Lesson Plan. Acids and bases. Essential Learnings. Teacher Preparation

Transcription

1 Acids and bases Lesson Plan Essential Learnings Lesson Objectives: By the end of this lesson, students should be able to: Distinguish between acids and bases by describing what happens when each is added to water. Identify acids and bases based on physical properties. Identify acids and bases using litmus paper. Describe the concentration of acids and bases using ph. Explain ph changes that occur when acids are added to bases or when bases are added to acids. Lesson Essential Questions: What are acids and bases? What are some properties of acids and bases? What is ph and how does it measure the concentration of acids and bases? How do acids and bases react with each other? Key Vocabulary: acid, base, ph, neutralization, salt, indicator litmus paper, solution, ion Teacher Preparation Background for the Teacher: Acids and bases are classified by the ions that are formed when they are added to water. Acids are compounds that produce hydrogen ions (H + ) in solution. Bases are compounds that produce hydroxide ions in (OH - ) solution. Acids and bases can be identified by their properties. Acids taste sour and turn litmus paper red. Bases feel slippery, taste bitter, and turn litmus paper blue. Both conduct electric current in solution. Acids react with metals to produce hydrogen gas. Acids and bases react with each other to form a salt and water. This type of reaction is called a neutralization reaction because the products of the reaction tend to have a ph around seven. Acidity is measured on the ph scale. The ph is a measure of the concentration of hydrogen ions in a solution. The ph scale ranges from one to fourteen. Solutions with a ph below seven are acidic and contain more H+ ions than OH- ions in the solution. Solutions with a ph above seven are basic and contain more OH- ions than H+ ions in the solution. Neutral solutions have a ph = 7 and have an equal amount of OH- and H+ ions in the solution. Common Misconceptions: Be aware that students may have the following misconceptions about acids and bases: Students might think that acids contain only H+ ions. In fact, acids contain both H+ and OH- ions. Acids simply contain more H+ ions than OH- ions, and bases contain more OH- ions than H+ ions. Students might think that strong acids that separate into ions easily, such as hydrochloric acid, will always have a low ph in solution. In fact, ph is a measure of how many more H+ ions are in solution than OH- ions. If the solution contains a small amount of hydrochloric acid (it is dilute), it will have a ph closer to seven. Students might think that all bases contain hydroxide ions (OH-). In fact, ammonia (NH3) is a base that doesn t contain a hydroxide ion. However, it produces OH- ions when it is mixed with water. Many students believe that acids (and perhaps bases) are dangerous, exotic chemicals found only in chemistry labs. Many common substances, however, are acids and bases. Prior Knowledge for Students: Students should come to this lesson with an understanding that a compound is made of atoms of two or more elements chemically joined together. Students should understand that some compounds dissolve in water by Acids and Bases Page 1 3i Education

2 separating into ions. Students should understand that an ion is a charged particle and that ions Teacher can have Guide a positive or negative charge. Materials to Prepare Teacher Demonstration Materials: Computer station with projection device Student Materials: For each group: o safety goggles for each student o at least five pieces each of red and blue litmus paper o one set of at least five labeled paper cups containing five ml each of acidic household liquids (orange juice, lemon juice, carbonated cola, vinegar) neutral household liquids (water) basic household liquids (dilute ammonia solution, glass cleaner) Computer access Writing paper or science notebook for recording ideas Notes to Teacher: Estimated Time for Preparation: one day to gather materials and label cups Session 1 ENGAGE (25 minutes) Activate Prior Knowledge Open the session by asking students to recall the definition of a compound. If needed, remind students that compounds are substances made of atoms of two or more elements joined together. Ask students how many compounds they think exist. Tell students that well over 50 million compounds exist! Tell students that one way to keep track of all of these compounds is to classify, or sort, them based on their properties, and that they will learn one way to classify compounds in this lesson. Ask students if they have heard of an acid. If so, ask students what they know about acids. Students will likely have heard of acids and might say that acids can burn your skin. Ask if they know of some substances that are acids and some substances that are not acids. They might say that some cleaners are acids. They might note that water is not an acid. Ask students if they ever eat acids. It s likely that many students will say no. Ask if they have heard of a chemical base, other than the base in baseball. Students will probably not have heard of a base in this sense of the word. Tell students that a base can be thought of as the opposite of an acid. Ask students what the term neutral means to them. They will likely have an idea that the term neutral means in the middle or balanced. Tell students that compounds can be classified as acids, bases, or neutral substances. Some neutral substances are called salts. Ask students what they know about salts. Students will likely think of table salt. They might know that table salt is composed mostly of NaCl. Tell students that many other compounds are also classified as salts. Stimulate Interest Have students view the video segment Acids, Bases,& Salts(1:22), which introduces the terms acid, base, and salt, and shows why these types of compounds are important when studying underwater caves. The video shows that the citric acid in a lemon is an acid. Ask students if their idea of an acid has changed. Ask students if it surprised them that the scientist was diving in water that was acidic and touching a substance that is a base? Tell students that they will learn about what makes an acid, a base, and a salt in this lesson. Acids and Bases Page 2 3i Education

