CHEMISTRY NOW: pH and Acidity (Grades 5-8) Print

Objective:

Students will investigate the pH of household substances. Students will design and carry out an investigation that visually illustrates the different pH levels of solutions, using indicators. Students will be able to observe the color changes produced by pH indicators in solutions with various pH values. Students will draw conclusions about the pH values of some common types of household substances like foods, drinks, and cleansers.


Introduction Notes:

 

CHEMISTRY NOW: pH and Acidity

 

Subject Area: Chemistry

Grade Level:  Middle School Chemistry

Lesson Title:  pH and acidity

National Science Education Standards:                                                 

  • Science as Inquiry: 5–8

  • Properties and Changes of Matter: 58

 

Suggested Prior Knowledge:  concepts of acids and bases, pH

 

Purpose: To give students an understanding of pH indicators, pH, and the acid/base properties of some common household mixtures

Key Vocabulary:  

acid—substance that has a pH less than 7; produces hydrogen ions (H+) in solution

base—substance that has a pH greater than 7; produces hydroxide ions (OH-) in solution

indicator—compound that changes color depending on the pH of the solution it is in

pH—measure of the acidity or basicity of a substance, a pH of less than 7 is acidic (lower number = more acidic); a pH of greater than 7 is basic. In general, the scale goes from 1 (very acidic) to 14 (very basic)

Objectives:      

1. Students will investigate the pH of household substances.

2. Students will design and carry out an investigation that visually illustrates the different pH levels of solutions, using indicators.

3. Students will be able to observe the color changes produced by pH indicators in solutions with various pH values.

4. Students will draw conclusions about the pH values of some common types of household substances like foods, drinks, and cleansers.

Materials:  

- Safety goggles

- Universal pH Indicator solution and/or pH test paper

- Dropper pipettes (or medicine droppers)

- Plastic sheet protector or plastic wrap to use as reaction surface

- Various household solutions to test, such as:

    • - Distilled water
    • - Tap water
    • - Skim milk
    • - Apple juice
    • - Light-colored soda
    • - Detergent solution
    • - Ammonia solution
    • -Vinegar solution
    • - Diluted orange juice
    • - Diluted lemonade
    • - Glass cleaner
    • - Bleach
    • - Drain cleaner
    • - Liquid hand soap
    • - Shampoo

Procedure:

1. Discuss with students that pH indicators are compounds that display different colors in different pH conditions. Briefly review pH, acids and bases, and pH indicators. Indicators can be used to differentiate acidic and basic substances. Begin with a leading question, and follow up:

  • - What is the difference between an acid and a base?
  • - What is a neutral compound?
  • - What pH values do acids have?
  • - What pH values do bases have?
  • - What kind of investigation can we do to determine the pH of various substances and to look for patterns in types of substances and their pH?
  • - Do certain types of mixtures (cleaners, food, drinks) have similar pH values?

 

2. Work with students to design a valid investigation, similar to the one outlined in this lesson plan, to determine the pH of various solutions and to look for patterns. Guide students to ensure a control (distilled water) and proper experimental set-up.

 

3. Work with students to develop questions to investigate, or a hypothesis to test. Ensure that students outline the steps of their procedure carefully. Some good leading questions include the following:

  • - How do we determine what the pH of a substance is?
  • - Can we make some predictions about the materials that we are going to test?
  • - Do things we ingest (foods and drinks) have similar pH values?
  • - Do cleaning products have similar pH values?
  • - How does tap water compare to distilled water? (rain water? snow?)

 

4. Lab safety equipment should be used and safety protocols followed. Drain cleaner and bleach need to be used with caution; avoid inhaling the fumes or skin contact. Dispense these solutions in dilute form (ideally already in a small pipette or dropper bottle).

 

5. Thicker solutions should be diluted so that they can be dispensed with a pipette or dropper.

 

6. A general procedure that could be used:

a. Obtain a reaction surface (plastic sheet protector or sheet of plastic wrap), and place a sheet of white paper in or under this surface.

b. Students can use droppers to place several small drops of each solution to be tested on this reaction surface, making sure these drops are far enough apart to avoid joining together.

c. Students might want to classify each mixture they plan to test by type (cleaners, foods, drinks, etc). They might also want to make predictions based on what they know about these materials. You might expect 5th graders to be able to group compounds into categories; 8th graders might be familiar with the idea that materials dissociate into ions.

d. Use two samples of each solution being tested. Test one with the pH test paper and one with the universal indicator solution. (Optional; investigation can be done with only one of these indicators)

e. Use the color chart that comes with the indicator to determine the pH of each solution.

f. Have the students record the data and observations from this experiment in a data table.

g. Students can determine approximate pH values from the color chart provided with the commercial pH indicator and use this information to determine which solutions are acidic       (pH < 7), which are basic (pH > 7), and which are neutral (pH = 7).

h. Students can draw conclusions about the acidity of certain types of solutions (cleaners, foods, drinks) and their uses.

 

7. After the students have finished their investigation, they may present their findings to their classmates and compare their results with those of their classmates.

 

Additional Resources:

         http://www.flinnsci.com/Sections/Safety/safety.asp

         http://chemistry.about.com/od/acidsbases/Acids_Bases_and_pH.htm

         http://www.erowid.org/archive/rhodium/chemistry/equipment/ph-indicator.html

         http://www.ehow.com/list_5931608_flowers-natural-indicators.html

         http://www.osu.edu/experiments/chemistry/pH.html

 

 

 

 

 

 

Student Worksheet for pH and Acidity Investigation

 

Experiment Title: __________________Date: __________Name: _________________

 

Student Hypothesis or Question:

 

_____________________________________________________________________

 

Materials:

_____________________________________________________________________

 

_____________________________________________________________________

 

Safety Precautions:

 

_____________________________________________________________________

 

_____________________________________________________________________

 

_____________________________________________________________________

 

Procedure:

 

Wear safety goggles for all lab work.

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________


Data and Observations:

 

Substance tested

Color with universal indicator/pH paper

Estimated pH

Acid or base?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Observations:

 __________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

Analysis of Data:

 __________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

Conclusion:

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

 

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