Summary

In groups of six to eight, students will observe the behavior of substances and mixtures to determine the relative strength of intermolecular forces between the particles in each substance or mixture. They will then arrange different cards representing ions and molecules based on intermolecular forces to determine the best molecular level representation of the physical samples they observed.

Grade Level

High School

NGSS Alignment

This demonstration 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 at the bulk scale to infer the strength of electrical forces between particles.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Constructing Explanations and Designing Solutions

Objectives

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

• Provide evidence that indicates that some intermolecular attractions are stronger than others.
• Explain how the strength of attractions can affect physical appearance.
• Give one possible explanation as to why some ionic compounds dissolve and others do not.

Chemistry Topics

This demonstration supports students’ understanding of:

• Covalent Bonding
• Ionic Bonding
• Lewis Structures
• Intermolecular Forces

Time

Teacher Preparation: 15 minutes

Lesson: 40-60 minutes

Materials

• White boards (1)
• White board markers
• Na⁺ Cards (4)
• Cl^– Cards (4)
• H₂O Cards (8)
• NH₃ Cards (4)
• Fe³⁺ Cards (2)
• O^2– Cards (3)

Per class, for teacher use:

• 250-mL beakers (3)
• Water
• Solid NaCl
• Solid Fe₂O₃
• Ammonia solution

Safety

• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• Prior to this demonstration, students should have an understanding of ionic bonding, Lewis Dot Structure, and identifying molecular polarity. (You may also require your students to know the VESPR Theory in order to properly draw the water and ammonia.)
• The teacher will present a sample of each substance/mixture to the students. Make sure each group is able to get a good look at each sample so they can record detailed observations. The students will then try do describe how that sample looks on the atomic level using the cards with their group. The group consensus will be written on the white boards and displayed for the class to see. The class will discuss the white boards and come to an overall consensus which will be recorded in the area provided in student handout.
  • The ammonia solution should be prepared in advance, but the NaCl-water mixture and the Fe₂O₃-water mixture should be prepared while the students observe.
  • If you allow students to observe the smell of the ammonia water mixture, be sure they safely waft the vapors to their nose, rather than sticking their noses in the beaker!
• The activity is designed for 4 groups of 6 to 8 students.
• Molecule and ion cards can be found in the “Cards” download in the sidebar. (Note that students will be drawing their own Lewis structures of water and ammonia so the boxes only include the formulas.) There are enough cards for 4 groups of 8 students, but you can print more cards if you want students to work in smaller groups.
• As you are discussing the models as a class to come to a consensus model, encourage students to think about the states of matter of each substance and how they could be affected by intermolecular forces. Students should reach the conclusion that the particles would only be fixed in place in a rigid structure (solid) if the attractions are strong enough to keep the particles from moving around one another. They might also draw the connection between full charges in ionic compounds having a stronger attraction and more rigid structure leading to their solid state, as opposed to the partial charges in the polar molecules that have a little more flexibility in where and how the molecules can move, hence the liquid state. Be sure their models show more space between the liquid particles, indicating that they have room to move around one another.
• Following question #1 you many want to discuss with the class why the NaCl molecules disassociate in water even though they have a relatively strong attraction to one another. The discussion should lead to the fact that the greater amount of water molecules overcome the stronger attraction of Na⁺ to Cl^– ions. The many water molecules with partial charges combine to allow for a stronger total attraction of Na⁺ to H₂O and Cl^– to H₂O than the attraction between Na⁺ and Cl^–.
• Students may not be able to answer analysis questions 3 and 4 if they don’t understand the answer to question 2.If your students struggle with either of those questions, you may want to have them talk through their answers to 2 with you to make sure they fully comprehend it, and then guide them from there to how those same concepts relate to questions 3 and 4.