Life Cycle - Natural Environment (6A)
Lab 

   
OBJECTIVES:
  • Exploring the chemical make-up of soil.
  • Comparing the pH of different soils.
VOCABULARY:
  • acid
  • base
  • ion
  • pH
MATERIALS:

 Students experiment with pH and soils.

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BACKGROUND:

Define pH on the board by telling students that the more reactive hydrogen in a liquid the more acidic it is; the more hydroxide ions (hydrogen + oxygen) the more basic it is.  Water is neutral because it has one H+ ion and one OH- ion, so they balance out and are neutral.  It is not important that the students really understand what pH is, but that they understand what the numbers refer to.  Advertisers refer to pH all the time, especially in soap and shampoo products.

Measuring whether a substance is an acid or a base is not difficult using litmus paper.  However, this is not quantitative, chemists use what is called a pH scale from 0-14 to measure the intensity of being an acid or base.  Water, which is neutral is a 7, 0 is a strong acid and 14 a strong base.  Lye is 13, bleach is 12, ammonia is 11, milk of magnesia is 10, borax is 9, baking soda is 8, blood and milk are 7, orange juice is 4, vinegar is 3, lemon juice is 2, and battery acid is 1.  These numbers refer to the concentration of hydrogen ions in a solution.  In this lab, students will use the pH indicator or litmus paper on solutions of soil to try to figure if the acidity or basicity of a soil will affect the plant growth on that soil. 

The pH of mineral soils varies from values of 3 or less in very acidic soil near the coastlines to more than 10 in alkali soils of some arid and semiarid areas.  In order for a soil to be productive in humid areas it must have a pH between 5-7 and arid regions must have a soil pH between 7-9.  An indicator is a substance that can determine the presence of an acid or base.  Indicators change color when they come in contact with an acid or base.  Litmus is red (pink) in acid and blue in base.  Citrus fruits and vinegar are examples of acids and bleach and ammonia are examples of bases.  In this lab, the students will determine the actual pH of a soil by making a solutions and comparing the color of  pH paper.  Litmus paper can also be used but the samples must be cleaner than what is needed for an indicator solution.

PROCEDURE:

  1. Prior to lab:  Collect soil near school.

    Students will look at two types of soil; soil from a granitic area and soil from a serpentinite area.  They will determine whether each of the solutions from the soil is either basic or acidic.  They will examine the vegetation on each of the soils in the next lab to determine if there are any differences between the vegetation that occurs in each of the two different soils.  Climatic conditions are the same, so they will have to determine why there is such a difference.  The key point to emphasize is that there is some kind of chemical difference which makes one soil much better for growth than another.  
      

  2. The following diagram may help you explain the range of pH in soils.

  

  1. This lab has students actually do a chemical determination.  Sometimes impurities can make the experiment produce different results.  If the beakers or spoons are not clean, the results may be different.
      

  2. Students should follow their lab sheet.  It is important that they just use a small amount and grind it with their mortar and pestle.  Illustrate to students how to use the pestle by grinding it with a circular motion in the mortar  Mix the ground soil with water as directed.  Make sure that students clean the instruments before they grind another soil.  Just a dry wipe is sufficient. 

    The granitic soil is more acid than the serpentinite, but not dramatic.  Granitic soil can be from 7 to 6, while serpentinite ranges from 7 to 8.  The pH of your local soil will vary.  The key objective is for students to realize that there are chemicals in soil which makes soils different.  

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