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