Dissolution of Ionic Compounds in Water

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Objective

SWBAT to diagram and describe what occurs at the molecular level during the dissolution of an ionic compound in water (a polar substance).

Big Idea

Water is polar, with regions of slight positive and negative charge; this polarity attracts and pulls apart cations and anions of an ionic compound.

Why This Lesson?

This lesson focuses on allowing students to explain the process of dissolution by examining polarity.  Students understand that some thing dissolve on a bulk scale, but may not fully comprehend what is occurring on a molecular scale.  This lesson is designed to help students see patterns in attraction between opposite charges and the outcome of those attractions.  In particular, this activity addresses one of the Crosscutting Concepts related to PatternsXC-P-HS-1.  

Additionally, this lesson incorporates two different Science and Engineering Practices.  Students are drawing a model of an ionic compound dissolving in water (SEP 2) and providing explanations for how the dissolution occurs (SEP 6).

This lesson occurs at the end of this unit at a point where students are already expected to understand ionic bonds, ions, and polarity.  Diagramming and explaining their individually created models helps to synthesize pockets of learning that have occurred throughout the unit and adds to a greater overall understanding of how all of these concepts tie in together to explain this particular phenomenon.

Warm-Up

5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use today's warm-up to probe for students' prior knowledge about the day's upcoming lesson and to have them start thinking about what they will be learning.  (To read more about Warm Up and Reflection Books, please see the attached resource.)

Today's Warm-Up: "Why do you think ionic compounds dissolve in water? (Hint: It has to do with water's polarity!)"

I am hopeful that some students might recall that we discussed in prior lessons that ionic compounds are formed by ions.  We have also discussed briefly that ionic compounds have a crystal structure with alternating cations and anions that are held together by electrostatic attraction.  Yesterday, I drew a Lewis Dot Structure of water and we discussed its polarity due to both the electronegativity difference between oxygen and hydrogen as well as oxygen's lone pairs of electrons.  I anticipate that students will express a vague understanding, but will not articulate it very well.  

As students complete the warm-up, I walk around and read student responses.  I stamp the books of students who demonstrate any type of deeper thinking about the prompt.  Answers like, "because they do," or "that's how it works," do not get a stamp.

I ask students if anyone cares to share what their answer was, and call on one student once there are at least 5 hands in the air.  After that student answers, I ask if anyone else had other ideas or if they agreed with the prior student's response.  At no time during this warm-up discussion do I give students a "right" or "wrong" indication regarding their answers.

 

Intro to Activity

10 minutes

First, I draw a Lewis dot structure of a water molecule on the whiteboard.  After drawing the molecule, I ask students the following questions to help them walk through the reasoning why water is a polar molecule:

  • Which side of the water molecule has a slightly negative charge?  (The oxygen side.)
  • Why?  (Oxygen is more electronegative than hydrogen, and oxygen has two lone pairs of electrons adding to the electron density around it.)
  • Which side of the molecule is slightly positive?  (The side with two hydrogens.)
  • Why?  (Hydrogen is less electronegative so oxygen pulls the electron density from it.)

Previously, students learned that ionic compounds consist of nonmetal and metal atoms that have transferred electrons leaving metallic cations and nonmetallic anions.  Students also know that these ions are held together by electrostatic attraction.  

Before I diagram an ionic compound crystal structure, I ask students the following questions:

  • How are ionic compounds arranged?  (I pause because most of my students do not know what I mean.)
  • What are ionic compounds made of?  (Ions.  Note: Some of my students will answer, "a nonmetal and a metal."  I follow up that answer with these questions:  What happens to the electrons in an ionic bond?  They are transferred from the metal to the nonmetal.  What happens after the electrons are transferred?  Ions are formed.)
  • How are those ions arranged?  (If students do not readily answer, I follow with "Do all of the cations hang out together?"  The positively charged ions alternate with the negatively charged ones.)
  • Why?  (Like charges repel each other, but opposite charges attract.)

After diagramming an ionic compound, complete with alternating ions with charges indicated, I use a different colored marker to outline the atoms in the Lewis dot structure of water previously drawn on the whiteboard.  I show students that a water molecule looks like a Mickey Mouse head (if they don't immediately say that themselves--which sometimes they do!).  I ask them what the charge density is on the "ears" of the water molecule (positive).  I also ask them what type of ion those "ears" or hydrogens would be attracted to (the anions).  Then I explain that as the water molecules surround an anion in the crystal structure, once there are enough water molecules surrounding the ion, it is pulled from the crystal because it is no longer as electrostatically attracted to the surrounding cations.  I say the water molecules almost "shield" the charge that the anion feels.  Then I diagram an anion surrounded by water molecules with the "ears" closest to the anion.  I also diagram a cation surrounded by water molecules.  As I begin to surround the cation with water, I ask students which side of the water molecule should be closest to the cation (the negative side) and make sure to draw it as such.

A picture of the whiteboard when I am done diagramming is included here: 

Individual Drawing Activity

35 minutes

I handout the Dissolution of an ionic solid drawing handout to my students.  I explain that I expect them to diagram similarly to mine, not exactly the same.  This is a key distinction as I expect my students to demonstrate clear understanding of the content and not just ability to copy my diagram.  I also explain that sometimes accompanying text can help explain a diagram more clearly, and I encourage them to add explanations where necessary for clarity.

Some student samples are shown below. 

 

Here is student work that mirrors my diagram, has no explanation, and does little to show me that this student understands:

These two student samples have some explanation included, but still miss the mark.  

 

These next two samples include enough diagramming coupled with explanation to show that these students understand the process of dissolution of an ionic compound in water:

 

This last sample is actually from a student who struggles with the English language, having just moved here from Armenia a year ago.  See the attached reflection for more about how this activity helped her demonstrate her understanding and make connections to the English academic vocabulary.

 

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students.  

Today's Reflection Prompt:  "Why are cations and anions attracted to different sides of a water molecule?"

Desired student responses should indicate that:

  • Cations have a positive charge and anions have a negative charge; cations and anions are oppositely charged.
  • Water is polar--the lone pairs of electrons on the oxygen atom coupled with oxygen's higher electronegativity value means that side of the molecule is slightly negative, leaving the hydrogens' side slightly positive.
  • Positive areas are attracted to negative charges and negative areas are attracted to positive charges