19 Introducing particle models 11-14 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

By interpreting diagrams on cards that show representations of particles, students reinforce their understanding of the terms mixture and pure substances (elements and compounds). They explore the use of models, including three-dimensional models.

The cards may be used at different levels of sophistication. For example, compounds are seen as made up of two or more different types of particles, but in a given compound the ratio of these is always the same.

Learning objectives

Students will be able to explain that:

  • substances are made up of tiny particles
  • differences between mixtures and pure substances can be modelled using particles.

Sequence of activities

Give each student a copy of the worksheet Mixtures or pure substances?.
Ask students to:

  • work in pairs
  • explain one to the other what a mixture is and the other student to explain what a pure substance is
  • agree and write down their explanations.

Open a large packet of Smarties and spread them out on a sheet of white paper. Invite students to describe what they see.

  • Ensure that the word mixture is used and understood.
  • Separate into piles of ‘like’ sweets.
  • Describe how this might be used as a model for the particles of which substances are made.

Share the objectives with the students and explain that they will look at modelling more carefully.

Give each pair a pack of Particle cards. Ask them to sort the cards into two heaps: mixtures and pure substances.

When finished, lead a class discussion using the prompt questions.

How does this help you ‘picture’ the difference between mixtures and pure substances?

How does it fit with the explanation you wrote earlier?

What might you want to add to your explanation?

Go around the pairs and ask students in turn to hold up a card that represents either a mixture or a pure substance (it must be different to others that have been shown). Ask the remainder of the class if they think each card is correct, and if not why they think differently.

Reinforce the same ideas using 3‑D models. Fill large beakers with different ‘particles’ made from suitable molecular modelling kits. Invite students to identify which are mixtures and which are pure substances, by using thumbs up or traffic light cards.

Explain the value of using 3‑D models of particles rather than 2‑D pictures.

Ask each pair to sort the piles of pure substances cards into further ‘like’ piles.
With the whole class, invite pairs to explain the reasons for sorting the cards as they did.
This is an opportunity to draw out the ideas of (a) elements and compounds, and (b) solids, liquids and gases.

Assessment for learning commentary

Although working in pairs, students are making an assessment of their own understanding at each stage of this session. Any insecurity about individual learning is not, however, exposed to the whole class as they give their responses jointly.

Teacher questions have an important role in confirming and reinforcing the concepts.

Resources

Stimuli

  • A large packet of Smarties and A2 sheet of white paper.

Props

For each pair of students

Download Word Download PDF Mixtures or pure substances?
Download Word Download PDF Particle cards
  • Set of traffic light cards

Answers

Page

Top picture

Middle picture

Bottom picture

1

liquid; mixture; elements

gas; mixture; elements

solid; mixture; elements

2

gas; pure; element

liquid; pure; element

solid; pure; element

3

liquid; pure; compound

gas; mixture; compounds

liquid; mixture; compounds

4

gas; pure; element

liquid; pure; element

solid; pure; element

5

gas; pure; element

liquid; pure; element

solid; pure; element

6

gas; pure; element

liquid; pure; element

solid; pure; element

7

gas; pure; element

liquid; pure; element

solid; pure; element

8

gas; mixture; elements

gas; mixture; element and compound

gas; mixture; compounds

9

gas; pure; compound

liquid; pure; compound

solid; pure; compound

10

gas; pure; compound

liquid; pure; compound

solid; pure; compound

11

gas; pure; compound

liquid; pure; compound

solid; pure; compound

12

gas; pure; element

liquid; pure; element

solid; pure; element

13

liquid; mixture; elements

gas; mixture; elements

liquid; mixture; elements

14

gas; pure; element

liquid; pure; element

solid; pure; element

Acknowledgements

K. Taber, Chemical misconceptions – prevention, diagnosis and cure Volume 2: classroom resources. London: Royal Society of Chemistry, 2002.