Introduction

The numerical scale called pH indicates how acidic or basic an aqueous (water) solution is, or whether that solution is neutral. Many products we use daily for personal hygiene, home and auto care, or eating and drinking are suitable for pH testing. For this laboratory activity, you may bring to class as many products, in their original, closed containers, as you wish to test.

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Purpose

To determine the pH of common household products and classify each based on its acidic or basic properties using the pH scale.

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Safety

Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Read the labels on all products, record any warnings and take appropriate precautions for handling. Dispose of all solutions in the containers provided by your teacher. Wash your hands before leaving the laboratory.

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Procedure

Use the data table provided, or construct one.

  1. Obtain several strips of wide-range pHydrion paper and several strips each of various narrow-range papers (4-6, 6-8, etc.).
  2. Arrange the strips so they are convenient to use during testing.
  3. Set up an area for temporarily discarding the used test papers. (Not the sink!)
  4. First use the wide-range paper, then the narrow-range paper to test several products, one at a time. Transfer a few drops of the product to be tested to a 50 mL beaker or watch glass. Dip a clean glass stirring rod into the product and transfer one drop of it to the pH paper.
  5. Observe the color change. Then match the final color to those on the chart to estimate the pH value of the product.
  6. If you are unsure of the color, use two narrow-range papers that either overlap or follow. (Example: If pH approximates 8.0, use 6-8 followed by 8-10.)
  7. Rinse and dry the stirring rod before each test.
  8. Record the pH of each product.
  9. Wash hands thoroughly before leaving the laboratory.

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Data Analysis

  1. Trade data with classmates who tested different products. Gather data on at least 20 products.
  2. Formulate at least three distinct conclusions from your data. Look for any similarities or differences among product pH values.

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Imply, Apply

  1. Are the pH values of the products tested consistent? Do they make sense? Why or why not?
  2. State which, if any, pH values surprised you. Why were you surprised?
  3. Look up the pH value or range found on human skin. Also try finding the meaning of "pH balanced". (A skin-care book may be helpful.) Were any products "pH balanced?" Which one(s)? Cite any reference(s) used.
  4. Predict the mouth - pH range most likely to promote tooth decay. Extra Credit: Provide an answer from your dentist or one obtained through library research.
  5. What did you like about this laboratory activity? Why? Did you dislike anything about this activity?
  6. Suggest improvements for this laboratory activity.
  7. Do you have any questions about pH? If so, list them on a separate sheet of paper and submit them to your teacher.

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Teachers Guide

Preparing for the Laboratory Activity

Conducting the Laboratory Activity

Assessing the Laboratory Learning


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Preparing for the Laboratory Activity

Major Chemical Concept

Household products exhibit a wide range of pH values, ranging from highly acidic to highly basic.

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Level

Applied chemistry, General chemistry

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Expected Student Background

Since this activity is used as an introduction to the concept of pH, no prior knowledge is required.

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Time

50 min

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Safety

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Materials

Non-consumables (per lab team)

Consumables (for class)

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Advance Preparation

  1. You may need to supply many of the consumable products.
  2. Testing may be facilitated by placing a few products at each lab bench, with students moving from bench to bench. If a large, accessible area is available to hold all products, students may take products to their benches for testing.
  3. Use either container caps, 50 mL beakers, or watch glasses to hold products. Label all containers.
  4. Place several pieces of various pH papers at each student station.
  5. Provide students with data tables (constructed with a metric scale, using 1 cm per pH unit) or have them construct their own.

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Conducting the Laboratory Activity

Pre-Lab Discussion

Before any discussion of pH, ask students to freewrite (5 min maximum) what they know about pH. This activity should be done in class to insure individual, spontaneous answers. Writing samples reveal naive ideas students have about pH ("pH is an additive"; "only acids are harmful"), as well as what they correctly know. Samples may be saved for later comparison.

Demonstrate how to test products and how to match colors.

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Teacher/Student Interaction

Encourage students to test a variety of products, not just the ones they have brought to class. Twenty to twenty-five pH values provide sufficient data from which to draw conclusions.

Be watchful for students mixing different products or not cleaning their stirring rods.

