Try this class activity to explore how alloying can be used to change the properties of a metal
In this practical, students mix plasticine (or Play-Doh) with varying amounts of sand. They then measure its ductility in a simple test. The plasticine is used to represent the main metal in the sample (eg iron) and the sand an added substance (eg carbon in steel).
The practical is suitable for students of all abilities and can remain at the level of observation for less able students, or act as a springboard to explain the properties of alloys to more able students.
The model alloy is a mixture of plasticine (or Play-Doh) and varying quantities of sand. These can either be made by students or made up in advance. The plasticine (or Play-Doh) can be used and reused many times, so once the mixtures are made-up for the first time – which can take a while – the actual practical work is very straightforward.
- Different coloured lumps of plasticine or Play-Doh, 35 g, x4 (see note 2 below)
- Sand, 12 g (see note 3)
- Rough paper
- Access to a balance
- Magnifying glass
Health, safety and technical notes
- Read our standard health and safety guidance.
- The samples can be prepared by a technician, or by the students. The samples can be used several times, so it is worth ensuring that they are colour coded so that they are easy to identify – such as all 2 g samples in blue. For the experiment to give good results, it is very important that the sand is mixed thoroughly and evenly with the plasticine or Play-Doh. If the samples are to be stored, it is worth wrapping them in clingfilm or placing them in plastic bags, to prevent the plasticine or Play-Doh from drying out.
- Sand from a builders merchants is the best, as the particle sizes tend to be fairly even. Sand from a fire bucket usually has a wide range of particle sizes and is also often dirty, so is not recommended.
Making the model mixture
- Weigh out 2 g of sand onto a piece of rough paper.
- Take one of the plasticine (or Play-Doh) lumps and mould it until it is warm and malleable. Work it into a flat shape about 0.5 cm thick.
- Sprinkle the sand onto the plasticine (or Play-Doh) and roll it up. Working over the rough paper, mould and work the solid, adding back in any sand that falls out, until the sand is evenly distributed.
- Repeat with 4 g and 6 g of sand in the plasticine (or Play-Doh).
Using the mixture
- Mould each of the solid samples for about a minute until they are at hand temperature.
- Shape each of the samples into cylinders of about the same size and shape – 6 cm long and 1.5 cm in diameter.
- Hold the ends of the cylinder firmly and pull the ends of the piece of plasticine (or Play-Doh) apart, slowly and steadily. If your hands fly apart it is a failed test and you need to re-mould the cylinder and try again.
- Repeat for each specimen in turn, pulling with about the same force each time.
- Examine the fracture surface of the solid surface with a magnifying glass.
If the same plasticine (or Play-Doh) samples are used repeatedly they always snap in the same place. To solve this simply remould the solid samples for a couple of minutes until all the sand is evenly distributed again and the sample is warm.
The plasticine (or Play-Doh) and sand can be investigated further, and other factors such as temperature can be tested to see the effect they have on the samples’ properties.
Another idea is to remove the 6 g sample from students at the start of the experiment. Students do the test on the 0 g, 2 g and 4g samples, and then have to predict what they expect for the 6 g sample. Alternatively, give them a ‘mystery’ sample with an unknown amount of sand in it (either 3 g or 5 g.) They have to determine the quantity of sand present by performing the same test and comparing the results to the other tests they have done.
Students often get the terms brittle, malleable or ductile, and strong muddled up. It is worth ensuring at some stage during the lesson that they are happy with the use of these. A material that can be stretched or drawn into wires is ductile (malleable means that it can be moulded into shape when cold). If it does not stretch but snaps instead then it is brittle. A material can be strong but brittle – and indeed many are. The opposite of ductile is brittle – not weak.
It is worth asking students to describe the results in detail, and to focus particularly on: the size of the fracture surface; what happens to the plasticine (or Play-Doh) before it breaks; comparing the fracture surface with a piece of solid sample which has just been snapped.
Students should be able to notice that:
- The size of the fracture surface increases as the amount of sand increases.
- The plasticine (or Play-Doh) thins before it breaks.
- The fracture surface has a larger amount of sand in it than a piece which has just been snapped.
When the sand is added to the solid, the properties of the solid, including its malleability and ductility, are altered. This models the way that alloying a metal will alter its properties. When a metal is alloyed, the added material interferes with the neat packing of the original, pure metal. This prevents the layers from sliding over each other and reduces the malleability and ductility.