Developing understanding of alkane boiling points

Developing understanding is a series of resources that encourages learners to connect their thinking at the macroscopic, sub-microscopic and symbolic levels.

 Learning objectives

1. Write the formula and displayed formula of any of the first four alkanes.
2. Determine the state of an alkane at a given temperature from boiling point and melting point data.
3. Explain why the alkanes have different boiling points.
4. Explain the trend in boiling point of the alkanes.

How to use this resource

This resource aims to develop learners’ understanding of the different boiling points of alkanes. The questions encourage learners to think at the macroscopic, sub-microscopic and symbolic levels about why different alkanes have different boiling points and the trend in boiling points. As a result, learners should develop more secure mental models to support their thinking about this topic.

• When to use? Use after initial teaching or discussion of this topic to develop ideas further. You can also use as a revision activity.
• Group size? Suitable for independent work either in class or at home. Or use the questions for group or class discussions.
• How long? 15–30 mins

Johnstone’s triangle

Johnstone’s triangle is a model of the three different conceptual levels in chemistry: macroscopic, sub-microscopic and symbolic. You can use Johnstone’s triangle to build a secure understanding of chemical ideas for your learners.

Introduce learners to Johnstone’s triangle with our Alkane boiling points Johnstone’s triangle worksheet which guides learners to connect their understanding of the macroscopic properties of octane and butane with their symbolic representations.

Johnstone’s triangle and this resource

The icons in the margin indicate which level of understanding each question is developing so as to help prompt learners in their thinking.

• Macroscopic: what we can see. Think about the properties that we 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 using symbols and diagrams.

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. Our approach has been to apply icons to questions based on what the learners should be thinking about.

Questions may be marked with two or all three icons, indicating that learners will be thinking at more than one level. However, individual parts of the question may require learners to think about only one or two specific levels at a time.

Support

This worksheet is ramped so that the earlier questions are more accessible. The activity becomes more challenging in the later questions, so you may want to give extra explanations for these. If completing the worksheet as an in-class activity, it is best to pause and check understanding at intervals, as often one question builds on the previous one.

Give learners physical models to use and manipulate, such as Molymod kits.

Additional support may be needed for any learners still lacking in confidence in the required symbolic representation, for example, by sharing and explaining a diagram or a simulation that can show movement of the particles.