Materials Included In Kit

Additional Materials Required

Safety Precautions

All of the acids and bases used in this lab are very corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid all body tissue contact. Acetic acid, hydrochloric acid and ammonium hydroxide are also toxic by inhalation. Avoid breathing the vapors and dispense these chemicals in a fume hood. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult material safety data sheets and acid/base safety handling and disposal procedures for additional safety and handling techniques. Keep spill control materials on hand to neutralize acids or bases in case of spills. Use sodium carbonate or sodium bicarbonate to neutralize acid solutions. Use citric acid to neutralize base spills.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. All of the solutions may be flushed down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. The used paper strips should be discarded in the solid waste disposal. The used calcium carbonate and metal pieces should be rinsed with water and either reused or discarded in the solid waste disposal.

Teacher Tips

• This kit is intended to be used in an introductory chemistry curriculum to aid in teaching the properties of acids and bases. The laboratory instruction sheets, data tables and questions and analysis pages are designed to be reproduced for student use.
• This kit contains materials for a class of 30 students working in pairs.
• The lab activities included in this kit may take multiple lab periods to complete. Look for logical places to stop based upon the level and maturity of the class group.
• It is suggested that you precut the magnesium ribbon and copper wire to 1-cm lengths for students to use.
• A solid waste container should be made available for students to dispose of their indicator paper strips, solid calcium carbonate pieces and metal pieces. Caution the students that these materials are not to be thrown down the drain.
• Each group will need a 24-well microplate to perform this lab. These microplates may be purchased once and then reused. Note: If these plates are not available, all of the tests may be performed in test tubes. Alternately, the tests may be performed on clear acetate sheets using smaller volumes of solutions.
• Distilled water has a pH of 7. If tap water is used, pH results may vary.
• If the pH of distilled water is found to be higher than 7, the ammonium hydroxide vapors may be the cause. Try testing the water away from the ammonia.
• Cassette tape cases are useful for transporting and holding Beral-type pipets.
• The use of pH meters is an excellent addition to this lab. If available, incorporate pH meters into each part of the lab where pH is measured.
• If a conductivity meter is available, a worthwhile addition to this lab is to either perform a conductivity test as a demonstration or to have students perform the following suggested test as part of their procedure
  {12827_Tips_Figure_2}
  An optional in-class or home project would involve the testing of household products. Students could bring in household samples, predict whether the product is an acid or a base, and then test the product with litmus paper or phenolphthalein indicator solution. Then the student could predict the approximate pH of the product and test the product with pH paper or universal indicator solution.
• For other exciting pH labs and activities, refer to A pH⁴ Laboratory & Activity Manual: pHysiology, pHarmacology, and other pHantastic pHenomena by Carla R. Krieger available through Flinn Scientific, Inc. (Catalog Number AP4571).
• For additional acid–base labs, try Acid–Base Test Kit II: Examining Reactions of Acids and Bases available from Flinn Scientific, Inc. (Catalog Number AP4568).

Sample Data

Table 1. Indicators, pH and Reaction with Carbonates

Table 2. Reactions of Acids and Bases with Metals Table 3. Results of Burning Splint Test

