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Biology:
“The Chemistry of Life”
The pH Scale
pH SCALE
Every living organism must maintain a certain
pH balance in order to survive.
pH scale indicates how acidic or basic a
substance can be.
pH scale ranges from 0 to 14.
Below 7 is acidic
7 is Neutral
Above 7 is basic
pH SCALE
Acids Compounds that form hydrogen ions (H+)
when dissolved in water.
Acids have a pH below 7
Acids turn blue litmus paper RED
The lower the pH number the more H+ and the
stronger the acid.
Examples: stomach acid, citric acid,
battery acid
Bases Compounds that form hydroxide ions (OH-)
when dissolved in water.
Bases have a pH above 7
Bases turn red litmus paper BLUE
The higher the pH number the more OH- and
the stronger the base.
Examples: soapy water, ammonia, bleach
pH – Human Body
The human body must maintain an internal pH
Most organisms must keep a pH close to 7.0
To prevent illness or death, organisms have a
built-in safety net to resist pH change called
buffers.