SAM Teachers Guide Solubility Overview In this activity students explore solutions. They create models of solutions and discover the dissolving process involves a consideration of the intermolecular attractive forces. Students explore the solubility rule “like dissolves like” by mixing different substances. Finally, students take into consideration such factors as temperature and number of molecules as they relate to the rate of dissolving and saturation. Learning Objectives Students will be able to: • Create a solution using a model and identify examples of solutions. • Reason that, for dissolving to occur, the attractions between molecules of a solute and water molecules must be stronger than the solute’s intermolecular attractions. • Discuss how temperature affects the rate of solutes dissolving in solvents and the molecular reasons for saturation. Possible Student Pre/Misconceptions • The process of dissolving is a chemical reaction, not a physical change. • Only water can be a solvent. • Covalent bonds are broken when a substance dissolves. • Melting and dissolving are the same thing. Models to Highlight and Possible Discussion Questions After completion of the activity: Models to Highlight: • Page 1 – Creating an Ink Solution o Use this first model to review the definition of a solution including the terms solute and solvent. Remind students that when ink mixes with water, ink is still ink and water is still water. No chemical reaction has occurred. • Page 3 – Evaporating Water Model o Highlight that dissolving is not a chemical process. As the water evaporates, the sodium and chlorine ions are free to rejoin to form a solid salt precipitate. Also, compare this process to the dissolving of sugar in water. The covalent bonds of sugar are not broken. Dissolving is a physical change, not a chemical reaction. •
•
Page 4 – Mixing Polar and Non‑Polar Molecules o Link to other SAM activities: Intermolecular Attractions. Students will need to understand that different substances have different intermolecular attractions and difference in strength accounts for properties such as melting point and dissolving. o Discuss with the students the results of mixing the polar and non‑polar compounds. Compare this to the results when the charge is removed. Take this opportunity to review intermolecular attractions. Page 5 – Like Dissolves Like o Discuss the outcome of running the models. Have students analyze what happens to salt, sugar, and benzene in water versus what happens to these substances in oil. Discuss the connection to polarity and intermolecular attractions. Possible Discussion Questions: • Have students come up with other examples of substances that dissolve easily in water and those that do not. Have students apply their understanding of polarity and the principle of “like dissolves like” to explain why different levels of solubility exist. • Can you think of any other factors besides temperature that might play a role in how fast a solute dissolves? • Demonstration/Laboratory Ideas: o Have students create saturated solutions. Students can then draw and explain what is happening at the molecular level. o Have students mix oil and water. Students can draw and explain what is happening at the molecular level. Connections to Other SAM Activities This activity explores why things dissolve and the principle of “like dissolves like.” The activity explains why non‑polar things tend to dissolve in a non‑polar environment while polar molecules tend to dissolve in polar environments. This activity is supported by Heat and Temperature. Temperature affects the rate at which substances dissolve and the motion of the particles. The Electrostatics activity explains the nature of positive and negative charge interactions and why molecules “stick” together. The Intermolecular Attractions focuses on the attraction between molecules and is directly tied to why things dissolve and the rule of dissolving: “like dissolves like.” Finally, Chemical Bonds outlines the nature of polar versus non‑polar bonds and how these bonds impact the polarity of substances. The Solubility activity supports many other activities as well. In Diffusion, Osmosis, and Active Transport, it helps explain why certain substances can cross a lipid membrane. In order for students to understand the role of transmembrane proteins in Protein Partnering and Function students must understand how non‑polar sections of proteins can dissolve in the lipid membrane. Lipids and Carbohydrates explores the formation of micelles and how non‑polar materials will congregate together in a cluster when in water. Activity Answer Guide Page 1:
1. Take a snapshot when the model
represents a solution (all molecules
uniformly distributed) and place it below.
Sample snapshot:
2. Describe how the intermolecular
attractions are responsible for salt
dissolving in water.
The intermolecular attractions between the ions
are what make the salt crystal fairly stable.
However, when you add water molecules to the
model the intermolecular attractions between the
polar water molecules and the ions are greater
than the intermolecular forces between the ions
themselves. This means that ions break away
from the crystal structure.
Page 3:
1. Describe why a crystal of salt formed as
the water evaporated from a drop of salt
solution.
This snapshot represents a solution because the
droplets of ink are evenly distributed throughout
the water particles.
2. Consider the mixtures listed. According to
your everyday experience, which would meet
the definition of "solution" given above, after
they are allowed to settle? (Check all that
apply.)
(b) (d)
Page 2:
1. Take a snapshot that shows some part of
salt has been dissolved in water.
Sample snapshot:
As the water evaporates, the positively charged
sodium ions and negatively charged chlorine
ions are attracted to one another. They rejoin to
form a salt crystal.
Page 4:
1. Which of the following forms
intermolecular attractions?
(d)
2. In the above model, which of the following
did you find to have the strongest
intermolecular attractions?
(b)
3. How do intermolecular attractions explain
why the polar molecules and non-polar
molecules do not mix?
The polar molecules are more attracted to each
other than they are to the non-polar molecules.
There are intermolecular attractions among all
the molecules; however, the polar molecules
attractions to each other exclude, or prevent it
from mixing with the non-polar molecules.
Page 5:
This sample image shows that both sodium and
chlorine atoms have dissociated from their
crystal structure and are now dissolved in the
water molecules.
1. Does salt dissolve in benzene? Why or
why not?
Salt does not dissolve in benzene. Benzene is
similar to oil because they are both non-polar.
Salt stays attracted to itself in crystal form and is
not dissociated by the non-polar benzene
molecule.
Sample snapshot:
2. Explain how the model explores the
following rule for dissolving: “Like dissolves
like.”
The model shows that polar molecules and ionic
molecules are attracted to each other, as they
both have charges, and mix. Non-polar
molecules do not mix or dissolve in this case.
