Laboratory Practice: pH Indicators – Acids and Bases

 

Laboratory Practice: pH Indicators – Acids and Bases

General Objective

To study the behavior of acids and bases through the preparation and use of a natural pH indicator made from red cabbage, allowing the identification of acidic and basic substances and the development of experimental skills in chemistry.

Specific Objectives

1. Prepare a natural pH indicator using red cabbage.
2. Identify acidic and basic substances through the color change of the natural indicator.
3. Evaluate the properties of different acids and bases by observing neutralization reactions.
4. Promote the use of natural resources for experimental chemistry learning in everyday contexts.

Introduction

The chemistry of acids and bases has been a fundamental area for understanding the reactions and properties of substances. Classical definitions describe acids as substances that release hydrogen ions (H⁺) in aqueous solution, while bases are substances that release hydroxide ions (OH⁻). These properties give them unique characteristics and distinctive reactions.

pH is a fundamental parameter that measures the degree of acidity or alkalinity of a substance on a scale from 0 to 14. Values below 7 indicate an acidic medium, values above 7 indicate a basic medium, and a pH of 7 is considered neutral. Acid-base indicators, such as red cabbage extract, can change color depending on the pH of the medium, providing a quick visual method to determine the nature of the substance being analyzed.

This experimental practice uses red cabbage to prepare a natural indicator and evaluate different samples. In addition, neutralization reactions between acids and bases are observed, demonstrating the ability of the indicator to visualize changes in the acidity and alkalinity of solutions.

Materials and Reagents

• Container for boiling
• Strainer
• Cups or beakers, one for each sample
• Spoons, forks, or stirring rods
• Knife

Samples

• Water
• Red cabbage leaves
• Lemon juice
• Detergent
• Vinegar
• Baking soda
• Shampoo
• Disinfectant
• Drain cleaner
• Antacid
• Hydrochloric acid
• Chalk
• Lime
• Boric acid

Experimental Procedure

1.     Prepare the natural pH indicator by boiling red cabbage leaves until a colored solution is obtained.

2.     Place the same amount of the prepared indicator into each cup or beaker.

3.     Add small amounts of the samples to the indicator and observe the color changes.

4.     Record the color change and classify each sample according to its acidity or basicity.

5.     Carry out a neutralization reaction by adding an acidic sample and a basic sample to the same container with the indicator, observing the change toward a more neutral pH.

Analysis of Results

Complete the following table with the observations obtained:

Sample	Color Change	Nature of the Substance Acid or Base	Type of Acid or Base Lewis, Brønsted, or Arrhenius

 

Questions for the Laboratory Report

1.- Why do you think it is important to measure the pH of substances we use in everyday life?

2.- Classify each of the following substances as acidic or basic: H₃PO₄, Ca(OH)₂, H₂S, H₂SO₄, NaOH, HCl, KOH, HNO₃, Mg(OH)₂, H₂CO₃.

Acids:

Bases:

3.- What is an acid-base indicator used for?

4.- Write the type of pH, acidic or basic, that the following substances may have:

Coffee: ________

Gastric juice: ________

Blood: ________

Urine: ________

Orange juice: ________

Toothpaste: ________

Saliva: ________

Water: ________

5.- What is the scientific name of red cabbage?

6.- What causes its purple color?

7.- Why can red cabbage extract be used as an acid-base indicator?

8.- Is it possible to prepare another natural acid-base indicator besides red cabbage? If so, mention some examples.

Evidence

Include your impressions of this practice by describing:

• The preparation of the indicator.
• The cups or beakers containing the indicator.
• The cups or beakers containing the indicator and the samples.
• The neutralization reaction.

Bibliographic References

• Brown, T. L., LeMay, H. E., Bursten, B. E., & Murphy, C. J. (2009). Chemistry: The Central Science. Pearson Education.
• Chang, R., & Goldsby, K. (2012). Chemistry. McGraw-Hill.
• Petrucci, R. H., Harwood, W. S., Herring, F. G., & Madura, J. D. (2017). General Chemistry. Pearson Education.
• Zumdahl, S. S., & Zumdahl, S. A. (2017). Chemistry. Cengage Learning.