Chromatography | Structure strip

Learning objectives

1. Construct and interpret diagrams of the apparatus used for chromatography experiments.
2. Describe how you can tell that a sample is a mixture from its chromatogram.
3. Explain why samples travel different distances in chromatography experiments.

Introduction

Chromatography is a useful separation technique based on chemicals’ solubilities – in other words, how well chemicals dissolve in particular solvents. In the pharmaceutical industry, chromatography is used to check the purity of medicines. In the food and drink industry, it is used to detect contaminants in batches of produce. And forensic scientists use chromatography to detect the presence of tiny amounts of chemicals in crime scene samples, such as hair or soil.

There are many different types of chromatography, but paper chromatography is the technique you focus on at school, and it’s the type of chromatography we’ll discuss in this structure strip. 

If possible, get learners to carry out a chromatography experiment before attempting this structure strip. Look at the paper chromatography practical video for more information.

How to use structure strips

Structure strips are a type of scaffolding you can use to support learners to retrieve information independently. Use them to take an overview at the start of the topic, to activate prior knowledge, or to summarise learning at the end of a teaching topic. For more ideas on how to use structure strips with your learners, visit 5 ways to use structure strips effectively. 

Structure strips have sections containing prompts, sized to suggest the amount that learners must write. Learners glue the strips into the margin of an exercise book and write their answers next to the sections, in full sentences. When learners have finished using the structure strip, they should have an A4 page set of notes and examples.

The strips are printed five to a page and will need to be trimmed to size. Find them on the second page of the student sheet.

Scaffolding

• Encourage learners to use the suggested key words and phrases in their answers.
• To further support learners, include additional prompts in the structure strip. If learners are struggling to engage with the task, supply them with sentence starters created from the model answers.
• As learners grow in confidence, ask them to attempt the extension question first and then use the structure strip to improve or self-assess their answer.

Key words

Chromatography, mobile phase, stationary phase, chromatogram, R_f value, soluble, solvent, solubility.

Metacognition

This resource supports learners to develop their metacognitive skills in three key areas.

• Planning: the strips provide scaffolding to plan the written response. Learners will decide where to gather information from (textbooks, own notes, revision websites). Ask learners: is the source of information you are using reliable?
• Monitoring: learners are prompted by the questions in the structure strip and can check their own answers against the prompts. Ask learners: have you covered all of the prompts in the space provided? Do you need to change anything to complete the task?
• Evaluation: learners can self-assess or ask a peer to check their work against the answers. Ask learners: did you achieve what you meant to achieve? What will you do differently another time?

Answers

Suggested answers for the structure strip activity are given in the downloadable teacher notes.

Answer to extension question

Learners are tasked with identifying and explaining the errors in a chromatography set up, and suggesting improvements. Example answers are included in the teacher notes.