3 Finally, post the Essential Questions that constitute what students will be learning. Students Teacher may read Guide them or you may wish to read them aloud together. What are acids and bases? What are some properties of acids and bases? What is ph and how does it measure the concentration of acids and bases? How do acids and bases react with each other? Inquiry Tip: Students are more likely to be engaged in the rest of the lesson if they are investigating a question they have devised. If students come up with a related question whose investigation will lead to an understanding of how scientists classify compounds as acids and bases, let them use the Explore to try to answer it instead. EXPLORE (Guided Inquiry: 30 minutes, or Directed Inquiry 25 minutes) Exploration Student Resources for Guided Inquiry Reading Passages Acid Test (the ph scale and examples of acids and bases) Video Segments Acids, Bases, & Salts (1:22 introduction to acids, bases, and salts) Properties of Acids & Bases (start to 1:36, and 1:43 to 2:59 to learn about how acids behave in water and the properties of acids and bases) Weak or Strong (1:30-1:57 introduces the ph scale) Acids & Bases: ph Scale & Indicators (00:50-1:13 shows how indicators and litmus paper can be used) Video Segments Compounds: Acids & Bases Directed Inquiry and Guided Inquiry are two alternate instructional pathways through Explore. For Directed Inquiry, the specific digital resources that students will explore and the order in which students will explore them are specified for the instructor. In this option, you might choose to present the specified assets to the entire class, or you may choose to have students individually explore each asset as you introduce it. For Guided Inquiry, students are given a list of assets to explore on their own. Your choice between the two should be made based on your students level of familiarity with Guided Inquiry as a process and their need for teacher assistance, as well as on such practical matters as the number of computers in your classroom. Note that the time estimates in this lesson are based on Directed Inquiry; a Guided Inquiry session may take longer for students to complete. Essential Question(s): What are acids and bases? What are some properties of acids and bases? What is ph and how does it measure the concentration of acids and bases? Guided Inquiry Have students respond to the Essential Questions by exploring the i3 Education resources listed in the Explore section. Ideally, each student will use more than one resource for information, but no student is expected to use all of the resources within the time allotted. Encourage students to take notes as they explore. If your students have never before performed a Guided Inquiry using the DE resources, you will likely need to lead them through the process of selecting appropriate assets, reviewing them, and taking notes. It is appropriate to use a think aloud to model this process for them yourself to demonstrate how to gather the information. Review the Essential Questions that they will be answering from this section. Ask students what kinds of resources will likely help them understand the difference between acids and bases. Students might want to begin by learning about the properties of acids and bases. Video segments that contain both terms might provide a good Acids and Bases Page 3 3i Education

