Teaching sequence

Lesson objective

In this lesson students identify common mixtures and their components. They will explore how the properties of a mixture change when the ratio of the components change.

Introduction

Display the words or show images of salad dressing, cough mixture, soil and sand.

Salad dressing.

Source: © David Munns/ Science Photo Library.

Soil.

Source: © ASTA.

Cough mixture.

Source: © ASTA.

Sand dunes.

Source: © Tetra Images/Corbis.

Ask the students if they can think of what these things have in common.

Explain that all these things are mixtures and that most naturally occurring substances are mixtures.

Core

1. Define mixture as ‘a combination of two or more different substances whose basic components remain unchanged when mixed’. Explain that substances that are not mixtures are pure substances because they are made up of only one type of matter.
2. Ask students to write down examples of five mixtures and five pure substances and give reasons why they think they are mixtures or pure substances.
3. Discuss students’ examples and relate to definition
4. Explain that some mixtures are easy to identify because the individual components can be easily seen (heterogeneous mixture), for example, chocolate chip biscuits but in other mixtures the components are so small that it is difficult to determine if it is a mixture (homogeneous mixture), for example, milk, vitamin tablets.
5. Use the learning object Types of matter: pure substances and mixtures to show students the differences in mixtures and pure substances at the particle level.
6. Explain that the same type of mixture can have different ratios of components and this will change the properties, for example, salt water could have a teaspoon of salt or a tablespoon of salt but it is still salt water.
7. Conduct an investigation into how the proportions of components change a mixture by making and testing sherbet. Students record their findings on the worksheet Making sherbet (Word, 390 KB). 
   Teacher note: The procedure for making sherbet can be found in Background information (PDF, 908 KB).
8. Discuss the effect/s of changing the proportions of the different ingredients on the taste of the mixture.

Conclusion

Review students’ understanding of mixtures using the worksheet Common mixtures (Word, 390 KB).