This focus idea is explored through:
Contrasting student and scientific views
Student everyday experiences
Students today are surrounded by a wide variety of materials that are often classified as natural or processed materials. This classification should be seen as a continuum rather than an either/or classification. Natural materials are ones that occur within the natural environment and have undergone very little modification. Processed materials are often modified from natural materials or do not occur at all in the natural environment, but have been designed and manufactured to fulfil a particular purpose. On a continuum using wood as the material, timber would be the natural material and with an increasing level of processing you would have plywood, MDF (medium density fibre board) and paper.
It is less common for students to experience natural materials and increasingly difficult for them to identify the source of raw materials needed in the manufacture of many processed materials. Their common experience is with toys, clothing and other materials made from plastics, synthetics, polymers and resins and they are less likely to be able to identify natural materials like cotton, rayon, silk, wool and mohair derived from plants and animals.
Students broadly identify substances and materials by how they are used and generally can only identify one or two properties that make the material well suited to its application. They also tend to associate substances and materials with objects and even though some of the obvious properties of these objects may undergo chemical reactions resulting in a change of colour, smell and composition, they continue to believe that the object and the material from which it is composed remain the same, i.e. students believe that when iron rusts, the rust, although it has changed colour, still remains iron.
de Vos & Verdonk (1987)
In our increasingly commercial world, students are less likely to have experiences which require them to select or even identify materials based on their suitability to perform a given task. The modern consumer is more likely to purchase a garment based on cost, aesthetics or availability and leave issues of material selection designed to enhance functionality to the product manufacturers. A comparative study of Year 3 students in Western Australia revealed that 1 in 3 students could not identify that plastic wrap is waterproof. The inference is that students perceive that plastic wrap is used to wrap sandwiches because it is ‘sold for this purpose’ not because it has important properties designed to make it well suited to the task, i.e. plastic wrap is waterproof, light and flexible, transparent, easily folded and unfolded, creates an air tight seal on contact and is low cost.
Students frequently have tacit knowledge of the properties of a wide range of materials that they have encountered. Because materials they use have been chosen by others based on their properties students seldom have a need to identify specific properties and in most cases they do not see the need to ask such basic and sometimes obvious questions as:
- why are windows made from glass?
- what properties of bricks make them so suitable for constructing walls?
- why do we select fluffy or hairy materials to construct garments to keep us warm?
The properties and structure of materials are interrelated and determine their behaviour. Their uses are determined by their properties, some of which can be changed and enhanced by processing.
Throughout history, humans have found the need to modify naturally occurring materials to enhance useful properties. Increasingly humans’ reliance on processed materials has required modification of materials to create new ones that have not previously existed. Plastics are an important example of this. The more processed a material is, the more likely the material will create problems of waste disposal.
Critical teaching ideas
- Different materials have different properties, such as colour, strength, texture, smell, hardness, flexibility and also cost which determine their applications and likely use.
- Natural materials are often selected for applications which exploit their properties and are also used because of their availability or cost of production.
- Natural materials can be combined, mixed, heated or treated in a combination of ways to produce processed materials with changed or enhanced properties.
Explore the relationships between ideas about natural and processed materials in the
Concept Development Maps (Atoms and Molecules, Chemical Reactions, Conservation of Matter,
States of Matter).
Collect evidence/data for analysis and open up discussion via a shared experience
This activity aims to introduce students to a range of natural and
synthetic materials and promote discussion about the enhanced properties of synthetic dyes. Students could also discuss and identify some of the disadvantages of synthetic dyes in comparison with natural dyes, such as waste disposal, safety handling issues and allergies.
Assemble a number of natural foods which have a strong colour and could be used as possible sources of natural dye (such as beetroot, blackberries, saffron, red currants, raspberries and flower petals) and purchase a few commercial synthetic cold water dyes. Prepare a range of clothing samples from both natural and synthetic materials (such as cotton, silk, nylon, terylene and polyester), identify each and place samples into the containers with the different dyes. Leave these for a couple of days and then using gloves remove them and rinse them under cold water and air dry.
Encourage the students to evaluate how effective the natural and synthetic dyes were in creating a permanent colour change in the cloth. Which of the natural materials were most affected by the dyes and which ones appeared to fade when washed? Discuss the advantages and disadvantages of natural and synthetic dyes and their ability to effect colour fast changes in a range of different materials.
Consider investigating stories about the development of processed material, such as Tyrian ‘Royal’ Purple and the accidental discovery of Mauve in 1856 by Sir William Perkins.
Clarify and consolidate ideas for/by communication to others
This activity aims to encourage students to identify and discuss the physical properties of a range of materials and suggest on the basis of the properties identified how they could be best used.
Collect and introduce the class to a range of natural and processed materials. Avoid presenting them with easily identifiable objects designed for a specific purpose (such as a building brick). Encourage the class to identify a number of individual properties associated with each material and then invite them to select a material best suited for a specific task.
For example, assemble a number of small pipes or tubes made from the following materials: glass, PVC, cardboard, metal, ceramic and bamboo. After the students have discussed and identified the different properties of each material, have them work in groups to identify the best material to perform the following tasks/purposes: transport cold water, transport boiling water, make tent poles, hold electricity wires, construct playground climbing equipment, construct packaging for sending mail and make a fire.
Encourage students to discuss and identify the properties which make the material well suited to the task. Students could also construct a list of other natural and processed materials well suited to perform a required task.
Another approach to this type of activity is presented in the PEEL vignette
Why don’t we make underwear out of glass?
Challenge existing ideas and collect evidence/data for analysis
Collect a range of plastic and paper bags suitable for collecting supermarket shopping. Cut a similar size strip from each and devise ways of measuring the force needed for them to break.
For example, attach a bucket using a length of string and compare the numbers of cups of sand required in the bucket before the strip breaks. Encourage students to draw a simple picture graph to represent the number of cups required to break each strip. Discuss the advantages and disadvantages of each of the materials tested.