This is the second Investigation of the LabLearner CELL Properties of Compounds. In it, students will examine the solubility of solutes in solvents. They will also explore the chemical concept of concentration and saturation of solutions.

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A. Begin the lesson by asking students to recall what they have learned about elements and compounds in Investigation One. The following questions may aid in the review:

1. Ask students: 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.

2. Ask students: Scientists have developed a chart that organizes and gives information about all of the known elements.  What is the name of this chart? The Periodic Table of the Elements organizes the elements.

3. 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 temperature.

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

5. 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.

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

Encourage students to refer to their Student Data Record for Investigation One. 

Ask students: Which seven compounds did you observe in Investigation One? Students investigated baking soda, salt, sugar, vinegar, rubbing alcohol, hydrogen peroxide, and cream of tartar.

2. Ask students: Which properties of these compounds did you observe or measure? Students observed the state of matter and color of each of the compounds. In addition, they measured the mass and volume of the compounds.

3. Tell students that they will continue to investigate four of these compounds in the lab in this Investigation – sugar, cream of tartar, salt, and baking soda.  In addition, they will investigate an additional compound, water.

4. Ask students to think about what they already know about water:  What are some properties of water? Student answers may vary.  Students may know that water is a liquid, is clear, is wet, and can be poured. They may also know that water can move matter (pushing dirt or sand in a stream or on the beach), can vary in temperature, and has a chemical formula of H₂O.

5. Remind students that, during Investigation One, they looked at the chemical formulas for the compounds as they explored their properties.

6. Tell students they will continue to investigate the properties of these five substances – sugar, cream of tartar, salt, baking soda, and water – during the lab.

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C. Students will use vocabulary throughout Investigations 2 and 3, including solubility, soluble, and dissolve. Students may find the following activity helpful in reviewing the definitions and use of such vocabulary.

READ:  It is a hot summer day and you have just finished helping your grandparents work in their yard.  The three of you stand up, brush off the dirt, and go inside.  Your grandmother decides that the best way to cool off is to drink some lemonade.  She opens the jar of powdered lemonade and adds several scoops to a pitcher of water.  Most of the powdered lemonade sinks to the bottom of the pitcher.  As she stirs the water, all of the powdered lemonade seems to disappear.

1. Tell students that using this scenario, you will help them understand and define three words that they will use throughout this Investigation:  solubility, soluble, and dissolve.

2. Explain to students that you will read a sentence using the word solubility. Students can follow along by reading the sentence in Problem 1 of the Student Data Record. By reading this sentence and recalling the story, students may be able to begin to understand the term solubility:

READ: The grandmother is testing the solubility of the powdered lemonade when she watches how it mixes with the water.

a. Ask students: What do you think the term solubility means? Student answers may vary.  Students may suggest that solubility refers to how something mixes with something else or how well it dissolves.

b. Instruct students to refer to the definition of solubility in their Scientist’s Glossary. After students have read the definition,

Ask students:  Does this definition match what you thought the term meant? If not, how is it different? Student answers may vary.

3. Explain to students that you will next read the second sentence from Student Data Record Problem 1.  This sentence uses the term soluble.

READ:  The grandmother knows that the lemonade powder is soluble in water because it seems to mix.

a. Ask students: What do you think the term soluble means? Student answers may vary.  Students may suggest that soluble means to mix with or dissolve.

b. Instruct students to refer to the definition of soluble in their Scientist’s Glossary. After students have read the definition,

Ask students: Does this definition match what you thought the term meant?  If not, how is it different? Student answers may vary.

4. Tell students you will read the final sentence from Student Data Record Problem 1. This sentence uses the term dissolve.

READ:  The lemonade mix dissolved in water as it mixed with it.

a. Ask students:  What do you think the term dissolve means? Student answers may vary.  Students may suggest that to dissolve means to mix with or to become part of something else.

b. Instruct students to refer to the definition of dissolve in their Scientist’s Glossary. After students have read the definition,

Ask students: Does this definition match what you thought the term meant?  If not, how is it different? Student answers may vary.

c. Ask students the following questions to aid them in differentiating between the terms disappear and dissolve.

Ask students: Did you think the lemonade powder disappeared in the water? Student answers may vary.

Ask students: What does it mean to disappear? Student answers may vary.

Explain to students that when something disappears, it goes away completely. No evidence of its existence remains.

• Ask students: Why don’t scientists say that the lemonade powder disappeared instead of dissolved? After the lemonade powder was mixed into the water, it still existed. It did not go away completely. Evidence that it still existed was in the flavor and color. The three people in the story were able to taste the lemonade powder in the water, so they knew it did not disappear.

• Ask students: Why should you not use the word disappear when referring to dissolved solutes? If something disappears, it no longer exists. However, when a solute is dissolved in a solvent, it may change state or appear differently, but it still exists.

5. Tell students that they will continue to use these terms throughout Investigation Two as they explore the property of solubility.  By investigating how compounds act in water, students will be able to draw conclusions regarding the solubility of each substance in water.

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D. Introduce students to the concepts of concentration and saturation.  Conduct the following activity to aid in this introduction.  Students may work in pairs or individually for this activity.

1. Provide each pair or individual student with approximately 20 to 30 small, flat manipulatives, such as pennies, paper clips, or squares of paper.

2. Ask students to find the rectangle drawn in Problem 2 of the Student Data Record.

3. Instruct students to place ten pennies (or other small objects) in the square. Tell students that when they do this, the concentration of pennies in the square is 10 pennies per square.

4. Ask students: How can you increase the concentration of pennies in the square? Student answers may vary. Students could add more pennies to the square to increase the concentration of the pennies in the square.

5. Instruct students to add five more pennies to the square.

Ask students:  What is the concentration of pennies in the square now? The concentration is 15 pennies per square.

6. Ask students: How can you decrease the concentration of pennies in the square? Student answers may vary. Students could remove pennies from the square to decrease the concentration of pennies in the square.

7. Tell students that just as the number of pennies per square can vary, the number of atoms dissolved in a liquid can vary. For example, in the story of the grandmother making lemonade, she could have made the lemonade strong with a lot of lemonade powder, or she could have made it weak with only a little lemonade powder. Strong lemonade has a high concentration or a lot of lemonade powder per water.  Weak lemonade has a lower concentration or less lemonade powder per water.

8. Tell students that they will test the solubility of salt, sugar, baking soda, and cream of tartar by varying the concentrations of the compounds in water.

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E. Tell students that during the lab, they will investigate solubility and concentration to answer the following questions:

• Do compounds made of the same elements have the same solubility?

• Do compounds made of different elements have different solubility?