General ---------Biology ------------------Online------------------ Ch. 3 Water and the fitness of the environment

The pH scale is a measure of whether a solution is basic or acidic.

As the pH scale goes up, the basic nature of the solution increases.

As the pH scale goes down, the acid nature of the solution increases.

The chemistry involved is shown on page 47-48 of your textbook.  In summary:

Water contains some H2O molecules that have dissociated (fallen apart) into H+ ions and OH- ions. 

The dissociation is reversible (  ), i.e. some of the ions are associating into water, while some of the waters are dissociating into ions.  A solution will reach an equilibrium where the number of waters, hydrogen ions, and hydroxide ions stay the same, even though association and dissociation are proceeding.

In pure water the number of H ions  equals the number of OH ions, 
(H+ = OH-), and the solution is called "neutral."  Chemicals can be added to the water and shift the balance.  If the shift causes more H+, then we say the solution is acidic, if more OH-, then we say it is basic.

Click on solutions and determine which solution is acidic __, which is basic __, and which is neutral __.  Check your answers.

Our blood is "buffered" to help keep the pH in the range where our chemical reactions function best.  Organisms are sensitive to changes in pH; it does not take much of a change in acidity for "the chemistry of life" to not work at all, causing death.

The following pH scale (page 48 of text) shows the pH of some common solutions.  Notice that as the acidity (# of hydrogen ions) increases, the number on the pH scale gets smaller.  Warning: This seemingly inverted scale was made by chemists to trick biology students on exams.  Click here to see pH scale.

Summary:  Students should write summary statements below as bullet points. 
 
 
 
 

Test your knowledge:  Practice Quiz # 1, 7, 39.