecture progressed, Faraday slowly disappeared as a silver mirror spread across the glass screen. The audience found itself viewing its own reflection.

In 1850, a Mr Drayton recommended a mixture of one part ammonia, two parts silver nitrate, and three each of water and alcohol. When grape sugar was added, a brilliant silver mirror appeared. This sixth form version gives a glass tube that is silvered inside, just like a vacuum flask.

Method: The chemical reagent needed for this experiment is rather unstable and must be prepared immediately before use. If left to stand, it can form explosive products. Some sugars contain reducing groups, for example glucose contains an aldehyde group. It is this group that reduces the silver compound to silver metal for the mirror finish. The tube must be absolutely clean or the mirror will not adhere to the glass. Place a few millilitres of silver nitrate solution in the tube followed by enough dilute sodium hydroxide to give a brown solid. This is silver oxide. Add dilute ammonia solution until the solid dissolves. This is the material often referred to as Tollen's reagent.

Next, add a solution of a reducing sugar such as glucose and warm the tube in a water bath. The silver mirror will quickly appear coating the inside of the tube.

Post-16 students should be able to research the chemistry of this reduction and provide equations to explain it, certainly in terms of a simple aldehyde such as ethanal (acetaldehyde). They might also research the extraction of silver itself, often found as an impurity in ores of other metals such as lead.

Key stage 4: The thermit reaction

This experiment is a version of the industrial Goldschmidt Process, originally used to extract metals such as chromium. The same chemistry has been applied to the chemical welding of broken railway lines and even to incendiary bombs. The reaction produces so much heat that the metal melts and glows white hot.

For on-site welding, the thermit reaction was carried out in an insulated vessel with a hole at the base so that the liquid iron dripped onto the damaged rail below. As it cooled and set solid, the gap in the rail was mended. This is a spectacular, if hazardous, example of applied chemistry.

The thermit reaction is also a good demonstration of the reactivity series of metals. The more reactive aluminium metal displaces the less reactive iron from its oxide. The heat is so intense that a clay surround is used since clays are refractory materials that can withstand high temperatures.

Method (safer in a fume cupboard with safety screen): Fill a large tin with dry powdered clay and use a test-tube to make a shallow hole in the powder.

Add a mixture of dry aluminium powder (4g) and iron (III) oxide (13g). To help the ignition, top the mixture with a little of the oxidising agent barium peroxide. Use a piece of magnesium ribbon as a fuse, then stand well back. The reaction goes off like a spectacular firework. The glow of the white hot iron can be seen for several minutes afterwards.

Key stage 3: The exploding tin

Gas and air mixtures are notoriously hazardous and continue to cause problems in coal mines. The use of candles for illumination in 19th-century coal mines resulted in many catastrophic explosions. The gas responsible was fire damp, now called methane, or natural gas.

In 1816, Humphrey Davy's safety lamp was adopted by miners and the problem was solved. This experiment deliberately sets out to produce just the right gas-air mixture to cause an explosion.

Method: Use a empty catering size metal coffee tin with a press-fit lid.

Make a hole in the lid, about 6mm in diameter. Make a second hole in the base, large enough to fit some Bunsen tubing. Connect the tube to the gas supply and switch on for about 30 seconds. Switch off the supply and remove the tube. Place the tin on a metal tripod and cautiously ignite the gas as it escapes from the lid. As the methane burns, air enters the tin from the hole in the base. When the gas-air mixture inside is about 110, the gas explodes. The lid flies off and may bounce off the ceiling.

It is more fun to set up two tins simultaneously and ask for guesses about which will blow up first. Note that the tin will not explode until the flame has almost disappeared. Mixtures of natural gas and air only explode within certain narrow composition limits. You need between 5 per cent and 15 per cent gas in the mixture before it goes bang.

Ray Oliver is head of chemistry at St Alban's girls' school, Hertfordshire

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