Summary

In this activity, students will view an animation that explores how ionic and molecular compounds dissolve (or don’t) in water. They will see that if an ionic compound such as salt dissolves, the ions dissociate, whereas the molecules in a molecular compound such as sugar remain intact but are separated from one another by water molecules. They will also see that some ionic compounds such as chalk do not dissolve, and the cations and anions remain stuck together.

Grade Level

Middle School, High School

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Developing and Using Models
  • Engaging in Argument from Evidence

Objectives

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

• Discuss why some compounds dissolve and others do not.

Chemistry Topics

This activity supports students’ understanding of:

• Solubility
• Ionic and covalent bonding
• Intermolecular forces

Time

Teacher Preparation: minimal
Lesson: 10-30 minutes

Materials

• Computer and projector with internet access
• https://teachchemistry.org/classroom-resources/solubility-animation
• Student handout

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• All of the animations that make up the AACT Animation collection are designed for teachers to incorporate into their classroom lessons. Intentionally, these animations do not have any spoken explanations so that a teacher can speak while the animation is playing and stop the animation as needed to instruct.
• If you assign this to students outside of class time, you can create a Student Pass that will allow students to view the animation (or any other video or ChemMatters article on the AACT website).
• We suggest that a teacher pause this animation at several points or watch it more than once to give students the opportunity to make notes, ask questions, and test their understanding of the concepts presented. The animation is about a minute and a half long and moves quickly, so students will likely require pausing or multiple viewings to successfully complete the student activity sheet if you choose to use it. Here are some of the points at which you might want to pause the video to allow students to answer the questions on the activity sheet:

• This animation can be used in a unit on solubility to help students understand that some compounds dissolve easily and others do not, and it depends on the types of bonds and intermolecular forces in the solute. This animation uses examples of everyday substances to show a soluble ionic compound (salt, NaCl), a soluble molecular compound (sugar, C₁₂H₂₂O₁₁), and an insoluble ionic compound (chalk, CaCO₃). For this animation to be most helpful for students, they should have been introduced to the concepts of ionic bonding, covalent/polar covalent bonding, and “like dissolves like” prior to viewing.
  • In more advanced classes, the more nuanced concept of varying degrees of solubility can be introduced as well. While many low-solubility compounds, including CaCO₃ (chalk), are generally labeled “insoluble,” most substances will dissolve to some extent, even if it is an extremely small amount. For general purposes, the term “insoluble” is applied to substances with very low solubility. The solubility product constant (K_sp) can be used to evaluate which of the “insoluble” compounds are most/least soluble. Conversely, even compounds labeled as “soluble” generally reach a point at which, with the addition of enough solute, the solution becomes fully saturated, and no more of that solute will dissolve.
    
• The sugar and chalk examples are a good way to start a discussion about some of the nuances of solubility that are often glossed over when introducing ionic vs. covalent bonds. Often when first learning about the properties of ionic compounds vs. molecular compounds, students are told that, generally, ionic compounds dissolve in water and molecular compounds tend not to (perhaps with the caveat of “unless they are polar”). However, sugar is an example of a (polar) molecular compound that does dissolve, and chalk is an example of an ionic compound that does not dissolve, seeming to break the “rules” they may have learned previously.
  • For more advanced students, this may be a good way to introduce the idea of a spectrum of polarity (and the concept of electronegativity values) rather than distinct “buckets” of ionic, polar covalent, and covalent bonds.
    
• The conclusion questions require students to propose explanations and explain their reasoning on questions they may not know the correct answers to just yet – that’s ok! These questions can be used as discussion prompts (maybe in a think-pair-share activity) to help students practice deep thinking about why some substances are soluble, and others are not, and how this relates to their inter- and intramolecular forces. Students may or may not draw accurate conclusions initially, but the value is not so much in making sure they write down the correct answer; rather, these discussions can give them an opportunity to practice making and evaluating scientific arguments and supporting their claims with evidence and reasoning based on what they know about solubility and intermolecular forces. Some links are also provided in the answer key to provide further background information related to those questions.
• Related classroom resources from the AACT Library that may be used to further teach this topic:
  • Unit Plan: Aqueous Solutions Unit Plan
  • Activity: The Chemistry of Water Video Questions
  • Activity: Basic Modeling of the Dissolving Phenomenon
  • Lab: Solubility & Compound Type
  • Lab: Experimental Determination of the Solubility Product Constant for Calcium Hydroxide