This was the third Investigation of the LabLearner CELL Properties of Compounds. In it, students examined the impact of stirring and temperature on the solubility of solutes in solvents.

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[Start by going over the Prediction Cognitive Tool to begin the discussion.]

A. Begin this part of the Investigation by reviewing the experiments students performed in the lab.  Ask the following questions to promote discussion of the experiments:

1. Ask students: What property of elements and compounds did you investigate during the experiments? Students investigated the property of solubility.

2. Ask students: Is solubility a quantitative or qualitative property? How do you know? Solubility is a quantitative property because it refers to how much solute can be dissolved in a certain amount of solvent. This is a measurable property. Students measured this in Investigation Two by determining the maximum amount of each compound that could dissolve in 100 ml of room-temperature water. 

Note to Teacher: However, it is important for students to understand that simply saying whether or not a compound is soluble in a solvent without mentioning an amount is a qualitative property.  For example, if a student were to say that salt is soluble in vinegar and water, they would be describing a qualitative property. If, however, they were to say that the solubility of salt is 30 g in 100 ml of water, they would be describing a quantitative property.

3. Ask students: You investigated two factors that may affect solubility and the rate of solubility. Which two factors did you investigate? Students investigated how stirring and temperature affect solubility and the rate of solubility.

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B. Tell students that in order to analyze the data they collected in Trials 1 and 2 each group must share their collected data with the other four groups.

1. Divide the students into cooperative groups. Each group should contain one representative from each lab group so that the group contains at least one student who explored each of the four compounds: baking soda, cream of tartar, salt, and surge.

2. Instruct students to work together to complete the Table in Problem 7 of their Student Data Record. To complete the table, students should write “yes” if the compound was soluble in 100 ml of water and “no” if the compound was insoluble for both the “stirred” and “not stirred” beakers.

3. Allow students sufficient time to complete the table.

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C. Begin a discussion of the results of stirring and the observations made in Trials 1 and 2.

1. Encourage students to recall Trials 1 and 2.  Ask students the following questions to aid in a review of the experiment. Students may wish to refer to Problem 7 of their Student Data Record as they answer the following questions.

a. Ask students: Think about Trials 1 and 2. What question did you investigate when performing Trials 1 and 2? Students investigated the question: How does stirring affect the solubility of compounds in water and the rate of solubility of compounds in water?

b. Ask students: How did you answer this question? To answer this question, students added water to two beakers containing one of the four compounds. By stirring the water in one beaker and allowing the other to remain still, students could compare and determine how stirring affects the solubility of compounds in water. Students tested two of the four different compounds using this method.

c. Refer to the Table on this slide.  Ask student volunteers to record their “not stirred” data from Problem 7.

Ask students: What did you observe in the beakers containing water and a compound that were not stirred?  Did any of the solutes dissolve completely? Students should indicate that none of the solutes dissolved completely. Although the water was clear, there was a layer of solute that remained on the bottom of each of the beakers.

d. Ask students: Do you think that any of the compounds dissolved in the beakers that were not stirred? Student answers may vary. Encourage students to realize that a small amount of the compound likely did dissolve into the water in the beakers that were not stirred.

e. Ask student volunteers to record their “stirred” data from Problem 7. 

Ask students: What did you observe in the beakers containing water and a compound that was stirred?  Which solutes dissolved completely? Students should indicate that, with stirring, all four solutes dissolved completely. The water was clear, and no solute remained at the bottom of each of the beakers.

f. Ask students: What effect did stirring have on the solubility of the four compounds? Was it the same effect for all four compounds?  Use your data to support your answer. Students should indicate that stirring increased the solubility of all four compounds because, in each case, more solute dissolved in the water when stirred compared to when not stirred. Encourage students to answer Problem 7c in their Student Data Record.

g. Ask students: What was the stirring time for each beaker? Students should indicate that the stirring time was 2 minutes for each beaker.

