Summary

In this activity, students explore the process of salt dissolving in water using cut-outs of ions and water molecules to model interactions between them. They then use their model to make a prediction about the relative solubility of salt in isopropyl alcohol compared to the solubility in water and design an experiment to test their prediction.

Grade Level

High and Middle school

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

Objectives

By the end of this activity, students should be able to

• Design a 2-dimensional model of the interactions of water molecules and ions during the dissolving process.
• Explain, on the molecular level, why water dissolves salt.
• Use their model to predict the relative solubility of salt in isopropyl alcohol and water.
• Design an experiment to test their prediction.
• Refine their model based on observations made while conducting their experiment.

Chemistry Topics

This activity supports students’ understanding of

• solutions
• solubility
• dissolving
• intermolecular attractions
• solute
• solvent

Time

Teacher Preparation: 15 minutes

Lesson: 90 minutes

Materials

• Student activity sheet with ion and molecule cut-outs (one per group)
• Scissors (one per group)
• Water^*
• Isopropyl alcohol^*
• Table salt^*
• Plastic cups (4 per group)
• Graduated cylinder
• Electronic Balance

^*Amounts will vary depending upon the lab plans designed by your students

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• Read more about this activity, and the related activity, Advanced Modeling of the Dissolving Phenomenon, in the associated article, Using Learning Progression to Improve Scientific Modeling in Chemistry in the May 2018 issue of Chemistry Solutions.
• Acknowledgements: This lesson is based on the “Why Does Water Dissolve Salt?” activity from Chapter 5, Lesson 3 on the Middle School Chemistry website. This website, brought to us by the American Chemical Society, allows students to investigate the world of atoms and molecules. All of the images used in the student documents are also from the Middle School Chemistry website.
• The student cut-outs for this activity are available as a separate file download.
• Images are Copyright © 2018 American Chemical Society. Used with permission from middleschoolchemistry.com
• Engage: Engage your students by asking them to describe what happens when salt dissolves in water. Provide them with an image of salt and water and ask them to think about how the structure of each might be part of the dissolving process.
• Explore: You students will then explore the dissolving process by building a two-dimensional model of a salt lattice with cut-outs of sodium and chloride ions. Ask them to explain what is holding the solid structure together.
• Explain: Use the image below to show your student what happen during the dissolving process. Ask them to describe the interaction between the charged ions and the water molecules. Then ask them to move their cut-out ions and molecules to demonstrate the dissolving process.

• Elaborate: Using the image below, have students discuss the difference in structure between water and isopropyl alcohol molecules. Have them make a prediction based on the following question, “Thinking about the polarity of water molecules and alcohol molecules, do you think alcohol would be just as good, better or worse than water at dissolving salt?” Their prediction must include information about their original dissolving model, polarity, interparticle forces, and structure.

• Evaluate: Students design an experimental procedure to test their predictions. Each group should record their procedures and get them approved before conducting their experiment. Based on their observations, have them answer the following question, “Is alcohol just as good, better, or worse than water at dissolving salt?” They should fully explain their answer using their observations and the model of dissolving that they developed at the beginning of the lesson.