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.
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
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.
This demonstration supports students’ understanding of:
- Covalent Bonding
- Ionic Bonding
- Lewis Structures
- Intermolecular Forces
Teacher Preparation: 15 minutes
Lesson: 40-60 minutes
- White boards (1)
- White board markers
- Na+ Cards (4)
- Cl– Cards (4)
- H2O Cards (8)
- NH3 Cards (4)
- Fe3+ Cards (2)
- O2– Cards (3)
Per class, for teacher use:
- 250-mL beakers (3)
- Solid NaCl
- Solid Fe2O3
- Ammonia solution
- Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
- 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.
For the Student
Have you ever wondered why a solid is a solid or a liquid is a liquid? Recall you have already learned about ionic bonding, molecular compounds and polarity. Now we will look at some of the interactions between these particles.
- In this activity, you will be observing the four different substances, listed below. Classify each one as an ionic or molecular compound:
- Water, H2O___________________
- Sodium chloride, NaCl___________________
- Iron(III) oxide, Fe2O3___________________
- Ammonia, NH3___________________
- Draw the following Lewis structures and identify dipole moments, if any are present, using ∂+ and ∂–.
Why are some materials solids and others liquids?
- White board
- White board markers
- One Na+ card each for half the group members and one Cl– card for the other half
- One H2O card per person
- NH3 cards for half the group members
- Two Fe3+ cards per group and three O2– cards per group
- With your group, discuss your Lewis structures for water and ammonia from the pre-lab questions and develop a consensus structure for each molecule. Draw the appropriate structure on each of your water and ammonia cards.
- Observe the sample of solid NaCl presented by your teacher and record your observations in the table below.
- Arrange your group’s Na+ and Cl– cards to represent that sample on the atomic level. Once your group has created the arrangement, draw a model on the white board to represent your arrangement.
- When finished, place your board in the display location.
- Discuss the models as a class to develop a consensus model and record it in the table below.
- Complete steps 2 – 5 observing the water and using the water molecule cards.
- Complete steps 2 – 5 observing the water and ammonia mixture. To make your arrangement half the group should have water cards and half the group should have ammonia cards.
- Complete steps 2 – 5 after you observe what happens when NaCl solid is placed in the water. To make your arrangement one student should have Na+, one student should have Cl–, and the rest of the group members should have water molecules.
- Complete steps 2 – 5 after you observe what happens when Fe2O3 solid is placed in the water. To make your arrangement two students should have Fe3+cards, three students should have O2– cards, and the rest of the group members should have water molecules.
Na+ and Cl–
H2O and NH3
H2O, Na+ and Cl–
H2O, Fe3+ and O2–
- What has a stronger attraction, sodium to chloride ions in a salt sample or water molecules to one another? Justify you answer.
- The NaCl dissolved in the water, but the Fe2O3 did not. What can be concluded from this about the strength of the attraction in NaCl and in Fe2O3? Give a possible reason for these results based on your knowledge of ions and formulas of ionic compounds.
- You have two samples of white powder. One of them is KBr and one is AlPO4. Based on what you observed in this activity and your knowledge of ionic compounds and their formulas, design a test to determine which sample is which.
- What do you think would happen if you put NaCl in a sample of a non-polar liquid, such as hexane, C6H14? Support your hypothesis with evidence from this activity.
Using concepts from this activity, explain in a short paragraph why certain materials are liquids and others are solids. Additionally, predict what this might mean for the attractions between particles in a gas sample.