h. Ask students: In the beakers that were stirred, what was the rate of solubility? Encourage students to record their answers in Problem 7d of their Student Data Record. Students should indicate that the rate of solubility for baking soda was 8 g/2 minutes, cream of tartar was 0.5 g/2 minutes, salt was 26 g/2 minutes, and sugar was 80 g/2 minutes.

i. Ask students: In the beakers that were not stirred, what was the rate of solubility? Student answers may vary. Encourage students to realize that although they can not get an exact amount for the rate of solubility, they can make an estimate. Students should indicate that the rate of solubility in the beakers that were not stirred was: baking soda was less than 8 g/2 minutes, cream of tartar was less than 0.5 g/2 minutes, salt was less than 26 g/2 minutes, and sugar was less than 80 g/2 minutes.

Encourage students to record their answers in Problem 7e of their Student Data Record.

j. Ask students: How are the rates of solubility different in the beakers that were stirred and the beakers that were not stirred? Encourage students to record their answers in Problem 7f of their Student Data Record.  Students should indicate that the rate of solubility was higher in the beakers that were stirred and lower in the beakers that were not stirred.

2. Encourage students to summarize their findings for Trials 1 and 2 by answering Problems 8a and 8b in their Student Data Record.

3. Allow several students to share their answers with the class.

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D. Begin a discussion of the results of temperature and the observations made in Trials 3 and 4.

1. Encourage students to recall Trial 3.  Ask students the following questions to aid them in recalling their experiments. Students may refer to Problems 4e and 4i of their Student Data Record.

a. Ask students: What question was Trial 3 designed to answer? Trial 3 was designed to answer the question:  How does temperature affect the solubility of compounds in water?

b. Ask students: How did you answer this question? Students added cold water to one beaker containing a compound and hot water to another beaker containing the same compound. By stirring each beaker for the same amount of time, students could compare the amount of the compound remaining in each beaker. Students performed the same procedure with the remaining three compounds, allowing them to make similar observations concerning the solubility of compounds in cold and hot water.

c. Ask students: What did you observe in the beakers containing cold water?  Which of the solutes dissolved completely? Students should indicate that even with stirring, none of the compounds dissolved completely in cold water. Although the water was clear, a small amount of each compound remained at the bottom of the beakers.

d. Ask students: What did you observe in the beakers containing hot water? Which of the solutes dissolved completely? Students should indicate that, with stirring, all of the compounds dissolved completely in hot water.  The water was clear, and none of the compounds remained at the bottom of the beakers.

e. Ask students: What do you think was the effect of temperature on solubility? Students should indicate that as temperature increases, solubility increases. As temperature decreases, solubility decreases. That is, more solute dissolved in hot water rather than in cold water.

f. Ask students: How long was each beaker stirred? Students should indicate that each beaker was stirred for two minutes.

g. Ask students: What was the rate of solubility in hot water? Can the rate of solubility in cold water be estimated? (Instruct students to record their answers in Problems 9a and 9b of the Student Data Record.)  Students should indicate that the rate of solubility in cold water can be estimated as less than 80 g/2 minutes for sugar, less than 26 g/2 minutes for salt, less than 8 g/2 minutes for baking soda, and less than 0.4 g/2 minutes for cream of tartar.  Students should indicate that the rate of solubility in hot water for sugar was 80 g/2 minutes, salt was 26 g/2 minutes, baking soda was 8 g/2 minutes, and cream of tartar was 0.4 g/2 minutes.

h. Ask students: How does temperature affect the rate of solubility of compounds? (Encourage students to record the answer in Problems 10a and 10b of their Student Data Record.) Students should indicate that hot water increases the rate of solubility of compounds and that cold water decreases the rate of solubility of compounds.

i. Ask students: Are these general rules or specific rules about the effect of temperature on solubility? Is there a difference between compounds that are made of similar elements and different elements? Students should indicate that this is a general rule. In general, higher temperatures will increase solubility and rate of solubility of compounds, and lower temperatures will decrease solubility and rate of solubility. Students should also indicate that compounds that are made of similar and dissimilar elements react in the same way to temperature.

j. Encourage students to summarize their findings for Trial 3 by answering Problems 10c and 10d in their Student Data Record.

