6.8 Measuring the Acidity of Solutions Page 160 PRESCRIBED LEARNING OUTCOMES measure substances and solutions according to ph, solubility, and concentration KNOWLEDGE ph is the measure of the tendency toward acidic or basic conditions SKILLS AND ATTITUDES demonstrate curiosity, skepticism, creativity, open-mindedness, accuracy, precision, honesty, and persistence as important scientific attributes ICT OUTCOMES use a variety of information technology tools to create, modify, explore, and present electronic documents that express ideas or concepts analyze information retrieved from a variety of electronic sources synthesize information from a variety of electronic sources for presentations SCIENCE BACKGROUND Acids and Bases Although acids and bases appear to be chemical opposites for some properties, they share other properties. Both acids and bases are water-soluble and are good conductors of electricity. A compound that is neither an acid nor a base is neutral and has a ph of 7.0. The ph scale is a measure of the hydrogen ion (H + ) concentration in a solution. ph stands for the power of H, which means the concentration of H + ions in the solution. Low ph corresponds with a high concentration of hydrogen ions (an acidic solution) and a high ph indicates a low concentration of hydrogen ions (a basic solution). The ph scale is a logarithmic scale; that is, each point on the scale is 10 times as great as the previous point. In other words, a solution that has a ph of 4 is 10 times as acidic as a solution with a ph of 5 and 100 times as acidic as a solution with a ph of 6. Definitions of Acids and Bases In 1884, Svante Arrhenius defined acids as materials that can release a proton or hydrogen ion (H + ) when dissolved in water. For example, hydrochloric acid (HCl) dissolves in water to produce chloride ions and hydrogen ions. H 2 O HCl H (aq) Cl (aq) Arrhenius also defined bases as substances that dissolve in water to release hydroxide (OH ) ions. Sodium hydroxide (NaOH) dissolves in water to produce sodium ions and hydroxide ions H 2 O NaOH Na (aq) OH (aq) The Arrhenius definition of acids and bases explains why acids and bases can be combined together in a neutralization reaction. If acids release H + and bases release OH, these can combine to create water (H 2 O). Time 45 60 min Key Ideas Solutions can be measured by concentration, solubility, and acidity. Vocabulary acids bases ph neutral acidic basic Program Resources BLM 0.0-2 Venn Diagram (comparing two items) SM 6.8 Measuring the Acidity of Solutions Nelson Science Probe 7 Web site www.science.nelson.com HCl NaOH NaCl H 2 O Chapter 6 Matter can be classified. 279
The neutralization reaction of an acid and a base produces a salt and water. The Arrhenius definition of a base works in most cases; however, it cannot explain why some substances, such as common baking soda (NaHCO 3 ), can act like bases even though they contain no hydroxide ions. Bronsted and Lowry defined acids as substances that can donate a proton (H + ) and bases as substances that can accept a proton. This definition explains why baking soda can act like a base. Baking soda can accept a proton (H + ) from an acid. HCl NaHCO 3 H 2 CO 3 NaCl There are strong acids and bases and weak acids and bases. Strong acids and bases ionize completely in water. The common strong acids are nitric acid hydrochloric acid sulfuric acid perchloric acid hydrobromic acid hydroiodic acid Strong bases include lithium hydroxide sodium hydroxide potassium hydroxide rubidium hydroxide cesium hydroxide magnesium hydroxide calcium hydroxide TEACHING NOTES 1 Getting Started Check for Misconceptions Identify: Students may think that acids are more dangerous than bases. Clarify: Explain that strong acids and strong bases are equally dangerous. We all know the dangers of strong acids. Strong bases such as toilet bowl cleaners are caustic and very good at dissolving organic material. Ask What They Think Now: Ask students what substances they can think of that are bases (household products) that are extremely good at dissolving materials. Some possible answers are drain cleaners and most household cleaners (ammonia, bleach, Lysol cleaner, hand soap (mild), shampoo, conditioner, and detergents). Phenolphthalein is a sensitive ph indicator. It is colourless in acidic solutions and pink in basic solutions. Use a phenolphthalein indicator (you can make your own by dissolving an Ex-Lax tablet in rubbing alcohol) to help capture students interest at the beginning of the section by dramatically changing the colour of a solution a few times. Start with a clear, colourless acidic solution (e.g., diluted white vinegar) containing some phenolphthalein. Add base (e.g., diluted ammonia solution) until it suddenly turns bright pink. Then add vinegar to make it colourless again. Repeat this a few times without any explanation. Do the following invisible writing demonstration: Use phenolphthalein for the demonstration. It turns red in the presence of a base such as ammonia, NaOH (glass cleaner) Tell students that the clear liquid in this bottle (phenolphthalein) can be used like invisible ink. Ask students if they have written a 280 Unit B: Chemistry
2 secret message using a magic pen before. Students may have used one before but may not know how the pen works. Show students a sheet of white paper and a paintbrush. Don t use acid-free paper. Ask for a volunteer to paint a word or a simple picture using the paintbrush dipped in the clear solution. When the liquid evaporates the page will remain blank. Have another student carefully squirt the blank sheet of paper with glass cleaner until the message appears. Ask the students to observe the message carefully because something else may happen (the message fades). Explain the process by saying that chemical indicator solutions such as phenolphthalein detect the presence of bases such as ammonia (NH 3 ). When the student painted the word or picture using the phenolphthalein solution, the clear liquid quickly evaporated. Once the paper was sprayed with glass cleaner, the phenolphthalein quickly reacted to the base, turning the word or picture red. As the ammonia soaked into the paper, another reaction occurred with the acid content of the paper. The red quickly faded as the acid reacted with and neutralized the ammonia (base). Guide