This was the fourth Investigation of the LabLearner CELL Properties of Compounds. In it, students examined the pH value of solutions made from compounds.

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A. Begin the analysis of this Investigation by encouraging students to summarize the experiments they conducted in the lab and the purpose of those experiments. Pose the following questions to prompt student discussion:

1. Ask students: What were the main questions we wanted to investigate in this lab? The main questions investigated in the lab were: Do all liquid compounds have the same pH? Do all solid compounds have the same pH? Do compounds that are composed of similar elements have the same pH? Do compounds that are composed of different elements have different pH?

2. Ask students: How would you summarize the types of experiments that you performed to investigate these questions? Students tested the pH for four liquid compounds: rubbing alcohol, hydrogen peroxide, vinegar, and water, using the pH indicator strips.  Students then created solutions of sugar, baking soda, salt, and cream of tartar and tested the pH using the pH indicator strips.

3. Ask students: Is pH a qualitative or quantitative property of compounds? Why? Students should indicate that pH is a quantitative property because it can be measured and assigned a specific value.

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B. Begin analysis of the lab by directing students’ attention to the results of Trial 1. The following discussion is designed to encourage students to think about pH differences in liquid compounds. Students may wish to refer to the Table in Problem 3 of their Student Data Record when answering these questions:

1. Ask students: Which liquid compounds were acidic? Basic? Neutral?

Students should indicate that rubbing alcohol, hydrogen peroxide, and vinegar are acidic. The water was neutral.

2. Ask students: Would you consider rubbing alcohol to be a weak or strong acid? Hydrogen peroxide? Vinegar? Students should indicate that rubbing alcohol has a pH of 6 and hydrogen peroxide has a pH of 5 which indicates that they are weak acids, and vinegar has a pH of 2 which indicates that it is a strong acid.

3. Ask students: Did all liquid compounds have the same pH? Students should indicate that all liquid compounds did not have the same pH.

4. Ask students: Think about the chemical formulas of rubbing alcohol, vinegar, hydrogen peroxide, and water. What elements do they have in common? Students should indicate that they all contain hydrogen and oxygen.

5. Ask students: Based on the results of this Trial, do you think that liquid compounds composed of similar elements have a similar or dissimilar pH? Use the data to support your answer. Students should indicate that liquid compounds composed of similar elements can have a similar or dissimilar pH. All of the liquid compounds tested in the lab contained hydrogen and oxygen. Vinegar was found to be a strong acid, rubbing alcohol and hydrogen peroxide were found to be weak acids, and water was found to be a neutral compound.

6. Ask students: Do you think that liquid compounds that are composed of different elements have a dissimilar or similar pH? For example, think about the chemical formula for rubbing alcohol (C3H8O) and hydrogen peroxide (H₂O₂), or think about the formula for vinegar (CH₃COOH) and hydrogen peroxide (H₂O₂).  Use the data to support your answer. Students should indicate that compounds made up of different elements can have similar or dissimilar pH. For example, rubbing alcohol (C₃H₈O) contains carbon, while hydrogen peroxide (H₂O₂) does not.  However, the pH of rubbing alcohol and hydrogen peroxide are similar in that they are both weak acids. On the other hand, vinegar (CH₃COOH) and hydrogen peroxide (H₂O₂) also contain different elements and have different pHs.

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C. Continue the discussion of pH by encouraging students to consider what they learned from Trial 2. Students may wish to refer to the Table in Problem 4 of their Student Data Record when answering these questions:

1. Ask students: How did you test the pH of a solid compound? Students should indicate that the pH indicator strip could not be used to test the pH of a solid compound. Therefore, a solution with a neutral solvent, in this case, water, was made using each solid compound.

2. Ask students: Why was it important that the water was neutral or very close to neutral before making your solutions? Student answers may vary. Encourage students to realize that if the water is neutral or very close to neutral, then the pH they recorded when making solutions was the pH of the solid, not the water.

3. Ask students: Which solid compound was acidic? Basic? Neutral?

Students should indicate that the baking soda was basic, the salt and sugar were neutral, and the cream of tartar was acidic.

4. Ask students: Would you consider baking soda a weak or strong base? Would you consider cream of tartar to be a weak or strong acid? Students should indicate that baking soda has a pH of 9, indicating a strong base.  The cream of tartar has a pH of 4, indicating it is a weak acid.

5. Ask students: Do all solid compounds have the same pH? Students should indicate that all solid compounds do not have the same pH.

6. Ask students: Think about the chemical formulas of baking soda, salt, sugar, and cream of tartar. What elements do they have in common? Students should indicate that baking soda, sugar, and cream of tartar all contain carbon, hydrogen, and oxygen. Baking soda and salt both contain sodium.

