Lab: Detecting ph of Commonly Used Acids and Bases FOR THE TEACHER Summary In this lab, students will use their knowledge of acids and bases to determine the acidity and basicity of every day items by measuring ph using several different methods for determination. Indications of ph may be determined using red & blue litmus paper, ph strips, and ph probes or ph meters. Grade Level Middle and High School Submitted by Genia Harris Angleton High School Angleton, Texas Thanks to: Dow Chemistry Teacher Summit Objectives By the end of this lab, students should be able to Idenitify which common daily use and household items are acidic or basic. Determine what methods of determining ph are situationally more accurate. Properly use various lab methods of testing ph including using red & blue litmus paper, ph strips, and ph probes or ph meters. Chemistry Topics This lab supports students understanding of Acids and Bases ph Indicators Time Teacher Preparation: 30-45 minutes Lesson: Two 50 minutes periods Materials (per group) Red litmus paper Blue litmus paper Universal ph paper ph meter & probe Paper towels Ten 100 ml beakers Labelling pen or marker One 250 ml beaker Wash bottle with distilled water 50 ml solutions of the following: o Distilled water o Lemon juice o Tomato juice o Maalox o Ammonia o Coke o Corn Oil American Association of Chemistry Teachers 1-265 1
o o o Shampoo Vinegar Dishwashing Liquid Safety Always wear safety goggles when handling chemicals in the lab. Students should wash their hands thoroughly before leaving the lab. When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals. When working with acids, if any solution gets on students skin, they should immediately alert you and thoroughly flush their skin with water. Teacher Notes Materials can be substituted and suggested amounts may be reduced. Some additional ph background information can be found at this website. Make sure ph meters are working properly and are calibrated before students use them. Teacher may have to also have to calibrate (depending on the quality of the probe) between uses if lab is done during different lab periods. Go over, in detail how students are to use the ph meter and probe. Be sure students have access or a copy of a ph scale. Classroom management tips: This lab may be best utilized if students are in groups of 4. This lab can be separated into stations and students can rotate to different stations (Ex. Create a station for just blue litmus paper with all the solutions already present. Each ph indicator can be in its own station.) Differentiation: Teachers may or may not have students use ph to calculate the H + and/or the OH - ions in solution. FOR THE STUDENT Lesson Detecting ph of Commonly Used Acids and Bases Background Many people think of acids and bases as substances used only in chemistry laboratories or in manufacturing plants. In fact, acids and bases can be found in medicines, household bleaches, detergents, and drain cleaners. They also are used as preservatives in food. Acids differ from bases by the relative amounts of H + and OH - ions they contain. A ph scale is a numeric scale from 0 to 14 that is used to distinguish acids from bases and to measure their strength. An acid has a ph less than 7. The lower the value for ph, the greater the concentration of H + ions and the stronger the acid is. A base has a ph greater than 7. The higher the value for ph, the greater the concentration of OH - the base is. ions and the stronger American Association of Chemistry Teachers 1-265 2
A ph of 7 indicates a neutral solution. A solution with an equal number of H + and OH - ions is neutral. Measuring ph Special paper, called ph paper is used to identify acids and bases. Two types of ph paper are going to be used today: litmus paper and universal indicator paper. Both of these papers contains certain dyes that change to different colors according to the concentration of the H + ions present in the substance being tested. A ph meter is another device that will be used for measuring the ph and the concentration of hydrogen ions in an aqueous solution. In this investigation, you will use litmus paper, ph paper, and a ph meter to determine whether various household and common daily use substances are acids or bases. The strength of acidity or basicity of these items will also be detected. Prelab Questions Examine the ph scale given to you. This allows you to measure the ph of solutions. Note that the various colors (ranging from red to blue) and numbers on the scale corresponds to certain ph values. Answer the following questions before starting the procedure: 1. Which numbers indicate an acid? 2. Which numbers indicate a base? 3. Which number indicates a neutral solution? 4. Which number indicates the strongest acid? 5. Which number indicates the strongest base? 6. Which number indicates the weakest acid? 7. Which number indicates the weakest base? 8. What type of ions do acids release (word and abbreviation)? 9. What type of ions do bases release (word and abbreviation)? 10. Define neutral solution: 11. Define ph scale: Materials Red litmus paper Blue litmus paper Universal ph paper ph meter & probe paper towels ten 100 ml beakers labelling pen or marker one 250 ml beaker American Association of Chemistry Teachers 1-265 3
wash bottle with distilled water 50 ml solutions of: o Distilled water o Lemon juice o Tomato juice o Maalox o Ammonia o Coke o Corn Oil o Shampoo o Vinegar o Dishwashing Liquid Safety Always wear safety goggles when handling chemicals in the lab. Wash your hands thoroughly before leaving the lab. Follow the teacher s instructions for cleanup of materials and disposal of chemicals. When working with acids and bases, if any solution gets on your skin immediately rinse the area with water. Procedure 1. Label ten 100 ml beakers according to the substances listed on the data collection sheet. 2. Pour 50 ml of each substance in the corresponding 100 ml beaker. 3. Dip red litmus paper in each solution. Observe any change that did or did not occur in the color of the litmus paper. Record your observations and any other information in the data table. Save the ten strips on a sheet of paper towel until the end of the class period. 4. Dip blue litmus paper in each solution. Observe any change that did or did not occur in the color of the litmus paper. Record your observations and any other information in the data table. Save the ten strips on a sheet of paper towel until the end of the class period. 5. Dip the universal ph paper in each solution. Observe any change that did or did not occur in the color of the universal ph paper. Record your observations and any other information in the data table. Save the ten strips on a sheet of paper towel until the end of the class period. 6. Collect a wash bottle with distilled water and a 250 ml beaker. Read or Listen carefully to your teacher for instructions on using the ph meter and probe. Rinse the probe off with distilled water from the wash bottle and use the 250 ml beaker to collect the rinse. Then dip the end of the probe in one of the solutions. Record the ph in the data table. Take the probe out of the solution and rinse it again with the wash bottle. 7. Repeat step 6 for all solutions. 8. When you are finished, ask your teacher if you need to save or discard your results saved on the paper towels. American Association of Chemistry Teachers 1-265 4
9. Discard the solutions in all of the 100 ml beakers as your teacher directs. 10.Thoroughly clean up your lab station. Data Substance Red Litmus Paper (record color change) Blue Litmus Paper (record color change) Universal ph Paper (record color change and ph) ph Probe (record ph value) Is the substance as: Acid, Base, or Neutral? lemon juice distilled water tomato juice coke corn oil Maalox ammonia vinegar shampoo dishwashing soap Analysis 1. Which of the household solutions tested are acids? How was this determined? American Association of Chemistry Teachers 1-265 5
2. Which of the solutions were bases? How was this determined? 3. Which solutions were neutral? How was this determined? 4. Did the results for the indicators all agree with one another; for example, did a solution show the same or similar ph as it did with litmus ph paper, ph paper, or the ph sensor? 5. Which of the indicators could be used to give specific ph strengths of the solutions? Conclusion 1. Discuss in paragraph form, the advantages and disadvantages of each indicator. 2. Summarize how common household items are related to particular ph ranges. What is the importance of these ph ranges? American Association of Chemistry Teachers 1-265 6