Solicit written questions about pH (Imply, Apply, Question 7) before the end of the activity; incorporate them into lecture. For example, some may question the discrepancy between the numerical progression of the pH scale (0-14) and the "highly acidic-to weakly acidic-to neutral-to weakly basic-to highly basic" descriptions. Others may want to know what makes a product highly acidic or highly basic. The activity can lead to expansion of the concept of pH on various levels including the origin of the pH scale and development of the ion product of water (Kw).

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Anticipated Student Results

  1. Citrus fruits, soft drinks, and vinegar have pH values of 3-4.
  2. Many household cleaners have pH values of 10 to 12+, although some cleaners are acidic.
  3. Depending on the products tested, pH values will vary, but seldom are below pH 3 nor above pH 12.
  4. See sample data table on next page.

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Answers to Data Analysis Questions

2. Common conclusions include:

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Post-Lab Discussion

In a question-answer format, address questions that were submitted by students. Solicit answers from students themselves; use questions as a starting point for further discussion of acids and bases.

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Possible Extensions

Homework assignment: Based on data collected in this activity, invent a new product, to be sold commercially, whose most important property is its pH. Write an ad for this product, explaining its utility in terms of the pH value you have chosen. Create a name for your product.

pH of Familiar Products/Sample Data Table
0  
1  
2  
3 Vinegar, Sprite 2, Orange
4 Vinyl cleaner, Teacher's saliva, Orange
5 Fluoriguard2 anti-cavity rinse, Cepastat2 mouth spray, stdt saliva
6 Nail polish remover, Student saliva, shampoo
7 Liquid detergent, Skin cleanser
8 Tap water
9  
10 Window cleaner
11 Dow2 bathroom cleaner
12 Dishwashing detergent
13  
14  

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Assessing the Laboratory Learning

Pre-Lab/Post-Lab Writing Assessment

Soon after the laboratory activity, ask students to write once more about pH. Sufficient time should be given in class for this follow-up writing activity; allow students to refer to their pre-lab writing. After examining both pre-lab and post-lab samples, holistically assess increased understanding or corrected misconceptions about pH, and provide written comments or grades for the student work.

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Lab Practical Quiz

Have students determine the pH of an aqueous solution and draw a conclusion as to the type of product present in solution, using their data. Students must support their choices by explaining how they omitted certain products. (You can prepare a solution of detergent, laundry soap, or any product not easily recognized by its odor. Preparation may be qualitative, providing you check the pH prior to student testing.)

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Lab Activity-Homework Assignment

Mention to students that pH is only one way to think about the danger of certain products. Some inedible products have about the same pH value as that of beverages. Have students find the pH in class of a commercial plant food (preferably an acidic one). Then have them use pH values from the activity to find an edible product(s) with a similar pH. Provide the plant food label with list of ingredients, and have them research the properties of ingredients that would prohibit their ingestion. Find an acidic liquid plant food for pH testing or any similarly toxic liquid with a pH similar to any edible product tested.

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Lab-Based Quiz

Quiz 1

Next to each product category listed, indicate the pH value or range appropriate for that type of product. You may use your data from the laboratory activity.

  1. Laundry detergents
  2. Oral hygiene products
  3. Water
  4. Soft drinks
  5. Skin care products

Quiz 2

For each pH value listed (see table on previous page for an example), propose a product or category of household products that might have the pH value indicated. You may use your data from the laboratory activity.

Quiz 3 - "Identify the Product"

  1. Teacher selects one product, previously tested by students, to be identified.
  2. Students gather in teams; each team formulates a question that can be answered "yes" or "no" and is non-specific. (For example, "Is the pH 6.3?" is not an acceptable question.)
  3. Teacher answers one question per group, until sufficient clues have been supplied to enable all students to identify the product. This step may be repeated.
  4. Without further conferencing, each student submits an answer to the product's identity, with an explanation for the choice. Their explanations should reflect their ability to classify products according to acid-base properties, and correlate the classification to the product.

Example: Unknown product = Liquid Drano, pH = 13

Q1: Is the product acidic?
A1: No. (neutral or basic)
Q2: Is the product weakly basic?
A2: No. (neutral or strongly basic)
Q3: Is the product neutral?
A3: No. (not distilled water)
Q4: Is the product a dishwashing liquid?
A4: No. (further narrows choices)
Q5: Is the product used in plumbing fixtures?
A5: Yes

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