Science Junction

Water What-ifs
Water Quality and pH

pH Reference Desk
 * Teacher Notes, Lesson Extensions and Alternative Assessment Ideas
 * pH Color Charts
 * Making Your Own pH Indicator
 * Warning Signs of Pollution
 * Effects of pH Range on Aquatic Species
 * Instructions for Lamotte water Test Kits
 * CBL Instructions for pH and Temperature Probes

The determination of the pH of a substance is the measurement of the H+ ions found in that particular substance. pH is determined and recorded as a number between 0 and 14. Pure deionized water has a pH of 7 which is neutral. This means that the level of H+ and OH- ions in pure water are equal.

If the level of H+ ions increases, the substance is considered an acid and the pH number is below 7. If the level of OH- ions increases, the substance is considered to be alkaline or base and the pH number is above 7.

An acid has a range of 0 to any numerical value below 7. For example, 6.9 would be a weak acid. A base has a range of any numerical value above 7 to 14 with 7 being a neutral value. A one-unit change in the pH, from 4 to 3, is a ten-fold change in how acidic the substance has become. This means that a pH of 3 is ten times more acidic than a pH of 4.

In the United States the pH of most natural water systems range from 6.5-8.5, but wide variations can occur due to increases in the atmosphere of nitrogen oxides and sulfur dioxides from automobile and coal-fired power plant emissions. These oxides are converted to nitric and sulfuric acids in the atmosphere and fall to earth as acid rain or snow. This acidic precipitation can adversely affect the pH of aquatic ecosystems.

Most organisms are adapted to a specific pH level. When pH increases or decreases, the diversity of an ecosystem can be changed appreciably.

pH Lessons 1, 2, and 3 are intended for middle school and high school science students. These lessons focus on pH level and its determination, the effect of pH level on aquatic plants, and how an acid or alkaline environment effects brine shrimp eggs. These lessons highlight several aspects of the following competencies:

Lesson1 Introductory Lesson-How is pH Determined?
Lesson 2 Does a change in pH affect the growth and survival rate of aquatic plants?
Lesson 3 Determining the Effects of a pH Change on the Hatching and Development of Brine Shrimp Eggs

Download PDF version of all three pH lessons.


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Last Modified: 8/13/01

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