Developing understanding of fractional distillation

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. Explain why different fuels have different boiling point ranges.
2. Explain how diesel and petrol are separated in a fractionating column.
3. Explain why butane and bitumen are collected at the top and bottom of a fractionating column.
4. Explain the order in which fractions are collected in a fractionating column.

How to use this resource

• 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

This resource aims to develop learners’ understanding of the idea of the separation of crude oil with a fractionating column. The questions encourage learners to think at the macroscopic and sub-microscopic levels about how a fractionating column works. As a result, learners should develop more secure mental models to support their thinking about this topic. 

Johnstone’s triangle

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

Johnstone’s triangle and this resource

The icons in the margin indicate which level of understanding each question is developing 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 including 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. You can give extra explanations for the more challenging questions. If completing this 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.