Summary

In this activity students build a model of sodium chloride based on their own knowledge of ionic compounds. Then they construct a model of the interactions between water and their salt model to develop an understanding of what caused the salt to dissolve. After refining their models based upon class discussions and critiques, students then construct a model of the interaction between salt and a different solvent, alcohol. Using their models, students make predictions as to which solvent (water or alcohol) would be better at dissolving the salt. Finally students design an experiment to test their prediction. As an extension, students are asked to use their solubility models to explain why calcium carbonate will not dissolve in water, even though it is also an ionic compound.

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

• Construct a 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

• solubility
• dissolving
• intermolecular attractions
• solute
• solvent

Time

Teacher Preparation: 15 minutes

Lesson: 2 hours

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, Basic 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.
• Images are Copyright © 2018 American Chemical Society. Used with permission from middleschoolchemistry.com
• The advanced modeling version of this activity allows for students to develop a deeper understanding of the dissolution process as they try to make sense of this phenomenon, and also to highlight any misconceptions they have.
• Engage: Students will construct a model of sodium chloride salt crystal based on their knowledge of ionic compounds. Using this they will consider how the salt would interact with water in order to dissolve. Students will present their model to the class for discussion and evaluation. Afterwards groups will have the opportunity to discuss the pros and cons of their model and make any necessary changes.
• Explore: Students will then construct a model to explore the interaction between salt and a different substance, alcohol. An image of an alcohol molecule has been provided for their reference on the student sheet. Students should think about what is taking place at the molecular level.
• Explain: Students will make a prediction about what solvent is better at dissolving salt. Next they will design an experimental procedure to test their prediction. Each group should record their procedures and get them approved before conducting their experiment. Procedures should include safety concerns, a list of all materials and equipment you will need (including amounts), and a detailed list of procedures. Based on the experiment results, students should use claim-evidence-reasoning (CER) to formulate a scientific argument for their model based on their evidence.
• Elaborate: Students will visit each group and engage in argumentation of their claim. One student will stay behind to interact with the other groups. After this have concluded groups should be encouraged to discuss their findings and make any adjustments necessary to their model and argument.
• Evaluate: Students should now be able to construct a scientific explanation for the 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.
• Extension: Students use their models and observations to explain why calcium carbonate, which is also an ionic compound, does not dissolve in water.