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Introduction

Representing elements, compounds and mixtures using particle models can help us understand their properties. There are significant differences in properties between pure substances and mixtures. In a pure substance the atoms are usually strongly bonded together to form an element or compound molecule and all the 'particles' are identical. In a mixture there are at least two different particles, i.e. different atom combinations that may be elements or compounds.

Task 1: Particles in elements, compounds and mixtures


Model 1

  1. Open Yenka file Model 1. Select each of the vessels in turn and examine its atom viewer. Which are elements and which compounds?
    Answer
    Elements: aluminum, mercury, gold.
    Compounds: hydrochloric acid, potassium chloride, sodium sulfate.
  2. The diagram shows five particles of elements and compounds. Each circle represents an atom of an element. Match the pictures A to E to the following questions.


    Diagram

    (a) Which particle picture represents a mixture of two compounds? ____
    (b) Which particle picture represents a pure compound? ____
    (c) Which particle picture represents a mixture of an element and a compound? ____
    (d) Which particle picture represents a pure element? ____
    (e) Which particle picture represents a mixture of two elements? ____
    (f) Which particle picture could represent pure water? ____
    (g) Which particle picture could represent something dissolved in water? ____

    Answer
    (a) C; (b) D; (c) A; (d) B; (e) E; (f) D; (g) A


Summary

In particle pictures, a pure substance will be represented by a single type of particle, either a single circle for one atom (element) or several atoms joined together to form a molecule (element or compound).

In a particle picture of a mixture, there will be at least two different types of particles, which can be elements (single atoms or larger molecules of two or more atoms) or compounds (molecules of two or more atoms of different elements combined together).

Task 2: Differences between pure substances and mixtures

  1. The following statements apply either to pure substances or mixtures but not both. Decide which statement applies to which:
    (a) It is made up of elements in fixed proportions.
    (b) The components can be readily separated, e.g. by filtration or distillation.
    (c) The boiling point changes (rises) as the substance boils away in distillation.
    (d) It is very difficult to separate the components of the substance.
    (e) The elements or compounds are not in fixed proportions.
    (f) The substance melts or boils at a specific temperature.
    (g) The substance melts gradually over a few degrees temperature range.
    (h) There is no significant energy change on its formation.
    Answer
    Pure substances: (a), (d), (f)
    Mixtures: (b), (c), (e), (g), (h)

Summary

Pure substances melt and boil at a fixed temperature; their composition is fixed, i.e. the same atomic ratio or molecular formula; it is not easy to separate the atoms whether it is an element molecule or a compound.

In a mixture, the melting and boiling points vary depending on the proportions of the components (elements or compounds); the proportions of the components are not fixed and vary; the components are readily separated from each other; the mixture properties are often similar to those of the components themselves.

Teacher Summary

  • This activity extends the use of particle pictures from just using them as particle models for gas, liquid and solid.