4 start, for example Properties of Acids & Bases and Compounds: Acids & Bases. Teacher These videos Guide provide qualitative (non-numeric) descriptions. Next, students might want to watch videos that describe acids and bases quantitatively (using numbers), such as Acids & Bases: ph Scale and Indicators. Encourage students to proceed in this way. If needed, you may want to further guide students by asking more specific questions: What happens when acids are added to water? What happens when bases are added to water? Circulate among the students as they are exploring and challenge them to summarize for you what they have learned. Encourage students to use reading, viewing, and note-taking strategies to get the most out of each resource. Directed Inquiry Before students begin the inquiry, have them create a three-column chart. Have them label the first column Properties of Acids, the second column Properties of Bases, and the third column Properties of Both Acids and Bases. Students should add to this table as they learn about the properties of acids and bases. Have students take notes on all twenty-eight items in the interactive video Compounds: Acids & Bases (2:18) to learn about the properties of acids and bases. Point out to students that the video incorrectly shows people handling strong acids and bases with bare hands. Remind students that gloves should always be worn when handling strong acids and bases. Also, the video states that all bases contain negatively charged hydroxide ions, when in fact, some bases, such as ammonia (NH3) don t contain hydroxide ions but produce them in water. As a class, discuss the properties of acids and bases. Have students read Acid Test to learn about the ph scale. Ask students to add information about ph to their three-column chart. Ask students whether they think they ever eat acids or bases. They might notice several foods on the chart such as molasses, milk, tomatoes, and vinegar. Have students add a few examples of acidic and basic foods to the acid and base columns of their three-column chart. Have students take notes on the video segment Acids & Bases: ph Scale & Indicators (00:50 to 1:13) to learn about how litmus paper can be used to identify an acid or a base. Have students add this information to their three-column chart. Acids turn blue litmus paper red. Bases turn red litmus paper blue. (Note that the first fifty seconds promote the misconception that all solutions of HCl have a low ph.) Session 2 EXPLAIN (50 minutes) Hands-On Activity Have students complete the Hands-On Activity Identifying Acids, Bases, and Neutral Substances. Session 3 EXPLORE (Guided Inquiry: 25 minutes, or Directed Inquiry: 25 minutes) Exploration Student Resources for Guided Inquiry Video Segments Neutralization & Buffers (start to 2:01; The reaction between acids and bases produces a salt and water.) Exploration Mixing Acids & Bases (Students see what happens on the molecular level when acids and bases react.) Essential Question(s): How do acids and bases react with each other? Acids and Bases Page 4 3i Education

5 Teacher Guide Guided Inquiry Havestudents respond to the Essential Questions by exploring the i3 Education resources listed in the Explore section. Ideally, each student will use more than one resource for information, but no student is expected to use all of the resources within the time allotted. Encourage students to take notes as they explore. If needed, you may want to further guide students by asking more specific questions: What happens to the ions that acids and bases produce in water when these substances are mixed? Circulate among the students as they are exploring and challenge them to summarize for you what they have learned. Encourage students to use reading, viewing, and note-taking strategies to get the most out of each resource. Directed Inquiry Have students take notes on the video segment Neutralization & Buffers (start to 1:24) to learn about the reactants and products of neutralization reactions. Students might have trouble with some of the vocabulary in this video. Help students to understand the reaction. Start by writing the following general reaction between an acid and a base on the board or chart paper. HA (acid) + BOH (base) H2O (water) + BA (salt) Have students recall the definition of an acid. Acids contain excess H+ ions. (These actually exist in the form of hydronium ions, H3O+, but H+ is a simplified representation of this.) Bases contain excess OH- ions. Draw the acid and base as ions on the board or chart paper. H+ + A- + B+ + OH- HOH (or H2O) + BA Ask students what common compound can be formed by combining these ions. Students might recognize that H2O can be formed. Fill this in. Ask students to state the ph of H2O. They should recognize that it is ph=7. Then ask what other compound can be formed. Students might recognize that the remaining ions can combine. Tell students that this compound is a salt. Have students try to predict the products of the reaction between HCl and NaOH. Then have them watch the remainder of the next part of the video segment Neutralization & Buffers(1:24-2:01) to see if their predictions were correct. EXPLAIN (25 minutes) Have students interact with the Exploration Mixing Acids& Bases. Help students to recognize the ions that each acid and base produces in the reaction and to identify the formula for the salt formed in each reaction. Begin by having students create a four-column chart with six wide rows. Label the first column Acid, the second column Base, the third column Water, and the fourth column Salt. Have students write the word Reactants above the first two columns and Products above the second two columns. Before students click play, help them to identify the six possible combinations of reactants. For each combination, help students write the ions formed when each reactant is dissolved in water. Then have students predict the formulas of the salts that are formed from each combination. Ask students to identify the ph of the products and to explain their answer. Session 4 ELABORATE (50 minutes) Virtual Lab: The Mystery of Potter s Pond Virtual Labs, in addition to being an inquiry instructional tool, can be used to assess students skills in science investigation. For this concept, the Virtual Lab The Mystery of Potter s Pond is particularly appropriate. Before Acids and Bases Page 5 3i Education

