The Acid Test Grade Nine

Transcription

1 Ohio Standards Connection: Physical Sciences Benchmark B Explain how atoms react with each other to form other substances and how molecules react with each other or other atoms to form even different substances. Indicator 8 Demonstrate that the ph scale (0-14) is used to measure acidity and classify substances or solutions as acidic, basic, or neutral. Lesson Summary: After discussing vocabulary and watching the testing procedure, the students will test the ph of various household substances. After compiling and analyzing the data in class, students will view a demonstration on the relationship between concentration and ph. Estimated Duration: Two to three hours Commentary: This lesson is designed to provide students with hands-on experiences with acids and bases they find in their homes. Through individual testing and in-class demonstrations and discussions, students will get an overall understanding of the nature of acids, bases and the ph scale. This lesson was field tested and reviewed by teachers across the state. Some of their comments were: I like the idea of students testing at home; food is very real world to these students. They (my students) did enjoy the litmus tests. Pre-Assessment: Have students answer homework/discussion questions using a textbook, the library and/or Internet resources as needed. See Attachment A, Homework/Discussion Questions. Scoring Guidelines: Answers are included on Attachment A, Homework/Discussion Questions. Discuss the questions in class after students have done them for homework. Notice areas students know or do not know and adjust the lesson accordingly. 1

2 Post-Assessment: Have students answer test questions that address the properties of acids and bases, characteristics of the ph scale, methods for measuring ph and the relevance to society of acids and bases. See Attachment B, Post-Assessment. Scoring Guidelines: See Attachment C, Post-Assessment Answers. Instructional Procedures: 1. Discuss pre-assessment questions in class. Begin to construct a K-L-H chart (what I know, what I learned and how I can find out more) during the discussion that can be referenced and completed throughout the learning cycle. 2. Demonstrate the use of litmus paper and ph paper in the class. Test an acid, base, distilled water and de-ionized water. 3. Give each person 10 to 12 strips of ph paper to use at home. 4. Have students test 10 different substances at home. All students must test vinegar, baking soda and eight other household substances of their choosing from the following list: ketchup, mustard, cooking oil, salad dressing, fruit juices, cola, ashes, hand soap, coffee, milk, fertilizer, glass cleaner, dill pickles, potatoes, eggs, cottage cheese and soil. A student may test a substance that is not listed with prior permission from the teacher. For a teacher reference sheet, see Attachment D, ph of Common Substances. 5. Have students test a solution of the substance. To test a liquid, pour out a small sample into a clean container and insert the strip. To test a powder, break off a small piece and mix it with water to form a solution or a diluted mixture and then test it with a strip. 6. Have students record the brand names of the substances that are tested and their ph levels. 7. Have students record the information in a table with the ph test strip attached alongside the data. Tell students to cover the strip with clear tape (package tape) to minimize color loss. Alternatively, the data can be presented in bar graph format. 8. Compile the results in class and compare readings where there is duplication of testing. Instructional Tip: Test results of the same brands should have consistent results. If not, try to determine why (e.g., bad data, improper procedure, carelessness). Be sure to use a ph paper that has a separate color for each specific ph. 9. Construct a chart and place the results of the testing on the chart to show whether each tested material is an acid or a base. Acid Base

3 10. Ask questions that help students evaluate the results of the testing. For example: What kinds of materials are in the basic range? Acidic range? Ask students to update their K-L- H charts to include what I know and learned about acids, bases and the ph scale. 11. As a transition to the following demonstration, ask students to compare the acidity of apples to carrots where the ph difference is Perform a demonstration that illustrates the nonlinearity of the ph scale. Show through a series of dilutions that a 10-fold change in concentration results in a ph change of one. See Attachment E, ph of HCl and NaOH Solutions for a table of the concentrations and expected ph values after each dilution. 13. Give students a copy of the blank table. As each dilution is made, fill in the new concentrations and the measured ph in a table on the board or overhead. Ask students to fill in their tables at the same time. Instructional Tip: Even if students don t understand the concept of molarity, they can understand that it is a method chemists use to express the concentration of solutions. a. Start with 100 ml of 0.1 M HCl in a beaker that is labeled in large letters (0.1M and 10-1 M). b. Measure the ph of the solution using a ph meter and ph paper or universal indicator. c. Dilute the solution by pouring 10 ml of the 0.1 M solution into a 100 ml graduated cylinder and adding 90 ml of distilled water. Pour the new solution into a clean 250 ml beaker, labeled with the new concentration (0.01 M and 10-2 M), and stir. Measure the ph by the same method used in step one. d. Continue the dilution process five more times, labeling new solutions with the new concentrations. Instructional Tip: After the first two dilutions, ask students to predict the ph of the new solutions before they are tested. e. Repeat the process with 0.1 M NaOH. 14. Ask questions to help students conclude the relationship between concentration and ph. For example: What happens to the concentration of the acid as water is added to it? What happens to the ph of the acid as water is added to it? What is the relationship between the concentration of an acid and the ph? What is the relationship between the concentration of a base and the ph? Refer to the question about the relative acidity of apples and carrots. (Since there is a ph difference of one, the acid concentration in an apple is 10 times greater than in a carrot.) 15. Discuss with the students the use of diluted and concentrated solutions in real-life applications. Some common examples may include: the dilution of one cup of concentrated car wash solution in one gallon of water; the dilution of concentrated floor 3

