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

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A. Being the lesson by asking students to recall what they have learned about elements, compounds, and solubility in Investigations One and Two. The following questions may aid in the review.

Recall Tool:

1. What have you learned about elements?

Students have learned that elements are substances that cannot be broken down into other, smaller substances by ordinary chemical reactions.  In addition, students learned that compounds are made up of elements.  All known elements are organized in the Periodic Table.

2. What have your experiments with compounds taught you about elements? Student answers may vary. Students may indicate that their experiments have taught them that the same elements can make up different compounds with different properties.

Further Discussion:

1. Ask students: What is a property? A property of a substance is a characteristic of that substance that can be seen or measured, such as mass, color, or solubility.

2. Ask students: Do any two compounds have exactly the same set of properties? No, each compound has a unique set of properties.

3. Ask students: What makes each compound different from another? Each compound is made up of elements combined in a manner that is unique to that compound. No two compounds are made up of the same number of atoms of each element combined in the same way. This difference makes each compound different from another.

4. Ask students: Which properties of compounds have you examined thus far? Students should indicate that they have examined the state of matter, color, volume, mass, solubility, and saturation point.

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B. Continue the review by asking students to recall the compounds and the properties which they observed in Investigation Two.

1. Encourage students to refer to their Student Data Record for Investigation Two. 

Ask students: Which four compounds did you observe in Investigation Two? Students investigated baking soda, salt, sugar, and cream of tartar.

2. Ask students: Which properties of these compounds did you observe or measure? Students observed the solubility and saturation point of the four compounds in water.

3. Direct students to locate the table in Problem 9 of their Student Data Record for Investigation Two. 

Ask students: What was the greatest amount of baking soda that could be dissolved in 100 ml of water? Students should indicate that 5 g of baking soda dissolved in 100 ml of water.

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4. Tell students that after making a baking soda solution, a scientist would say that the ratio of solute to solvent was 5 g of baking soda to 100 ml of water or 5 g of baking soda in 100 ml of water. Point to the ratio on the slide (5 g in 100 ml.)

5. Ask students: Have you ever heard of the word ratio before? Student answers may vary.  Students may or may not indicate that they have heard of the word ratio during math lessons.

6. Tell students that a ratio is used to compare two things; in this case, the mass of the solute (baking soda) is compared to the volume of solvent (water) used to dissolve it.

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7. To give students practice with ratios, engage them in the following activity:

a. Examine the six circles and two squares on the slide.

b. Ask students: How many circles are drawn on the board?  How many squares? Students should indicate that six circles and two squares are drawn on the slide.

c. Ask students: How would you write the amount of squares as compared to the amount of circles as a ratio? Student answers may vary.  Students should indicate that it can be written as “two squares to six circles or two squares in six circles.”

d. Encourage students to realize that a ratio can be written in several ways. Another way to write the ratio is “two circles/six squares (2 circles/ 6 squares)” or “2 circles:6 squares.”

e. Direct students to refer to the table in Problem 9 of their Student Data Record for Investigation Two (lower part of the slide). 

Ask students: How would you write a ratio for the greatest amount of cream of tartar that was dissolved in 100 ml of water? Students should indicate that the ratio could be written as 0.5 g of cream of tartar to 100 ml of water or 0.5 g/100ml or 0.5 g:100 ml.

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8. Explain to students that using ratios can be helpful when people create solutions.

a. Ask students the following questions about ratios in everyday situations:

• • Ask students: What do you think would happen if the wrong ratio was used when giving medicine to a patient? Student answers may vary. The incorrect amount of medicine could make patients more ill or even put their lives at risk.
  • Ask students: What do you think would happen if you used the wrong ratio when adding sugar to tea or lemonade? Student answers may vary. The tea or lemonade would either become too sweet if too much sugar were added or become too tart or bitter if not enough sugar was added.

b. Explain to students that scientists also use ratios when creating solutions. For example, if a scientist knows the saturation point of cream of tartar in 100 ml of water than the scientist can determine how much water will dissolve a different amount of that solute. Walk students through these steps:

• • Ask students: What was the saturation point of salt in 100 ml of water? Students should indicate that they found that the saturation point was between 30 g of salt and 50 g of salt in 100 ml of water because 30 g of salt completely dissolved in water, but 50 g of salt did not completely dissolve in water.
  • Tell students that scientists did many more tests to narrow the range of values and found that the saturation point of salt is 30 g in 100 ml of water.
  • Ask students:  How can this be written as a ratio? Students should indicate that it can be written as 30 g to 100 ml or 30 g/100 ml or 30 g:100 ml.
  • Ask students: How much water do you think it will take to dissolve 60 g of salt? Student answers may vary. Encourage students to realize that 60 g is twice as much as 30 g. Therefore, if it takes 100 ml of water to dissolve 30 g, then it would take twice as much, or 200 ml of water, to dissolve 60 g of salt.
  • Ask students: How much water do you think it will take to dissolve 15 g of salt? Students should indicate that it would take 50 ml to dissolve 15 g of salt. 15 g is half as much as 30 g; therefore, it would take half as much water (50 ml) to dissolve half as much salt.

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9. Explain to students that scientists can also use ratios to explain how quickly something can happen. Scientists give this type of ratio a special name: rate.

a. Ask students: Have you ever heard of the word rate before? Student answers may vary.  Sample answers may include heart rate, respiratory rate, rate of speed, or hourly pay rate.

b. Ask students: Take the example of heart rate; what two things are being compared? Student answers may vary.

c. Refer to the heart rate shown on the slide:  “Heart rate = number of beats/minute.” Explain that in this ratio, the number of beats is being compared to time.

d. Ask students: How is this different from the ratios you worked with for the salt and water? How is this similar? Student answers may vary. Help students understand that a rate is similar to the ratio of salt and water because both are ratios. The difference is between which two things are compared in the ratios. In the example of salt and water, the amount of salt was compared to the volume of water. In the example of heart rate, the number of beats was compared to time.

e. Ask students: Think of some examples of rates such as respiratory rate, speed, and hourly pay rate. What are the units of these rates? Students should indicate that respiratory rate is expressed as breaths/minute, speed as kilometers/hour, and an hourly pay rate is expressed as dollars/hour.

f. Ask students: Do you think rates can be increased? Decreased? Can you give any examples? Student answers may vary. Students should indicate that heart rate and respiratory rate can be increased by increasing the amount of physical activity being performed. They can be decreased by decreasing the amount of physical activity being performed. The rate of speed can be increased by pushing on a gas pedal and decreased by pressing on the brake pedal.

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g. Tell students that solubility can also be expressed as a rate. The rate of solubility is expressed as the amount, or grams, of solute that can be dissolved in a solvent over a specific period of time.  The rate of solubility can be written as “___ g/____min.” Write this on the board.

h. Encourage students to think about the experiment from Investigation Two. 

Ask students: When testing the solubility of salt, how long did it take to dissolve the 30 g of salt in the water? Students should indicate that it took 2 minutes to dissolve 30 g of salt.

i. Ask students: What was the rate of solubility of the salt? Students should indicate that the rate of solubility was 30 g/2 minutes.

j. Ask students: Do you think that the rate of solubility can be increased? Decreased? Student answers may vary.

k. Ask students:  Why would you want to increase the rate of solubility? Student answers may vary. Some students may feel that increasing the rate of solubility would decrease the preparation time when making some foods or drinks. For example, increasing the rate of solubility would enable a person to make lemonade from a powdered mix more quickly.

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C. Tell students that they will continue to investigate four compounds in the lab in this Investigation: sugar, salt, baking soda, and cream of tartar. As students conduct their experiments, they should consider the following questions:

• How can the rate of solubility of each compound be increased?

• How can the rate of solubility of each compound be decreased?

• How does stirring affect the solubility of a compound?

• How does temperature of water affect the solubility of a compound?