Answers to Questions

1. 1. The acidic solutions tested are HCl and HC₂H₃O₂.
   2. These solutions turned the litmus paper red and remained colorless with phenolphthalein.
2. 1. The basic solutions tested are NaOH, NH₄OH and NaHCO₃.
   2. These solutions turned the litmus paper blue and turned pink with phenolphthalein.
3. 1. Yes, the water was neutral.
   2. The water did not change the color of the litmus paper and was colorless/light pink in phenolphthalein.
4. Litmus paper is red in acids and blue in bases.
5. Phenolphthalein is colorless in acids and pink in bases.
6. 1. The acids turned the pH paper red or orange.
   2. The acids turned the universal indicator to a red color.
   3. Acidic solutions have a pH range from 1 to 6, with 1 being the most acidic.
7. 1. The bases turned the pH paper green or blue.
   2. The bases turned the universal indicator to a teal or violet color.
   3. Basic solutions have a pH range from 8 to 14, with 14 being the most basic.
8. 1. The neutral solution turned the pH paper light green.
   2. The neutral solution turned the universal indicator to a green color.
   3. The numerical pH value of a neutral solution is 7.
9. 1. The acids, ranked from strongest to weakest, are HCl and then HC₂H₃O₂.
   2. The bases, ranked from strongest to weakest, are NaOH, NH₄OH and then NaHCO₃.
10. 1. Phenolphthalein indicates whether a solution is acidic or basic. Universal indicator provides a numerical pH value for the solution. pH is an indication of the strength of the acid or base. The further the pH value is from 7, the stronger the acid or base.
    2. Phenolphthalein would be sufficient when trying to identify an acid or a base. It may also be used in a neutralization titration reaction, for instance, when a base is added to an acid until the solution becomes neutral and the indicator just turns pink.
11. The acids, HCl and HC₂H₃O₂, react with the calcium carbonate while the bases and water do not.
12. The two reactions that occur are
    1. CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
    2. CaCO₃(s) + 2HC₂H₃O₂(aq) → Ca(C₂H₃O₂)₂(aq) + H₂O(l) + CO₂(g)
13. 1. The gas released in the reaction with carbonates is carbon dioxide gas, CO₂.
    2. If the burning wood splint test was conducted on the products from the reactions in 12, there would be no pop or explosion. Carbon dioxide does not support combustion, so the flame would be extinguished.
14. The metals, ranked from most reactive to least reactive, are magnesium, zinc and copper.
15. The acids, HCl and HC₂H₃O₂, showed a positive reaction with some of the metals while the base, NaOH, did not.
16. The reactions that occur are
    1. Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
    2. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
    3. Zn(s) + 2HC₂H₃O₂(aq) → Zn(C₂H₃O₂)₂(aq)H₂(g)
    4. Mg(s) + 2HC₂H₃O₂(aq) → Mg(C₂H₃O₂)₂(aq)H₂(g)
17. The zinc and the magnesium are more reactive metals and thus show a positive test with the burning wood splint.
18. Hydrogen gas, an explosive gas, is released when metals react with HCl.
19. The balanced chemical equation for the combustion of hydrogen gas is: 2H₂(g) + O₂(g) + heat → 2H₂O(l) + “pop.” This exothermic reaction between hydrogen gas and oxygen gas to form water requires an ignition temperature of 580–590 °C.
20. 1. B
    2. A
    3. C
    4. B
21. Use your textbook and your laboratory data to complete the following acid/base summary table.
    {12827_Answers_Table_4}
22. The faint pink color indicates that the solution is slightly basic (pH slightly greater than 7). Upon standing, the pink color slowly fades to colorless. This indicates that the solution is becoming slightly more acidic. Carbon dioxide gas (CO₂) from the air dissolves in water to form carbonic acid (H₂CO₃) according to the following equation

    CO₂(g) + H₂O(l) → H₂CO₃(aq)

References

Herron, J. D.; Sarquis, J. L.; Schrader, C. L.; Frank, D. V.; Sarquis, M.; Kukla, D. A. Chemistry; D. C. Heath: Boston, MA, 1996; Chapter 19.

Source Book, Version 1.0.; Orna, M. V.; Schreck, J. O.; Heikkinen, H., Eds.; ChemSource: New York, 1994; Vol. 1, Chapter 2.

Introduction

Battery acid, stomach acid, acid rain—just a few acids in our everyday life! What does it mean when something is acidic? How do acids differ from bases? Examine both the physical and chemical properties of a variety of acids and bases.

Concepts

• Acids vs. bases
• Indicators
• pH

Background

Acids and bases make up two groups of substances that can be categorized by their physical and chemical properties. Let’s take a look at the distinguishing properties