the Learning Show the class some common acids and bases (e.g., lemon, can of pop, vinegar, window cleaner, liquid Drano). From Figure 2 they may know which are acids and which are bases. If you use examples different from those in the student book, students will not know which are acids and which are bases without testing them with litmus or some other indicator. Remind students that some acids and some bases are safe and, in some cases, beneficial. Other acids and bases are extremely dangerous. The only sure ways to know which are safe is to read the labels and to test the substances to determine the ph. Remind students never to taste any solution. Some strong acids and bases are extremely dangerous and potentially fatal if swallowed. The mucosal linings of the mouth, esophagus, and trachea can be severely burned, which may lead to serious digestive and respiratory problems. Have students identify any safety symbols and read the warnings on the containers. They can use the Skills Handbook to determine what the safety symbols mean. Have small groups of students read the section. One person in each group should record the main ideas. Discuss the properties and uses of acids and bases. Students often use acidic and basic solutions in their daily lives. Ask them for examples. Have students write their responses in their notebooks. For students who need additional support with reading in this section, use SM 6.8 Measuring the Acidity of Solutions. Chapter 6 Matter can be classified. 281
3 Consolidate and Extend Review the terms acids, bases, ph, neutral, acidic, basic. Review with students the properties of acids and bases. Record their responses on chart paper. Assign the Check Your Understanding questions. Students can use BLM 0.0-2 Venn Diagram (comparing two items) to help them complete question 1. CHECK YOUR UNDERSTANDING SUGGESTED ANSWERS 1. Acids have a sour taste react with (corrode) metal have a ph less than 7.0 turn blue litmus red Examples: lemon juice, vinegar, cola cause serious burns on skin change colour of litmus paper Bases have a bitter taste react with (break down) fats and oils feel slippery have a ph greater than 7.0 turn red litmus blue Examples: ammonia, baking soda, drain cleaner 2. Occasionally, there may be a spill of a dangerous material that is an acid or a base. If you know what it is, you can neutralize it by adding a base or an acid to it. 3. Bases react with (break down) fats and oils, thus removing greasy residues. 6.8 Measuring the Acidity of Solutions LEARNING TIP Check your understanding of the properties of acids and bases. Work with a partner and take turns describing the properties. Have you ever wondered what makes lemon juice sour (Figure 1)? Lemon juice is a solution that contains dissolved compounds. Scientists classify some compounds by the properties of the solutions they form. Figure 1 Lemon juice has a sour taste. Acids are compounds that form solutions with the following properties: have a sour taste react with (corrode) metals can cause serious burns on skin Many acidic solutions, such as lemon juice and vinegar, are harmless. They can be used to give foods a tangy flavour. Other acidic solutions are extremely dangerous. Hydrochloric acid, for example, is used to etch concrete and would make holes in your skin or clothing. Bases are compounds that form solutions with the following properties: have a bitter taste feel slippery react with (break down) fats and oils can cause serious burns on skin Some basic solutions are harmless. You can drink a solution of baking soda and water to calm an upset stomach. Other basic solutions, such as drain cleaner, should be used with extreme care. They should never be allowed even to touch your skin. Reading and Thinking Strategies: Check Understanding Sometimes it is helpful to discuss the vocabulary words with a partner. Have students read the Learning Tip on page 160. Ask them to recall instances when working with a partner helped them understand ideas. Ask how best to work with a partner. Elicit guidelines for working together from the students. Then ask them to work with a partner to check their understanding of the properties of acids and bases. 160 Unit B Chemistry 282 Unit B: Chemistry
Extra Support Ensure that students work in mixed-ability groups so they can support each other with their individual strengths. Share the mnemonic device that summarizes the fundamental properties of basic solutions: bitter and slippery in cleaners. Have students work in groups to create a similar mnemonic for acids. For students who need additional support with reading in this section, use SM 6.8 Measuring the Acidity of Solutions. Meeting Individual Needs ESL Encourage students to participate. They may require a scribe to note their observations. Everyday knowledge that is shared by those familiar with the materials in the community is not readily accessible to students new to the community and the culture. Being able to refer to this knowledge as required in reading text is a complex skill, which will be beyond the scope of many learners without direct experience. For instance, household products and their uses may be unfamiliar to some ESL students. To provide students with concrete referents, whenever possible, bring these products to class and show students how they are used. Extra Challenge Have students research problems resulting from extreme ph levels, such as acid indigestion. In their report they should indicate the type and ph of acids found naturally in the stomach, what happens when indigestion occurs, and how the problem is treated. Other possible topics could include acid rain, the affects of household cleaners on septic fields and sewage treatment facilities, and the environmental problem associated with the disposal of old batteries. www.science.nelson.com GO ASSESSMENT FOR LEARNING What To Look For in Student Work Evidence that students can classify (e.g., use the ph scale to classify substances) create products that are complete (e.g., Venn diagram) use technical language correctly (acids, bases, ph, neutral, acidic, basic). display data in tables Suggestions for Teaching Students Who Are Having Difficulty Have students review the section with a resource person. Use examples that are familiar to the students. Chapter 6 Matter can be classified. 283