Materials Included In Kit

Additional Materials Required

Safety Precautions

All of the acids and bases used in this lab are corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Avoid inhaling vapors. Keep spill materials on hand to neutralize acids and bases in case of spills. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Phenolphthalein and universal indicator are alcohol-based solutions and are flammable. Keep away from flames and other ignition sources. Phenolphthalein and universal indicator are toxic by ingestion. Wear chemical-splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Consult your current Flinn Scientific Catalog and Reference Manual for general guidelines and specific procedures and review all federal, state and local regulations that may apply before proceeding. All of the solutions may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The used paper strips should be discarded according to Flinn Suggested Disposal Method #26a.

Lab Hints

• This experiment is designed as a qualitative introduction to the properties of acids and bases. The experiment can reasonably be completed in one 50-minute lab period.
• Stagger the starting points for lab groups. Have half the class start with Part A and the other half with Part B.
• Remind students to rip the litmus paper and pH paper into small pieces before using it in lab.
• Neutral litmus paper is provided. However, litmus tests are often carried out using blue and red litmus paper to detect acids and bases, respectively.
• The concentrations of acids and bases in this experiment were intentionally kept low (0.1 M). Since the concentrations are the same, it made it possible to give a range of pH values for strong versus weak acids or bases.
• This experiment can easily be adapted to an inquiry-based lab activity. The Supplementary Information in the Further Extensions section contains a set of questions that can be used as a working guide for students to study the properties of acids and bases.

Teacher Tips

• The focus of this experiment is observation and classification. This experiment allows students to classify acids and bases based on their interactions with various indicators.
• The difference between strong and weak acids is frequently misunderstood by students who may confuse strong acids with concentrated solutions and weak acids with dilute solutions. In order to avoid misunderstanding, the solutions in this experiment all have the same concentration. Strong and weak acids (or bases) can be distinguished by their pH values if the solutions all have the same concentration. If two acids have the same concentration, then a stronger acid solution will have a lower pH value than a weaker acid. Thus, 0.1 M hydrochloric acid has a pH of 1 and 0.1 M acetic acid has a pH of 3. If two bases have the same concentration, then a stronger base will have a higher pH value than a weaker base. Thus, 0.1 M sodium hydroxide has a pH of 13, 0.1 M ammonia has a pH of 11, while 0.1 M sodium bicarbonate would have a pH value of about 10. The pH value of an acid (or base) solution depends on both the strength of the acid and its concentration.

Further Extensions

Supplementary Information
This experiment may be adapted to an inquiry-based experiment by providing students the following questions to investigate. The materials required are the same as those provided in the student section of this write-up.

Introduction to the pH Scale and Measurement—Student Objectives

1. Compare and contrast the effects of indicators on acids and bases. Study a variety of indicators with a variety of acids and bases.
2. How can pH paper and universal indicator be used to distinguish between acids and bases? What are the numerical pH values for acids and bases, respectively?
3. What happens to the color of an indicator if you start with a fixed amount of a strong acid and add a strong base to it, one drop at a time?

Answers to Prelab Questions

1. What is the pH range for acids?

   Acids have a pH range of 0–6.

2. What is the pH range for bases?

   Bases have a pH range of 8–14.

3. How can you tell, when comparing two acids, which acid is stronger?

   If they are of the same concentration, the acid with the lower pH number will be the stronger acid.

4. The term “acid wit” is defined in the dictionary as humor that is sharp, biting or sour in nature. In the laboratory, acids present a hazard because they are corrosive. What is meant by the term corrosive? How does this relate to acid wit?

   The term corrosive refers to substances that may irritate and burn organic tissues, notably the skin and, when taken internally, the lungs and stomach. Corrosive substances, such as acids, will also attack and destroy many other kinds of materials, including metals and fabrics.

5. Acid rain is recognized as a growing danger to the environment. Briefly describe two problems associated with acid rain.

   Acid rain is dangerous to fish and other wildlife (reducing fish populations in affected lakes and streams, for example). It also causes economic damage (reacting with and corroding man-made buildings and statues).

6. (Optional) The phrase acid test has entered the popular vocabulary to describe a severe but conclusive test of whether something is authentic. What is the origin of the term acid test?

   Note: This is a hard question. The term acid test probably originated in the days of the Gold Rush. Suspect metals were treated with concentrated acid—base metals dissolve in concentrated acid, gold does not. Gold is, of course, considered a precious metal precisely because it is unreactive. Concentrated nitric acid is required to dissolve gold.

Sample Data

Classifying Acids and Bases

Answers to Questions

1. How can you use litmus paper to tell whether a solution is an acid or a base? Be specific.

   Neutral litmus paper turns red in acidic solutions, blue in basic solutions.

2. How can you use phenolphthalein to tell whether a solution is an acid or a base? Be specific.

   Phenolphthalein indicator is colorless in acidic or neutral solutions, pink or red-violet in basic solutions.

3. Compare the pH data for the solutions that you labeled as acids and bases. What pH values can be assigned to acids and bases, respectively?

   Acidic solutions have pH values less than 7 (between 1 and 7). Basic solutions have pH values greater than 7 (between 8 and 14).

