Modelling alloys with plasticine

Class practicalThis experiment enables students to experience how alloying can be used to change the properties of a metal. Plasticine (orPlay-Doh) is mixed with varying amounts of sand and its *ductility * is measured in a simple test. The plasticine (or Play-Doh) isused 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 aspringboard to explain the properties of alloys to more able students.

Lesson organisationThe model alloy is a mixture of plasticine (or Play-Doh) and varying quantities of sand. These can either be made by students ormade up in advance. The plasticine (or Play-Doh) can be used and re-used many times, so once the mixtures are made-up forthe first time which can take a while the actual practical work is very straightforward.

Apparatus ChemicalsTo make one set of models:Different coloured lumps of plasticine or Play-Doh, 35g, 4 (Note 1)Sand (12 g) (Note 2)Rough paperAccess to a balancePer pair or group:Magnifying glass

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Health & Safety and Technical notesRead our standard health & safety guidance

1 The samples can be prepared by a technician, or by the students. The samples can be used several times, so it is worthensuring that they are colour coded so that they are easy to identify such as all 2 g samples in blue. For the experiment to givegood results, it is very important that the sand is mixed thoroughly and evenly with the plasticine or Play-Doh. If the samples areto be stored, it is worth wrapping them in clingfilm or placing them in plastic bags, to prevent the plasticine or Play-Doh fromdrying out.

2 Sand from a builders merchants is the best, as the particle sizes tend to be fairly even. Sand from a fire bucket usually has awide range of particle sizes and is also often dirty, so is not recommended.

ProcedureMaking the model mixture

a Weigh out 2 g of sand onto a piece of rough paper.

b 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 0.5cm thick.

asusTypewritten TextUnit 2. Particles and Atomic StructureSyllabus Ref. 10.1

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c 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.

d Repeat with 4 g and 6 g of sand in the plasticine (or Play-Doh).

Using the mixturea Mould each of the solid samples for about a minute until they are at hand temperature.

b Shape each of the samples into cylinders of about the same size and shape 6 cm long and 1.5 cm in diameter.

c Hold the ends of the cylinder firmly and pull the ends of the piece of plasticine (or Play-Doh) apart, slowly and steadily. If yourhands fly apart it is a failed test and you need to re-mould the cylinder and try again.

d Repeat for each specimen in turn, pulling with about the same force each time.

e Examine the fracture surface of the solid surface with a magnifying glass.

Teaching notesIf the same plasticine (or Play-Doh) samples are used repeatedly they always snap in the same place. To solve this simplyremould 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 seethe 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 and4g samples, and then have to predict what they expect for the 6 g sample. Alternatively, give them a mystery sample with anunknown amount of sand in it (either 3 g or 5 g.) They have to determine the quantity of sand present by performing the sametest 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 thelesson that they are happy with the use of these. A material that can be stretched or drawn into wires is ductile (malleable meansthat it can be moulded into shape when cold). If it does not stretch but snaps instead then it is brittle. A material can be strongbut 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; whathappens to the plasticine (or Play-Doh) before it breaks; comparing the fracture surface with a piece of solid sample which hasjust been snapped.

Students should be able to notice that:

the size of the fracture surface increases as the amount of sand increases;lthe plasticine (or Play-Doh) thins before it breaks;lthe fracture surface has a larger amount of sand in it than a piece which has just been snapped.l

When the sand is added to the solid, the properties of the solid, including its malleability and ductility, are altered. This modelsthe way that alloying a metal will alter its properties. When a metal is alloyed, the added material interferes with the neat packingof the original, pure metal. This prevents the layers from sliding over each other and reduces the malleability and ductility.

Health & Safety checked, September 2014

CreditsThis Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015