4 cleaner in water; mixing fertilizer for gardens; or the use of various ph ranges in soaps and shampoos. 16. Have students update their K-L-H charts for the final time. Differentiated Instructional Support: Instruction is differentiated according to learner needs, to help all learners either meet the intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the specified indicator(s). All students benefit from hands-on activities especially kinesthetic learners. Active interactions within groups and time to discuss concepts engages all students in their own learning. Challenge students who are working beyond the indicator to conduct further independent study of an item of interest. Extensions: 1. Have students make an indicator solution from red cabbage juice. See Attachment F, Homemade Indicator Recipe. Use it to test several solutions from home. Ask them to compare the test results obtained using the ph paper and the cabbage juice indicator. 2. Have students research and make indicators from plants and use them to test for acids and bases. 3. Have students make a batch of diluted hydrochloric acid (0.1 M) to duplicate stomach acid and use it to design a controlled experiment to test the effectiveness of various brands of antacids. 4. Have students test rainwater to see if it is acidic. (It should have a ph in the range of ) Encourage them to compare results with persons in their region and/or across the country and explain patterns, if any. 5. Ask students to research the causes, effects and possible methods of reducing acid rain. A Web site with useful information is the Environmental Protection Agency (EPA). 6. Use a ph meter to test some of the common solutions used in this lesson. 7. Have students research and report on the role of acids in the body. Homework Options and Home Connections: Have students test the ph of several bodies of water and/or soil samples in the community. Have students contact a local biologist to find out how the ph affects the type of aquatic life that can live in the water or plants that may thrive in local soil. Have students complete the lab investigation at home with parental/ custodial involvement. Materials and Resources: The inclusion of a specific resource in any lesson formulated by the Ohio Department of Education should not be interpreted as an endorsement of that particular resource, or any of its contents, by the Ohio Department of Education. The Ohio Department of Education does not endorse any particular resource. The Web addresses listed are for a given site s main 4

5 page, therefore, it may be necessary to search within that site to find the specific information required for a given lesson. Please note that information published on the Internet changes over time, therefore the links provided may no longer contain the specific information related to a given lesson. Teachers are advised to preview all sites before using them with students. For the teacher: For the students: ph paper, litmus paper, hydrochloric acid ( HCl), sodium hydroxide (NaOH), baking soda, salt, de-ionized water, distilled water, indicators, ph meter, beakers, graduated cylinders. ph paper. Vocabulary: acid base neutral ph salt de-ionized water distilled water indicator anion cation hydronium Technology Connections: If available, demonstrate the use of a ph meter. Reference the EPA Web site for information about acid rain. Research Connections: Blachowicz, Camille, and Donna Ogle. Reading Comprehension: Strategies for Independent Learners. New York: Guilford Publications, Ogle, Donna. K-W-L: A Teaching Model that Develops Active Reading of Expository Text. Reading Teacher 40 (1986): The KWL is a teaching technique that helps students recall prior knowledge. It was originally developed as a model for active thinking during reading. It has been modified to include H to encourage students to think about how they can learn more. Marzano, R. et al. Classroom Instruction that Works: Research-Based Strategies for Increasing Student Achievement. Alexandria: Association for Supervision and Curriculum Development,

6 Homework provides students with the opportunity to deepen their understanding and skills related to content that has been presented to them. Nonlinguistic representations help students think about and recall knowledge. This includes the following: Creating graphic representations (organizers); Making physical models; Generating mental pictures; Drawing pictures and pictographs; Engaging in kinesthetic activity. General Tips: Be sure the students realize that the ph scale is based on powers of 10, i.e. a ph of 1 is 10 times stronger than a ph of 2, 100 times stronger than a ph of 3 and so on. Attachments: Attachment A, Homework/Discussion Questions Attachment B, Post-Assessment Questions Attachment C, Post-Assessment Answers Attachment D, ph of Common Substances Attachment E, ph of HCl and NaOH Solutions Attachment F, Homemade Indicator Recipe 6

7 1. What is ph? Describe the ph scale. 2. What is a hydronium ion? The Acid Test Grade Nine Attachment A Homework / Discussion Questions 3. How can the ph of a solution be determined? 4. What are the properties of acids? 5. What are the properties of bases? 6. What compounds are formed when an acid reacts with a base? 7. Describe properties of acids and bases that can cause harm or injury to self, others and/or the environment. 8. Which solution is more acidic, one with a ph of 2 or another with a ph of 6? Answer Key: 1. ph is the measure of the acidity of a substance. The scale runs from zero to fourteen where seven is neutral, below seven is acidic and above seven is basic. 2. The hydronium ion is a water molecule with an extra proton. It has a +1 charge. 3. ph can be measured with a ph meter (electronically) or with indicators such as litmus. 4. Acids have a sour taste, change litmus from blue to red, react with active metals to form hydrogen gas and a metal compound, usually contain hydrogen and are proton donors. 5. Bases have a bitter taste, are slippery to the touch, change litmus from red to blue, emulsify fats and oils (thereby making them useful cleaning agents), usually contain the OH - ion and are proton acceptors. 6. The compounds formed when an acid reacts with a base are a salt and water. 7. Since both acids and bases are corrosive, they can harm eyes and skin, as well as structural materials such as metals. Biological systems operate within a narrow ph range and can be negatively impacted by a change of ph. 8. The solution with a ph of 2 is more acidic than one with a ph of 6. 7