7. Ask students: Based on the results of Trial 2, do you think that solid compounds composed of similar elements have a similar or dissimilar pH?  Use the data to support your answer. Students should indicate that solid compounds composed of similar elements can have a similar or dissimilar pH. Baking soda, sugar, and cream of tartar all share elements.  Baking soda was found to be a base, while sugar was a neutral compound.  Cream of tartar was found to be an acid. In addition, baking soda and salt both contain sodium, yet baking soda is a base, and salt is a neutral compound.

8. Ask students: Do you think that solid compounds that are composed of different elements have a similar or dissimilar pH? Think about the chemical formula for sugar (C₁₂H₂₂O₁₁) and salt (NaCl) and baking soda (NaHCO₃), and cream of tartar (KC₄H₅O₆). Use the data to support your answer. Students should indicate that compounds made up of different elements can have a dissimilar or similar pH.  Sugar (C₁₂H₂₂O₁₁) contains carbon, hydrogen, and oxygen, while salt (NaCl) contains sodium and chloride. However, the pH of sugar and salt was found to be 7, or neutral.  However, baking soda (NaHCO₃) and cream of tartar (KC₄H₅O₆) are composed of different elements and differ in their pH. Baking soda was a base, and cream of tartar was an acid.

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D. Tell students that they will continue to analyze their experiment by adding their results to a pH scale.

1. Direct students to locate Problem 5 in their Student Data Record.

2. Direct students to transfer the pH of each of the compounds to the Data Table in Problem 5a of their Student Data Record.

3. Ask students: What is the pH range for acidic substances? Basic substances? Neutral substances? Students should indicate that a pH that is less than 7 indicates an acidic substance. A pH of 7 indicates a neutral substance. A pH that is greater than 7 indicates a basic substance.

Encourage students to label the pH scale in Problem 5c of their Student Data Record as “acidic,” “neutral,” and “basic.”

4. Ask students: Where do each of the eight compounds fall on the pH scale? Encourage students to add each of the eight compounds to the top of the pH scale in Problem 5c of their Student Data Record.

5. Ask students: Would you describe vinegar as a weak acid or a strong acid? Hydrogen peroxide? Rubbing alcohol? Cream of tartar solution? How do you know? Students should indicate that vinegar is a strong acid, while hydrogen peroxide, rubbing alcohol, and cream of tartar are weak acids. Students should indicate that the placement of each of the acids on the pH scale indicates if they are strong or weak. As the acid’s pH decreases, the acid’s strength will increase. An acid with a pH of 1 is much stronger than an acid with a pH of 6.

6. Ask students: Would you describe baking soda as a weak base or a strong base? How do you know? Students should indicate that baking soda is a weak base. Students should indicate that the placement of a base on the pH scale indicates if it is strong or weak. As the pH of a base increases, the strength of the base will increase. A base with a pH of 14 is much stronger than a base with a pH of 8.

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7. Divide students into groups of three. Ask student groups to draw conclusions about their experiment by answering the following questions in Problem 6 of their Student Data Record.

a. Ask students: Think about compounds, elements, and the elements that make up compounds. What do you think determines the pH of a compound? Students should indicate that it is the unique combination of elements and number of atoms of an element in a compound that determines pH.

b. Would you be able to figure out the identity of unknown substance by looking at its pH only? Why? Students should indicate that they would not be able to figure out the identity of an unknown substance if they only knew its pH. The results from the experiment showed that several different compounds might have the same pH. Thus knowing only the pH will not always allow a scientist to identify a specific compound.

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E. Encourage students to apply their knowledge from this Investigation to daily situations by engaging them in the following activity.

1. Divide students into groups of three.

2. Explain to students that they are going to be reading five short passages about common, everyday substances.  After reading the passages, they will be asked to decide whether each substance is an acid, base, or neutral.  If they need to review the properties of acids, bases, and neutrals, students should refer to the concept map they developed in Problem 2 of the PreLab.

3. Encourage students to read the passages in Problem 7 of their Student Data Record and answer the questions that follow each passage.

NOTE: The correct answers for teachers is included to the right.

4. Ask student groups to predict the pH of each of the liquids. Explain to students that there is a pH scale in Problem 7f of their Student Data Record. The scale has five arrows pointing to five different pHs. Each liquid has a corresponding arrow on the diagram (red arrows on this slide).

Encourage student groups to record their predictions on the pH scale. Allow sufficient time to complete this activity.

5. When students have completed their predictions, inform them of the following actual pHs:

6. Encourage students to check their predictions and make any necessary corrections. 

Ask students: Are you surprised at any of the pH values for the liquids? Why? Student answers may vary.

7. Ask students: What information did you use to make your predictions? Student answers may vary. Students may or may not indicate that they used the information from the pH concept map and data they collected in the lab.

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F.  Conclude this Investigation by asking students to consider the following question:

Ask students: Do compounds made of the same elements react in the same way?

In Investigation Five, Chemical Combinations, students will begin experimenting with chemical reactivity.