pH - Acids and Bases

Advance Preparation

pH paper is not pre-cut, so it will take at least 5 minutes to cut enough pH paper for one class. It should take ten minutes to assemble the other materials.

Introduction

Acids and bases are common, everyday substances.:

• Baking soda, sodium hydrogen carbonate (NaHCO3) is a weak base.
• Soda-pop, carbonic acid (H2CO3) is a weak acid.
• Stomach acid (HCl) hydrochloric acid is a relatively strong acid.
• Ammonium hydroxide (NH4OH) has a strong odor, but is not a strong base. Strong acids and bases are hazardous to skin and eyes and can cause serious burns.
• Sulfuric acid (H2SO4) is a strong acid.
• Sodium Hydroxide (NaOH) is a strong base. (See overhead)

The indicators used to identify acids and bases, are large organic molecules that change color in acid or base. The most common acid/base indicator is litmus paper which changes color in the presence of any acid or base. Other indicators are specific and will change color only at a particular pH (H+ ion concentration).

Once you have determined if a substance is an acid or a base, you need to determine how strong (or concentrated) it is. The simplest measure of the strength of an acid or base is the concentration of H+ ion in the solution. The fastest way to determine pH is to use pH paper. It uses a broad range indicator to turns a series of colors from pH 1 to 14. The color change is then compared with a color chart to determine the pH. A high pH indicates a base, and a low pH indicates an acid. A pH near 7 indicates a neutral substance. Students should be aware that shampoos and soaps and most skin products are pH balanced near 7, to protect skin, hair and eyes.

Student Objectives

The students will:

• use litmus paper to test for an acid, base or neutral substance.
• use pH paper to test for the pH of a substance.
• explain the difference between tests using litmus paper and pH paper.
• compile and evaluate the data in a class data table.

Materials

1. Litmus paper, pink and blue (neutral is available too, but not necessary)
2. pH paper (1-14), cut into 2 " pieces pH
3. color charts (one per lab group)

Procedure-1 Are you acidic or basic?

1. Students put a piece of both blue and red litmus paper into their mouths.
2. After five seconds, they take the papers out to see if either changed color. (Red to blue, means a base; blue to red, means an acid, no change means neutral)

Are you acidic or basic? _________ Class totals: Acidic______ Basic ______ Their saliva is acidic or basic; men are usually basic more often than women.

Students will test either acidic, basic or neutral. The red litmus paper will turn slightly blue for a base. The blue litmus paper will turn slightly pink in an acid. If nothing happens, it is neutral. In order to determine how acidic or basic they are, students need to use pH paper, which changes color to indicate the pH. pH paper is treated with a broad range indicator that changes color with varying pH. (This pH value is an approximate value based on color comparison. More exact pH values are found using pH meters or by titration using acids and bases.)

Depending on how this lesson is used, you may want to demonstrate a pH meter, or and acid/base titration for the class.

Procedure-2

1. Students place the end of a piece of pH paper into their mouths.
2. After five seconds, they remove it and compare the color of the paper with the pH color chart. (If the pH paper dries, the color comparison will be incorrect).

How acidic or basic are you? What was the pH __________?

Use a data table to compile the information for the class.

Questions

1. Explain the difference between the litmus test and the test using pH paper? Litmus paper tests for an acid or base. pH paper tests for the H+ion concentration and indicates the strength of the acid or base.
2. How many people tested acidic, basic or neutral with litmus paper? Which group was larger? The majority of the students will have saliva that is either acidic or basic, which helps with the breakdown of food in the mouth.
3. Was there a pattern in the results for the whole class? If so, what was it? The answers may lead to a discussion on whether gender influences a acidic or basic properties.
4. What substances could affect the outcome of the litmus test? Answers may vary . Any acidic substance (soda pop, a pickle or salsa) could change the outcome. Any basic substance (an antacid, Alka-Seltzer or baking soda toothpaste) would also affect the outcome.
5. What were the results with the pH paper? How many people were in each pH range? It might be interesting to see if there is a gender component in this answer too.
6. Was there a pattern in the pH values for the whole class. If so, what was it? Answers will vary, it might be interesting to compile a series of class data tables to compare results and look for a trend.