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pH Acid Base Indicators
Rubric
Contributed by: Council of Chief State School Officers (CCSSO)

Item Description:
This event assesses the student's understanding of acids and bases and the role of acid-base indicators as a function of pH.

Question 1 asks the students to explain how they determined the pH values of the four unknown solutions and why pH is an important piece of information about an unknown. A good response will show the student understands that the color of the indicator in the known solutions is directly related to the pH as measured with pH paper. The pH of the unknowns can be determined by comparing the color of the solutions plus indicator of the knowns to the unknowns.


The Item:
Question #1: Explain how you determine the pH values of four unknown solutions. Why would you want to determine the pH of a solution?

Question #2: Suppose that you were given a different acid-base indicator - one made from cranberries, blueberries, or red cabbage. How would you test the new juice to see if it would be a good indicator? Include in your discussion what properties you believe would make a solution a good acid-base indicator.

Question #3: Acid rain is a problem in the U.S. and around the world. Acid rain is also responsible for the death of some plants and animals. How could you test the rain in your area to see if it is acidic? Explain the procedure. How could you determine if the rain in your area is more or less acidic than the rain in other parts of your state?


SUMMARY TABLE

Criterion

  1. Understands concept - An indicator can be used to approximate pH based on color change.

  2. Explains how pH of unknowns was determined;

    OR

    Explains why you would want to determine pH.

  3. Test described to see if a juice is a good indicator;

    OR

    Properties of a good acid-base indicator are described.

  4. Method to test rain;

    OR

    Method to compare rain from different areas.


pH Acid Base Indicators Rubric

Criterion 1: The student understands that the color of a solution plus an indicator can be used to determine if a solution is an acid or a base or the approximate pH. The pH of a solution determines the color of a solution plus indicator.

This concept may be answered in Question 1:
  • The pH of the unknowns was determined by comparing the color of the unknown solutions plus indicator to the known solutions plus indicator.
  • The unknown pH values were found by comparing the different colors from the first part of the experiment after indicator was added with indicator in the unknowns to get an average pH.
This concept may be answered in Question 2:
  • A substance whose color depends on the pH of the material with which it is mixed can be a good indicator.
  • An indicator changes color if it is in an acidic or basic solution.
This concept may be answered in Question 3:
  • Add indicator to the rain and compare the color to a known acid or base with indicator added to determine if the rain is acidic.
  • Put indicator in rainwater from around the state and compare the colors to see which is acidic and which is neutral or basic.


Criterion 2: The student explains that the pH of the four unknown solutions was determined by comparing the color of the indicator in an unknown solution to the color of the indicator in a solution in which the pH has been determined. (Question 1)

For example:
  1. I matched the colors to the solutions that we used pH paper in. (This statement does not show an understanding of the concept described in Criterion 1, see NOTE below.)
  2. When indictor was added, we compared the colors to the solutions that we measured the pH and found the average pH which told us if they were acids or bases. (This statement satisfies Criterion 1.)
NOTE: [Statement 'a' does not explain what was done after the colors were matched as statement 'b' does (told us if they were acids or bases) to meet Criterion 1.]

OR


The student explains why you would want to determine the pH of a solution. (Question 1)

For example:

  1. To see if it is an acid or a base.
  2. To see if it is dangerous.
Neither a or b satisfy Criterion 1.



Criterion 3: A test is described to determine if an acid-base indicator made from cranberries, blueberries or red cabbage would be a good indicator. (Question 2)
For example:
  1. Add the juice to different liquids to see if the color is different. (This does not meet Criterion 1, see NOTE below.)
  2. Add the juice to two liquids that are the same color and that are known acidic and basic liquids, e.g. vinegar and ammonia, and see if two different colors are made by adding the juice. (This does not satisfy Criterion 1.)
NOTE: [Statement 'b' shows the student understands that the solutions tested must have high or low pH values because the color of the indicator (if it is good) will change with pH.]

OR

Properties of a good acid-base indicator are described. (Question 2)
For example:
  1. A small amount of indicator in a solution gives a very bright color. (This does not meet Criterion 1.)
  2. When an indicator is added to a solution with a low pH it should be a different color than when it is added to a solution with a high pH. (This meets Criterion 1.)


Criterion 4: A method is described to test the rain to see if it is acidic. (Question 3)
For example:
  1. Collect rainwater and dip a piece of pH paper in it and then match the color to the color chart.
  2. Put an acid-base indicator in the rainwater. (These examples do not meet Criterion 1, see NOTE below.)
  3. Add indicator to rainwater and compare the color to known acids and bases with the same indicator. (This meets Criterion 1.)
NOTE: [Criterion 1 is met in statement 'c' because it shows the student understands that an indicator can be used to differentiate between an acid and a base because of the color.]

OR

A method is described to determine if rain in an area is more or less acidic to rain in other areas. (Question 3)
For example:
  1. Collect rain from different areas and dip pH paper in each one.
  2. Add acid-base indicator to rain from different areas and compare the colors.
    (These examples do not meet Criterion 1.)
  3. Look at the same type of plant that grows in different areas and compare the colors of the flowers. Some plants are good indicators of the pH and the flowers will be different colors if the rain is acidic or basic. (This meets Criterion 1.)

 

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