Finally, the model shows non-polar molecules
mixing with other non-polar molecules.
Page 6:
1. Reset the model. Set the temperature to
"low" and run the model for 20,000 fs (note
the number near the clock at the lower-left
corner of the model window). Take a
snapshot and place it below.
Sample snapshot:
Snapshot taken with temperature set to "High"
for 20,000 fs.
3. Raising the temperature
(a)
4. Compare the dissolving rate of the green
molecules in the blue molecules at different
temperatures. Use the images above to
explain your answer.
Dissolving is represented by the even
distribution of the green molecules in the blue
molecules in the images. The images show the
green molecules more spread out after the same
amount of time when there is an increase in the
temperature. Therefore, one can conclude that
increasing temperature increases the rate of
dissolving.
Page 7:
Picture taken with temperature set to "Low" for
20,000 fs.
1. Place a snapshot of the model after a long
run below and annotate the image to point
out which region represents the part of
solute that hasn't been dissolved.
2. Reset the model. Set the temperature to
"high" and run the model for 20,000 fs. Take
a snapshot and place it below.
Sample snapshot:
The solute particles that have not dissolved are
circled in the image above.
2. Based on the above simulation, give a
molecular reason for why a solvent reaches
saturation.
Solvents reach saturation because there are
more solute particles than can physically interact
with solvent. Therefore, some excess solute
does not dissolve and remains a solid.
Page 8:
1. Water can dissolve ionic compounds.
Which idea best describes how this
happens?
(b)
2. For solids dissolving in liquids, an
increase in temperature
(c)
3. Is the statement “All liquids are able to be
mixed to create a solution” true? Explain
your answer.
No, it is not true. Water and oil do not form a
solution. The two liquids remain separate
because one is polar and one is non-polar. In a
solution, on the other hand, two substances are
so well mixed that the concentration of each
substance is the same everywhere.
4. Explain how attractive forces between
molecules and random motion determine if
one substance will dissolve another.
If the attractive forces among the solvent and
solute are greater then the attractive forces
among the solute molecules and the solvent
molecules encounter the solute molecules
frequently due to the random solute motion and
collisions that occur in liquids than dissolving will
occur. Examples include perfume mixing with air
and dye mixing with water.
5. Click the "Run" button to run the model to
the left. Watch carefully how the water
molecules around the gray ion interact with
it. Then click the following button to reverse
the charge on the gray ion: What happens to
the water molecules surrounding the ion?
You can reverse the charge as many times
as you want in order to observe more clearly.
Write down your observation.
The water molecules orient themselves around
the gray ion differently depending on the ion's
charge. When the ion is negatively charged, the
positive portion of the water molecules arrange
themselves nearby. When the gray ion is
positively charged, the negative part of the water
molecule is attracted. Intermolecular attractions
of polar molecules are illustrated in this model.
SAM HOMEWORK QUESTIONS
Solubility
Directions: After completing the unit, answer the following questions to review.
1. In the space below, draw a sugar and water solution. Label the solvent and solute. Use
squares to represent the sugar and circles to represent the water. Be sure that your
drawing aligns with the definition of a solution.
2. Predict what you think would happen if you placed sand in water. Use the terms polarity
and intermolecular attraction in your explanation of the outcome. Then sketch a diagram
that depicts your hypothesis.
3. Why do you think water is referred to as the “universal solvent”? Draw and label a water
molecule as part of your explanation.
4. Explain the phrase “like dissolves like.” What does it mean in the molecular world?
5. Does the picture represent a completely dissolved
solution? Why or why not? How do you know?
Annotate the picture as part of your answer.
*The larger circles are the solute and the smaller ones
are the solvent.
6. Career connection: Solution chemistry is extremely important for understanding the
spread of pollutants. Find an example of how computer models are used to understand the
flow of a pollutant through the environment.
SAM HOMEWORK QUESTIONS:
Solubility – With Suggested Answers for Teachers
Directions: After completing the unit, answer the following questions to review.
1. In the space below, draw a sugar and water solution. Label the solvent and solute. Use
squares to represent the sugar and circles to represent the water. Be sure that your
drawing aligns with the definition of a solution.
Student drawings should depict equal distribution of squares and circles to show that the sugar (solute) has
dissolved and is now mixed throughout the water (solvent).
2. Predict what you think would happen if you placed sand in water. Use the terms polarity
and intermolecular attraction in your explanation of the outcome. Then, sketch a diagram
that depicts your hypothesis.
Sand would not dissolve in water. Water, a polar molecule, is more attracted to itself than it is to the sand, in
which there is no polarity and, therefore, little intermolecular attraction to water. Student sketches should show
water as the solvent and sand as a solid solute.Likely the sketech will have sand at the bottom of the container.
3. Why do you think water is referred to as the “universal solvent”? Draw and label a water
molecule as part of your explanation.
Water is considered the universal solvent because its polarity makes it very good at dissolving many solutes.
Student drawings should show that the oxygen atom has a negative charge and the hydrogen atoms have a
positive charge.
4. Explain the phrase “like dissolves like.” What does it mean in the molecular world?
Non-polar solutes are more likely to dissolve in non-polar solvents. Polar solutes are more likely to dissolve in
polar solvents. This has to do with their ability to attract one another.
5. Does the picture represent a completely dissolved
solution? Why or why not? How do you know?
Annotate the picture as part of your answer.
No. The larger circles, the solute, are not evenly distributed
throughout the solvent. They are still solid within the solution because
the solution is saturated.
6. Career connection: There are many examples of how computer models are used to
understand pollution flow. Some possibilities include the flow of excess fertilizer from
fields into streams and lakes, or the Deepwater Horizon oil spill in which dispersants
(chemicals used to dissolve oil better into water) were used.