SAM Teachers Guide Solubility

Transcription

1 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.

2 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.

3 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.

4 Activity Answer Guide Page 1: 1. Take a snapshot when the model represents a solution (all molecules uniformly distributed) and place it below. 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. 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

5 not dissociated by the non-polar benzene molecule. 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. 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. 2. Reset the model. Set the temperature to "high" and run the model for 20,000 fs. Take a snapshot and place it below. 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. The solute particles that have not dissolved are circled in the image above.

6 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. 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. 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

7 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.

8 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.