1. Taste: Acids, such as lemons or oranges, taste sour. Bases, such as soap, taste bitter. Note: Taste should never be used to identify a lab chemical or unknown substance. This lab does not include a test using taste.
2. Feel: Acid solutions do not feel much different than water; however, they sting if they contact broken skin. Base solutions have a slippery feeling. Note: Lab chemicals or unknown substances should never be touched with the bare skin. This lab does not include a test using feel.
3. Indicators: An indicator is a chemical compound, either on a test paper or in a solution, that changes color depending on the acidity or basicity (concentration of H⁺ ions) of a solution and, thus, is used to test for the presence of acids or bases. There are many different kinds of indicators. Litmus paper and phenolphthalein indicator solution are two of the most common. Blue litmus paper turns red when dipped in an acid solution of pH 1–6. Red litmus paper turns blue when dipped in a basic solution of pH 8–14. In a neutral solution with pH 7, each paper retains its original color. Phenolphthalein indicator solution remains colorless in an acid solution, but has a distinct pink color in a basic solution.
4. Reaction with metals and carbonates: While most bases show no reaction with metals, acids react with active metals to release hydrogen gas. For example

   Fe(s) + 2HCl(aq) → H₂(g) + FeCl₂(aq)

   Similarly, most bases show no reaction with carbonates, while acids react with carbonates to release carbon dioxide gas. For example

   BaCO₃(s) + 2HCl(aq) → BaCl₂(aq) + CO₂(g) + H₂O(l)

5. Electrical conductivity: Strong acids and strong bases dissociate fully into their ions and both conduct an electrical current quite well. Weak acids and weak bases remain, for the most part, undissociated and in molecular form (with fewer ions in solution) and thus do not conduct an electrical current well.

Acids are defined as substances that release hydrogen ions (H⁺) in solution while bases release hydroxide ions (OH^–) in solution. The positive hydrogen ion, which is a proton, and the negative hydroxide ion combine together to form a neutral water molecule (H₂O) according to the following equation

H⁺(aq) + OH^–(aq) → H₂O(l)

To express the concentration of hydrogen ions in solution, a term called pH (the power of hydrogen ions) is used. If the concentration of H⁺ ion is greater than the concentration of the OH^– ion, then the substance is considered acidic and has a pH value of lower than 7 (i.e., 1–6). If the concentration of OH^– is greater than the concentration of the H+ ion, then the substance is basic and has a pH value greater than 7 (i.e., 8–14). If the H⁺ and OH^– concentrations are equal, the substance is neutral and has a pH value of 7. Special indicator solutions or indicator papers contain certain dyes which change color at various pH values, thus allowing the pH of a solution to be estimated. Figure 1 provides pH values for some common substances. Some acids and bases produce more ions in solution than similar amounts of other acids and bases. This is related to acid or base strength. A strong acid such as hydrochloric acid, HCl, dissociates nearly 100% into its ions, H⁺ and Cl^–. A weak acid, such as acetic acid, CH₃CO₂H, only partially dissociates into its ions, H⁺ and C₂H₃O₂^–, with the majority of it remaining in the molecular form, CH₃CO₂H. A strong acid thus donates a greater number of H⁺ ions to the solution than a weak acid and will have a lower (more acidic) pH. The large number of ions in a strong acid allows the solution to conduct electricity and is termed a strong electrolyte. A weak acid conducts an electric current to a lesser extent and is termed a weak electrolyte. Strong and weak bases can be defined in a similar manner, except that the bases produce hydroxide ions in solution. A strong base such as sodium hydroxide, NaOH, donates a greater number of OH^– ions to the solution than a weak base and will have a higher (more basic) pH. Thus, a strong base is a stronger electrolyte than a weak base such as ammonium hydroxide, NH₄OH.

Materials

Safety Precautions

All of the acids and bases used in this lab are very corrosive to eyes, skin and other body tissues. They are toxic by ingestion. Avoid all body tissue contact. Acetic acid, hydrochloric acid and ammonium hydroxide are also toxic by inhalation. Avoid breathing the vapors and dispense these chemicals in a fume hood. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part A. Acids, Bases and Indicators

1. Add about 10 drops of each of the following to 6 individual wells in Row 1 of your microplate
   • 3 M HCl
   • 1 M NH₄OH
   • 3 M CH₃CO₂H
   • Saturated NaHCO₃
   • 1 M NaOH
   • Distilled or deionized H₂O
2. Using a different piece of dry red litmus paper for each of the 6 solutions, dip the end of the litmus paper into the solution. Remove the paper immediately from each solution, record the color of the paper in Table 1, and place the test paper on a piece of paper towel for disposal. Do not dispose of the litmus paper in the sink!
3. Repeat the procedure in step 2, using a different piece of dry blue litmus paper for each of the 6 solutions. After each test, be sure to record the color of the litmus paper in Table 1.
4. To each of the 6 solutions in Row 1, add 3 drops of phenolphthalein indicator solution. Record the color of each solution in Table 1.
5. Use your results from steps 2–4 as well as the Background reading to classify each of the 6 solutions as acidic, basic or neutral. Record the classification in Table 1.