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2. Ask students to recall the experiment in Trial 4.

a. Ask students: What question was Trial 4 designed to answer? Trial 4 was designed to answer the question:  Can stirring and increasing temperature always increase the solubility of a compound?

b. Ask students: How did you answer this question? Students added hot water to one beaker containing 40 g of salt. The saturation point of salt is 30 g in 100 ml of water. By stirring the beaker for 2 minutes, students could observe the solubility of 40 g of salt in hot water.

c. Ask students: What did you observe in the beaker of 40 g of salt and hot water? Students should indicate that even after 2 minutes of stirring, salt remained at the bottom of the beaker.  It did not all dissolve.

d. Ask students: Why do you think that stirring and hot water did not dissolve the salt? Students should indicate that the salt did not dissolve because the solution had reached its saturation point, and the stirring and increase in temperature did not provide enough interaction between the solvent and solute to allow more dissolution.

e. Ask students: How did your predictions compare with the actual results of Trial 4? Student answers will vary based on their predictions.

f. Ask students: Do stirring and increasing temperature always increase the solubility of a compound? (Encourage students to record the answer in Problem 11 of their Student Data Record.)  Students should indicate that stirring and temperature may not always produce an increase in the solubility of a compound. The experiments showed that for all of the compounds tested, there was a concentration at which both stirring and increased temperature increased the solubility and rate of solubility of the compounds. However, Trial 4 showed that when working with solutions, there may be a concentration at which stirring and temperature can no longer produce increases in solubility or rate of solubility of the compound.

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3. Encourage students to apply the knowledge they gained from this Investigation to daily situations by answering Problems 12a – 12c in their Student Data Record with their groups.

4. After students have had sufficient time to discuss their experiments and answer Problems 12a – 12c, bring the class together to review their answers.

a. Ask students: Will the sugar dissolve quickly or slowly in Jamie’s iced tea?  How do you know? Student answers may vary. During Trial 3, sugar did not completely dissolve in cold water. Based on this observation, the sugar will either dissolve slowly or not completely. Jamie will be better able to dissolve sugar in the glass of tea if it is hot than when it is cold.

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b. Ask students: Should Grant’s soup be hot or cold in order to dissolve the salt most quickly and completely? How do you know? Student answers may vary. During Trial 3, salt dissolved more quickly and completely in hot water than in cold water.  Based on this observation, Grant will be better able to dissolve the salt in the soup when it is hot than when it is cold.

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c. Ask students: Why did the powdered juice mix not completely dissolve in the water? How do you know? Student answers may vary. During Trial 4, salt did not dissolve completely in hot water while stirring. This occurred because the solution was already saturated. Based on this observation, Minh will be better able to dissolve two scoops of the powdered juice mix in the water if she increases the amount of water she is using.

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E. As a conclusion to this activity, ask students to reflect on the results of Trials 1 through 4.

1. Instruct students to locate Problems 13a – 13d in their Student Data Record. Tell students to reflect on what they learned during Investigation 3 and answer the questions.

2. After students have had sufficient time, discuss the conclusions as a class, 

Ask students:  What conclusions can you draw based on your observations in the lab? Student answers may vary. Students should conclude that increased temperature of a solvent increases the solubility of a solute and the rate of solubility of a solute, a decreased temperature decreases the solubility and rate of solubility of a solute, and that stirring also increases the solubility and rate of solubility of a solute.

Students should also indicate that as a solution reaches its saturation point, there may be a concentration of the solution at which stirring and temperature may not be able to increase the amount of solute that will dissolve. Students should also conclude that compounds with similar and dissimilar elements respond in the same way to stirring and temperature. Finally, students should indicate that the property of solubility alone is not enough to identify an unknown compound.

3. Tell students that they will continue to investigate the properties of matter in Investigation Four.