8 Attachment B Post-Assessment 1. When an acid is dissolved in water, what ion does the water form? 2. What happens to the ph of an acidic solution as pure water is added? 3. What two types of substances are involved in a neutralization reaction? 4. What does the term salt mean to a chemist? 5. If the ph of a solution is 6.0, would you expect to find more or fewer hydrogen ions (H + ) than in a solution with a ph of 3.0? Explain your answer. 6. How can you use litmus paper to tell an acid from a base? 7. Why might a small piece of chalk reduce acid indigestion? 8. Describe the uses for several bases found in the home environment. 9. Describe the uses of several acids found in the home environment. 10. Correct this recent comment from the news: Acid rain is becoming less of a problem because studies have shown the rain s ph is dropping. 8

9 Assessment C Post-Assessment Answers Answers: 1. When an acid is dissolved in water, what ion does the water form? The hydronium ion (H 3 O + ) is formed when a hydrogen ion from the acid bonds to the water molecule. 2. What happens to the ph of an acidic solution as pure water is added? The ph of an acidic solution increases as pure water is added because the concentration of the hydrogen (or hydronium) ion decreases. 3. What two types of substances are involved in a neutralization reaction? Acids and bases are involved in neutralization reactions. 4. What does the term salt mean to a chemist? A salt is an ionic compound that is formed as the result of an acid and base neutralization reaction. It is formed from the anion of the acid and the cation of the base. 5. If the ph of a solution is 6.0, would you expect to find more or fewer hydrogen ions (H + ) than in a solution with a ph of 3.0? Explain your answer. A solution with a ph of 6.0 has fewer hydrogen ions than a solution with a ph of 3.0. Since ph is defined as the negative log of the hydrogen ion concentration, the higher the concentration of hydrogen ions, the lower the ph. 6. How can you use litmus paper to tell an acid from a base? Litmus paper contains a chemical that is red in the presence of an acid and blue in the presence of a base. To test a liquid, pour a small sample into a clean container and insert the strip. To test a powder, break off a small piece and mix it with water to form a solution or a diluted mixture and then test it with a strip. If the strip turns red, the substance is an acid, and if the strip turns blue, the substance is a base. 7. Why might a small piece of chalk reduce acid indigestion? Indigestion may be caused by excess stomach acid. Chalk, calcium hydroxide, is a base which neutralizes stomach acid. 9

10 8. Describe the uses for several bases found in the home environment. Answers will vary, but may include the mention of cleaning agents, such as soaps, ammonia and bleach, or antacids, drain cleaners, baking soda, etc. 9. Describe the uses of several acids found in the home environment. Answers will vary, but may include food, such as lemons, tomatoes and coffee, or batteries, etc. 10.Correct this recent comment from the news: Acid rain is becoming less of a problem because studies have shown the rain s ph is dropping. If the ph is dropping, the acid concentration is increasing, so acid rain is becoming more of a problem. 10

11 Attachment D ph of Common Substances Limes 1.8 Lemons 2.2 Vinegar 2.8 Grapefruit juice 3 Dill pickles 3.2 Apples 3.9 Tomatoes 4 Bananas 4.5 Green beans 4.6 Carrots 4.9 Cottage cheese 5 Potatoes 6.1 Milk 6.3 Distilled water 7.0 Eggs 7.1 Rubbing (isopropyl) alcohol 7.5 Baking soda 8.5 Ammonia 11.5 Coffee varies, but tends to be acidic Water varies, depending on the source 11

12 Attachment E ph of HCl and NaOH Solutions Acid/Base Concentration (Molarity) ph HCl HCl HCl HCl HCl HCl NaOH NaOH NaOH NaOH NaOH NaOH Decimal Exponential Predicted Measured 12

13 Attachment E (continued) Expected values: Acid/Base Concentration (Molarity) ph Decimal Exponential Predicted Measured HCl HCl HCl HCl HCl HCl NaOH NaOH NaOH NaOH NaOH NaOH

14 Attachment F Homemade Indicator Recipe Shred two leaves of red cabbage and boil them in water until the liquid gets very dark. When it has cooled, remove the leaves and pour the liquid into a container. Alternatively, put two leaves of red cabbage in a blender and cover with distilled water. Blend on high speed for about 1 minute. Pour resulting mixture through a paper towel. Store this cabbage solution in the refrigerator and watch it closely for spoilage. 14