4. How does pH vary with the “strength” of an acid or base, respectively?

   If the concentrations of two acids are the same, a solution of a stronger acid will have a lower pH value than a solution of a weaker acid. Thus, the pH of 0.1 M hydrochloric acid is 1, while the pH of 0.1 M acetic acid is 3. If the concentrations of two bases are the same, a solution of a stronger base will have a higher pH value than a solution of a weaker base. Thus, the pH of 0.1 M sodium hydroxide is 13, while the pH of 0.1 M ammonia is 10.

5. Complete the following table to summarize the properties of acids and bases.
   {12241_Answers_Table_4}

References

Cesa, I., Editor; Acids and Bases, Flinn ChemTopic™ Labs Series; Flinn Scientific: Batavia, IL, 2002; Vol. 13

Introduction

Acids and bases play an important role in nature and are also useful chemicals in the chemistry laboratory. What are acids and bases? What properties can be used to distinguish them? Explore the characteristic color changes of acid−base indicators and learn how to measure the pH of solutions using indicators.

Concepts

• Acids and bases
• pH scale
• Indicators

Background

The word acid is derived from the Latin verb, acere, which means “to (be) sour.” The origin of the word reveals a characteristic physical property of acids—they taste sour. Lemons and grapefruits are called citrus fruits because they contain citric acid, an acidic compound which gives them their sour taste. Although taste is an interesting property of the foods we eat, it is NOT a property that is used in the laboratory to classify compounds as acids or bases. NEVER taste or ingest any materials in the lab! Another property of acids and bases is how they change the color of indicators.

Indicators are interesting and useful chemicals used to identify acids and bases. Indicators are organic dyes that change color in acidic or basic solutions. One of the oldest known acid–base indicators is litmus, a natural dye obtained from lichens. Litmus paper, prepared by soaking paper in a solution of the dye, is often used as a general test for acids and bases. Almost any flower or fruit that is red, blue or purple contains a class of organic pigments called anthocyanins that change color with pH. The use of these natural dyes as acid–base indicators originated during the middle ages—painters made watercolor paints by combining flower and fruit extracts with vinegar, an acid, and limewater, a base.

The pH scale is a numerical scale that is related to the concentration of H⁺ ions in solution and is used to describe the relative acidity or basicity of a solution. The abbreviation pH stands for “power of hydrogen.” The pH scale ranges from 0–14, with 7 being neutral. Acids have pH values less than 7, while bases have pH values greater than 7. Strong and weak acids (or bases) can be distinguished by their pH values if the solutions all have the same concentration. If two acids have the same concentration, then a stronger acid solution will have a lower pH value than a weaker acid. If two bases have the same concentration, then a stronger base will have a higher pH value than a weaker base.

Experiment Overview

The purpose of this experiment is to explore the reactions of indicators with acid and base solutions.

Materials

Prelab Questions

1. What is the pH range for acids?
2. What is the pH range for bases?
3. How can you tell, when comparing two acids, which acid is stronger?
4. The term “acid wit” is defined in the dictionary as humor that is sharp, biting or sour in nature. In the laboratory, acids present a hazard because they are corrosive. What is meant by the term corrosive? How does this relate to acid wit?
5. Acid rain is recognized as a growing danger to the environment. Briefly describe two problems associated with acid rain.
6. (Optional) The phrase acid test has entered the popular vocabulary to describe a severe but conclusive test of whether something is authentic. What is the origin of the term acid test?

Safety Precautions

All of the acids and bases used in this lab are corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Avoid inhaling vapors. Notify your teacher and clean up all spills immediately. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills. Phenolphthalein and universal indicator are alcohol-based solutions and are flammable. Keep away from flames and other ignition sources. Phenolphthalein and universal indicator are toxic by ingestion. Wear chemical-splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Classifying Acids and Bases

1. Obtain a 15-well reaction plate and place it on a piece of white paper as shown in Figure 2.
   {12241_Procedure_Figure_2}
2. Using a dropper bottle, add 1−2 drops of phenolphthalein indicator solution to each well in the first row (1–5).
3. Add 1–2 drops of bromthymol blue indicator solution to each well in the second row (6–10).
4. Add 1–2 drops of phenol red indicator solution to each well in the third row (11–15).
5. Place 1–2 drops of the acids and bases listed in their respective columns. For example, HCl would be placed in the first column (wells 1, 6 and 11). Record all observations in the data table, on the Introduction to the pH Scale and Measurement Worksheet.
   {12241_Procedure_Table_1}
6. When finished with observations, rinse the contents of the reaction plate down the drain with plenty of excess water.
7. Place the well reaction plate back on the white paper as shown in Figure 2.

Part B. Classifying Acids and Bases, cont.

8. Add a small amount of litmus paper to each well in the first row (1–5).
9. Add 1–2 drops of universal indicator into each well in the second row (6–10).
10. Add a small amount of pH paper to each well in the third row (11–15).
11. Place 1–2 drops of the acids and bases listed in their respective columns. For example, HCl would be placed in first column (wells 1, 6 and 11). Record all observations in the data table.
    {12241_Procedure_Table_2}
12. Record all color changes. For the pH paper, use the color chart on the container to assign a numerical pH value for each solution. Record the pH value for each solution in the data table.
13. When finished with the observations, use forceps to remove any leftover litmus or pH paper. Dispose of the paper in the trash. Rinse the contents of the reaction plate down the drain with plenty of excess water.

Student Worksheet PDF

12241_Student1.pdf