Exhibitions pectacular demonstrations for teacher chs emistry

Demonstrations to capture the student’s imagination, by Declan Fleming of Pate’s Grammar School, Cheltenham. In this issue: Magic Metal

Fusible alloys students with the composition of the When teaching metals there are a mixture and ask them to predict number of ways to show how mixing what they expect to see from the metals together can alter their alloy: Do students expect the alloy to properties. Fusible alloys (ones which take on a melting point lower than melt at a low temperature) are an all three components or higher, or attractive and underused opportunity the same as the largest component, to show how far we can manipulate a or a weighted average of all three? material’s properties. The idea that mixtures can have a Fusible alloys are used for solders, lower melting point than a pure as temperature triggers in sprinkler substance is covered with younger systems (where the plug is held in students and is used with older place with a fusible alloy which melts students when discussing melting in a fire), as mercury replacements point determination. (eg galinstan thermometers) and even as experimental coolants in fast Storing energy neutron reactors. The result is unexpected both in Field’s metal is one such alloy which terms of how depressed the melting melts at a low temperature (61°C) point is and the fact that Field’s metal and, unusually for a fusible alloy, is a eutectic alloy – it melts at a contains neither lead nor cadmium. specific temperature rather than over Kit a range as will happen with other S ample of Field’s metal (~30 g is a Magic metal – see how to perform this demonstration at alloys. This suggests it has a single good size) www.rsc.org/EiC0412ec chemical composition as opposed to C eramic crucible being a mixture. This defined, 250 cm3 conical flask acrylic cut on the school’s laser cutter. Composition of depressed melting point has many O ptional moulding paraphernalia Jewellery moulding compound is Field’s metal and novel uses. (eg ice cube trays, plasticine, available from craft shops. This will melting points New solar molten salt technologies jewellery moulding compound) produce a reusable mould and you are being developed so that Procedure won’t need to spend time scratching the sun’s energy can be You can make Field’s metal yourself, out bits of plasticine from crevices in Sn used to melt a salt although it’s difficult to obtain small the metal. It is possible to produce a 16.5% (eg a eutectic quantities of indium, which can make mould for a ‘magic spoon’ but this (232°C) mixture of this prohibitively expensive. requires more metal. NaNO3 and Mindsets will supply the metal in In KNO3), sensible quantities at approx £1 per In front of the audience 51% effectively gram and it is available through other S imply place your piece of metal into Bi (156°C) storing online sellers. a beaker of freshly boiled water and 32.5% energy for If you’ve done this demonstration watch it melt! (271°C) use later (eg. before, you will potentially have a at night). piece of metal in the shape of the T eaching goal EMLA bottom of the container you last used Fundamentally the idea here is to (eutectic mixture – this will make it difficult for your show how mixing different elements of local anesthetic) audience to see what’s happening. I in an alloy can fine tune its is a mixture of normally prepare a specific shape in properties to a specific application. lidocaine and prilocaine. advance by steaming it in a ceramic You could, however, provide your Individually, these compounds crucible over a conical flask. This would be solids at room avoids over-heating the alloy and also temperature, but together they melt avoids having to decant off water if Safety to an oil at 18°C, avoiding the use of you melt it by submersion. Goggles should be worn for this demonstration. Wash your hands non-aqueous solvents for application I prepared a small mould in after touching the metal. and also increasing the dose that can plasticine by pressing into a piece of be applied. ■

12 | Education in Chemistry | July 2012 www.rsc.org/eic

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