Part B. Acids, Bases and pH

6. Again add about 10 drops of each of the following to 6 individual wells in Row 2 of your microplate
   • 3 M HCl
   • 1 M NH₄OH
   • 3 M CH₃CO₂H
   • Saturated NaHCO₃
   • 1 M NaOH
   • Distilled or deionized H₂O
7. Using a different piece of dry pH paper for each of the 6 solutions, dip the end of the paper into the solution. Remove the paper immediately and record the color of the pH paper in Table 1. Use the pH indicator color chart on the pH paper container to assign a numerical pH value to each solution. Record this value in Table 1.
8. To each of the 6 solutions in Row 2, add 3 drops of universal indicator solution. Record the color of each solution in Table 1. Use the pH indicator color card to determine the numerical pH value of each solution. Compare this value to that obtained from the pH paper.

Part C. Reactions of Acids and Bases with Carbonates

9. Again add about 10 drops of each of the following to 6 individual wells in Row 3 of your microplate
   • 3 M HCl
   • 1 M NH₄OH
   • 3 M CH₃CO₂H
   • Saturated NaHCO₃
   • 1 M NaOH
   • Distilled or deionized H₂O
10. To the solution in each well, add a small piece of calcium carbonate, CaCO₃. (Note: CaCO₃ is sometimes called marble chips.) Allow the reactions, if any, to proceed for 2 minutes. In Table 1, use “yes” or “no” to record the reactivity of each solution with calcium carbonate.
11. Use forceps to remove the pieces of calcium carbonate from the well plate. Rinse and discard each piece in the solid waste disposal.
12. Clean the well plate by pouring the solutions down the drain and rinsing the well plate with plenty of water. Caution: Take care when rinsing the plates so that solution does not splash out.

Part D. Reactions of Acids and Bases with Metals

13. Add the following to three individual wells in Column 1 of your microplate
    • Zinc mossy, 1 small piece
    • Magnesium ribbon, 1 cm piece
    • Copper wire, 1 cm piece
14. Add about 10 drops of 3 M HCl to each well containing a metal piece. (Be sure there is enough solution to cover the metal.) Compare the speeds of reaction of the metals with this acid. In Table 2, rank the reactivity (very fast, fast, moderate, slow, very slow, no reaction) of the HCl with each metal tested.
15. In Column 2 of your microplate, repeat steps 13 and 14, using 10 drops of 3 M HC₂H₃O₂. Be sure to rank the reactivity of the CH₃CO₂H with each metal as you did in step 14. Record the reactivity in Table 2.
16. In Column 3 of your microplate, repeat steps 13 and 14 using 10 drops of 1 M NaOH. Be sure to rank the reactivity of the NaOH with each metal as you did in step 14. Record the reactivity in Table 2.

Part E. Test for Release of a Gas

17. Place 1 small piece of zinc mossy in the first well of Column 4 of your microplate. Add about 10 drops 3 M HCl to the zinc. As the reaction proceeds, invert a clean, dry test tube held with a test tube holder over the zinc for about 1 minute. Light a wood splint (or have your lab partner light the splint) and, holding the test tube inverted, quickly insert the burning wood splint into the test tube. Record your observations in Table 3.
18. Repeat step 17 for the other two metals (magnesium and copper), using a separate well for each test. Be sure to record your observations in Table 3.
19. Use forceps to remove any metal pieces from the well plate, rinse them and discard them in the solid waste disposal.
20. Clean the well plate by pouring the solutions down the drain and rinsing the well plate with plenty of water. (Caution: Take care when rinsing the plates so that solution does not splash out.)

Student Worksheet PDF

12827_Student1.pdf