Cracking alkanes | Johnstone’s triangle

Learning objectives

1. Explain the equipment used for cracking alkanes in the laboratory.
2. Deduce a hydrocarbon product of cracking.
3. Use general formulas to identify the products of cracking.

How to use Johnstone’s triangle

Use Johnstone’s triangle to develop learners’ thinking about scientific concepts at three different conceptual levels:

• Macroscopic – what we can see. Think about the properties you can observe, measure and record.
• Sub-microscopic – smaller than we can see. Think about the particle or atomic level.
• Symbolic – representations. Think about how we represent chemical ideas including symbols and diagrams.

For learners to gain a deeper awareness of a topic, they need to understand it at all three levels.

When introducing a topic, don’t introduce all three levels of thinking at once. This will overload working memory. Instead complete the triangle over a series of lessons, beginning with the macroscopic level and then introducing other levels, in turn, once secure.

The levels are interrelated. For example, learners need visual representation of the sub-microscopic in order to develop mental models of the particle or atomic level.

Scaffolding

Share the structure of the triangle with learners prior to use. Tell them why you are using it and how it will help them to develop their understanding. Use an ‘I try, we try, you try’ approach when introducing Johnstone’s triangle for the first time.

More resources

To further develop learner’s thinking in all areas of Johnstone’s triangle, try our Developing understanding worksheets. These include icons in the margin reterring to the conceptual level of thinking needed to answer the question.