6 assigning the lab to students, however, you may need to model the appropriate process. The Teacher s Guide for this lab will help you achieve this. This Virtual Lab is divided into five segments: Introduction, Mission, Explore, Plan, and Experiment. Students should take two class sessions to complete the lab. During the first lab session, have students perform the Introduction, Mission, Explore, and Plan segments of the lab. Make sure students read the Briefing document. Students should create their own questions to explore in the Virtual Lab. Before students finish the Explore segment, make sure they: 1. Determine and record the ph of both ponds and of rain water, 2. Determine and record which of the two acids is strongest, and 3. Determine the effect of adding water on the ph of the water in the tank. While the results of the Exploration are still fresh in their minds, have students complete the Plan segment of the lab. Use the information in the Teacher s Guide and the Plan tab of the Virtual Lab to help students complete all parts of the plan: ask a question, write a hypothesis, and write a procedure. Approve students plans before they proceed with the experiment. Session 5 ELABORATE (25 minutes) Students should recognize that, though both ponds receive very acidic rain, one pond is able to maintain a ph that is closer to neutral than the other pond. After students have performed their experiments, help students to understand that adding water to an acidic solution increases the ph (and decreases the concentration of H + ions). Help students propose possible reasons for why Miller s Pond has a higher ph. Project Ideas : To help your students apply their understanding of acids and bases, you may wish to have them complete some or all of the following projects. The time required to complete each project will vary; some may require students to work outside the classroom. Have students make a list of items in their homes that contain acids and bases. For each item, have them write the name of an acidic or basic ingredient. For example, citrus fruits contain citric acid. Aspirin contains acetylsalicylic acid. Hand soap or shampoo might contain ammonium lauryl sulfate or sodium laureth sulfate. Students might need to research items to determine ingredients and their acidity or basicity. Expand on the Hands-on Activity. Have students research the actual ph of each of the substances they tested. They might use ph paper (paper that contains a universal indicator), other types of natural indicators, or a digital ph meter. Students can use Internet or classroom resources to verify the ph values. Students should be aware that the concentration of solutions affects the ph. For example, diluted vinegar will have a slightly higher ph than vinegar straight from the bottle. Have students complete the Activity on the second page of the Acid Test reading selection. The materials per student group are: three paper cups; two tablespoons (tbsp.) of vinegar, one tbsp. of dishwashing liquid, seven tbsp. of water, and three tarnished pennies. To the first cup, add the vinegar. To the second cup, add the dishwashing liquid and three tbsp. of water, and to the third cup, add the remaining water. Add one penny to each cup and allow them to sit for thirty minutes. Ask students which substance was the most acidic. They might want to test the solutions with red and blue litmus paper. Ask students which substance cleaned the pennies best. Ask students what other tests they can perform to determine the relationship between acidity and ability to remove tarnish. EVALUATE (25 minutes) Have students complete the Brief Constructed Response (BCR) item titled Acids & Bases. You may also wish to assign the online concept assessment and use the results in the student reports to guide you in assigning any remediation to students. In order to evaluate students understanding of science investigation, have them review the video, MythBusters: Chemical Changes: 101 Uses for Cola. Provide them with a copy of the student observation sheet and the critique Acids and Bases Page 6 3i Education

7 sheet so they can critique how well the Myth busters conducted the science investigation. Teacher Use their Guide results to monitor how well they are grasping the basic process of science investigation. Review Sheet: Students may review the information in this section using the Acids and Bases Review Sheet. Acids and Bases Page 7 3i Education