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

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A. Investigation Four continues students’ exploration of compounds by allowing students to examine another property of compounds: pH.  Begin the Investigation by reviewing some of the concepts examined in Investigation Three. Use the questions that follow to help students review.

1. Ask students: What property of elements and compounds did you investigate during Investigation Three, in which you added different amounts of compounds to water? Students investigated the property of solubility.

2. You investigated two factors that affect the solubility of compounds. Which two factors did you investigate? Students investigated how stirring and temperature affect solubility.

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B. Tell students that during this Investigation, they will explore another property of compounds: pH.

1. Direct students’ attention to the term pH in their Scientist’s Glossary.  Ask a student volunteer to read aloud the definition of pH.

pH: A property of matter that defines whether a substance is an acid, a base, or a neutral substance.

2. Ask students whether they have ever heard of the terms acid (or acidic), base (or basic), or neutral.  Allow students to recall possible exposure to these terms.

3. To give students an understanding of the concept of pH as a property of compounds, ask them the following questions:

a. Ask students: What is a quantitative property? Students should indicate that a quantitative property is a property of matter that can be measured using scientific tools.

b. Ask students: What are some of the quantitative properties that you have examined so far in this CELL? What did you use to measure those properties? Students should indicate that they have examined the quantitative properties of mass using a triple-beam balance, volume using a graduated cylinder, and solubility using a triple-beam balance and a graduated cylinder.

c. Ask students: How do you think pH is measured? Student answers may vary. 

Encourage students to realize that pH is measured using the numbers from the pH scale.

d. Ask students: Would you consider pH a qualitative or quantitative property? Students should indicate that pH is a quantitative property because it can be measured and assigned a specific value.

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4. Begin a discussion of the term scale.

a. Ask students: When have you heard or used the term scale? Students may have heard the word scale in reference to a scale to weigh objects, in reference to a measure or guide on a map, or as a method of rating something (scale of 1 to 100). As students respond, record their examples on the board.

b. Ask students: What do you think a scale is?  How is it used?  Accept and record varied responses. Student answers may vary.  Sample answers may include a bathroom scale that can measure a person’s body weight, a scale on a map that measures a specific distance, or a rating scale that gives a score or value to something.

c. Ask students: Do you see any common themes between each of the uses of the term scale? Student answers will vary.  Some students may indicate that all uses of the word scale refer to a measure of something.  Others may feel that the term scale refers to numbers.  Encourage varied responses.

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This slide shows two of the many different ways students might come across the pH scale. There are many others.

d. Explain to students that pH is measured or represented on a pH scale. Like the other sample uses of the term scale, the pH scale is a numbered scale that measures pH.

e. Ask a student volunteer to read aloud the definition of pH scale from the Scientist’s Glossary.

pH scale:  A scale of numbers from 0 to 14.  The scale describes the strength of an acid or a base.

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C. Begin a discussion of the pH scale and how it is used and read.  Students need to realize that the pH scale’s starting point, or “zero,” is at number 7.  As the pH moves away from 7, a substance becomes more acidic or more basic.

1. Ask students to locate the diagram of the pH scale in Problem 1 of their Student Data Record.

2. Tell students that the pH scale is a numerical scale that extends from 0 to 14. Explain to students that certain numbers on the scale represent neutral substances, certain numbers represent acids, and certain numbers represent bases.

3. Ask students: Think about some of the other scales we talked about.  What number is the starting point for each of those scales? Students should indicate that each of the scales they discussed (previous slide) always started at the number 0.

4. Ask students to use their Scientist’s Glossary to determine the pH of neutral substances. Place the label neutral under 7 on the scale (see slide). Encourage students to label their scale in Problem 1. Explain to students that 7 on this scale is the starting point or “zero.”

5. Ask students to determine the pH of acids. Place the label acid under 0 through 6 on the scale (see slide). Encourage students to label their scale in Problem 1.

6. Ask students to determine the pH of bases. Place the label base under 8 through 14 on the scale.  Encourage students to label their scale in Problem 1.

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7. Explain that the pH scale also indicates how acidic and basic a substance is.

a. Explain that a weak base is almost neutral. In other words, it has a pH close to 7, such as 8 or 9. A strong base is far from being neutral. In other words, it has a pH close to 14.

b. Ask students: What do you think is a pH of a weak acid? A weak acid is almost neutral and has a pH close to 7. A substance with a pH of 6 or 5 is considered a weak acid.

c. Ask students: What do you think is a pH of a strong acid? A strong acid is far from being neutral. A substance with a pH of 0 or 1 is a strong acid.

d. Encourage students to label their pH scale with the words “strong acid,” “weak acid,” “strong base,” and “weak base.” Tell students to draw an arrow from weak acid to strong acid and from weak to strong base.

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D. Engage students in the following activity. This activity has been designed to increase students’ knowledge of acids and bases. Students will look at some of the properties of acids and bases. This information will help them to determine whether a substance is an acid, base, or neutral.  They will also be able to apply this information to substances they encounter daily.

1. Ask students to locate the pH concept map in Problem 2 of their Student Data Record.

2. Explain to students that this concept map lists some of the properties of acids and bases. Students will decide which descriptions or properties match each.

3. Ask a student volunteer to read the property that is listed in the first column, “not an acid or a base.”

Ask students:  Do you think this is a property of an acid, base, or neutral substance? Students should indicate that this is a property of a neutral substance. Encourage students to write “neutral” at the top of the column and fill in the pH of neutral substances. Students may have to refer to their pH scale to determine the pH of neutral substances.

4. Ask students: Can you think of any examples of neutral substances? Student answers may vary. Sample answers include: water is generally a neutral substance. Encourage students to record their examples in the last rectangles of each column in Problem 2.

5. Ask a student volunteer to read the three properties that are listed in the middle column. Ask students: Do you think that these are properties of an acid, base, or neutral substance? Student answers may vary. Encourage students to realize that these are properties of basic substances. Encourage students to write the word “bases” at the top of the column and fill in the pH range of basic substances. Students may have to refer to their pH scale to determine the pH of basic substances.

6. Ask students: Can you think of any examples of basic substances? Student answers may vary. Sample answers include soap and baking soda as basic substances.

Encourage students to record their examples in the last rectangles of each column in Problem 2.

7. Ask a student volunteer to read the two properties listed in the last column. Ask students: Do you think that these are properties of an acid, base, or neutral substance? Student answers may vary. Encourage students to realize that these are properties of acidic substances. Encourage students to write the word “acids” at the top of the column and fill in the pH range of acidic substances. Students may have to refer to their pH scale to determine the pH of acidic substances.

8. Ask students: Can you think of any examples of acidic substances? Student answers may vary. Sample answers include lemon juice and vinegar as acidic substances. Encourage students to record their examples in the last rectangles of each column in Problem 2.

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E. Tell students they will be testing the pH of four liquid compounds and four solid compounds in the lab. As students conduct their experiment, ask them to consider the following questions:

Do compounds that are composed of similar elements have the same pH?

Do compounds that are composed of different elements have different pHs?

What makes compounds have different pH?