How can students learn the differences between mixtures and compounds with the aid of modeling clay?

Mixing iron powder and sulphur powder results in a mixture. When the mixture is heated strongly, it becomes the compound iron sulphide. This is a typical experiment done by secondary 4 chemistry students in Hong Kong when they learn the concepts of mixtures and compounds. Because the reaction produces toxic sulphur dioxide, some teachers carry out the experiment as a demonstration using a test-tube or a bottle cap.

However, there are two major limitations of the above experiment: (i) some secondary 4 students may mistakenly believe that simple contact of two materials will never produce a new chemical compound; and (ii) students cannot "see" the fixed proportion of iron and sulphur particles in the compound.

To supplement the experiment, you may plan a hands-on learning activity for students. The following procedures are adapted from Papageorgiou (2002):
  1. Provide students with yellow and blue modeling clay (these two colours will give very good results).
  2. Make small balls about equal in size of each colour of modeling clay to represent iron atoms and sulphur atoms.
  3. Place a random number of yellow-coloured and blue-coloured balls on a sheet of white paper. This is analogous to a mixture of iron and sulphur. The characteristics of the mixture are:


  4. Mix together a yellow-coloured ball and a blue-coloured ball (i.e., a 1:1 proportion). A green ball should be formed and is analogous to a new chemical compound. The characteristics of the compound are:
    • Its components (i.e., yellow-coloured and blue-coloured balls) are in fixed proportions (1:1).
    • It cannot be physically separated into its components (by separating the balls according to their colours).
    • The properties of the components are not retained (i.e., a new colour is formed).


  5. Ask students to solve this Conceptual Challenge Question: "Place some balls on a sheet of white paper to represent a mixture of the element sulphur and the compound iron sulphide."



This learning activity is interesting and effective. Modeling clay is available in toy stores. Remind students to wash their hands after using the modeling clay. Papageorgiou (2002) also suggested that different proportions of yellow-coloured and blue-coloured balls may be mixed together to produce water (H2O) and hydrogen peroxide (H2O2). Read his article for details.



Reference

Papageorgiou, G. (2002). Helping students distinguish between mixtures and chemical compounds. Science Activities, vol. 39(2), pp. 19-22.