6.2 PH SCALE AND INDICATORS
SNC2D P
Water Molecules
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Water is an important part of all life. The earth’s
surface is about 70% water
Humans are made up of about 66% water and the
amount of water needed varies depending on an
organism.
Nevertheless, water is one of the basic requirements
for life on earth.
Water Molecules
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A water molecule is made up of
three atoms: two hydrogen
atoms and one oxygen atom
(H2O)
The oxygen atom tends to be
highly attracted to the
electrons of the hydrogen atom.
This is due to the large
number of protons in its
nucleus.
As a result, oxygen pulls the
hydrogen’s electrons closer
causing it to have a stronger
negative charge.
These partial charges enable
water molecules to bind to
one another.
Water Molecules
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Water molecules are also able to dissociate from one another.
The dissociation of water molecules contributes to the formation of
acids and bases.
When water dissociates hydrogen ions and hydroxide ions are
released.
Remember!! The presence of hydrogen ions (H+) indicates an acid, the presence of
hydroxide ions (OH -) indicates the presence of a base.
Water Molecules
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Thus in pure water, there is an equal
concentration of hydrogen (H+) and hydroxide
ions (OH-).
The amount of H+ ions to OHions determines whether the
substance is an acid, base or
neutral.
A pH scale helps measure the
amount of H+ ions that are
present.
The Potential Hydrogen (pH)
The pH scale is a numerical scale, ranging from 0 to 14,
that is used to classify solutions as acidic, basic, or
neutral.
A pH scale can help to
monitor of a variety of
solutions (pool water, foods
and beverages, and solutions
from industrial processes) to
ensure that they are safe.
(Page 230)
Acidic: pH < 7
Basic: pH > 7
Neutral: pH = 7
The pH scale ranges from 0 to 14, where 0 is the
pH of very strong acids and 14 is the pH of very
strong bases.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Acidic Solutions: pH < 7
All acidic solutions have values
that are less than 7 on the pH scale.
Acidic solutions have a greater
amount of hydrogen ions than
hydroxide ions.
The lower the pH number the more
acidic the solution, thus the
greater the number of hydrogen ions.
Basic Solutions: pH > 7
All basic solutions have values that
are above 7 on the pH scale.
Basic solutions tend to have many
more hydroxide ions than hydrogen
ions.
The greater the pH number the
greater the number of hydroxide ions
and the more basic the solution.
Neutral Solutions: pH > 7
Neutral solutions have a pH value
of 7.
Neutral solutions have an equal
number of hydrogen and hydroxide
ions.
Pure water is considered to be
neutral.
A pH value relates to
the concentration of
hydrogen ions in a
solution. Values
increase or decrease
exponentially (by a
power of 10) as you
move up or down the
scale.
Learning Check!
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Questions 1-6 clicker technology
Determining the pH of a Solution
There are a variety of ways to determine the pH of a
solution.
1) pH Meter
2) Litmus Paper
3) Universal Indicator
4) pH paper
Example 1: pH Meter
pH Meters have a sensor or probe that
electronically produces a precise (real time)
reading of the pH of a solution that is displayed
digitally on the meter.
Example 2: pH Indicators
pH Indicators are substances that change colour to
show the concentration of hydrogen ions (H+) and
hydroxide ions (OH-) in a solution.
pH indicator: Litmus Paper
Red and Blue Litmus
Paper – A chemically
treated indicator paper.
Blue litmus turns red in
acids.
Red litmus turns blue in
bases.
This simple indicator can
determine whether a
solution is acidic, basic, or
neutral.
pH indicator: Universal Indicator
Universal Indicator and pH Paper are composed of a
mixture of indicators that change to different colours
under different pH conditions. These indicators cover
the entire pH range from 0-14.
As the pH increases the colour
changes to help quickly
identify the pH.
pH paper has a colour scale
which can be compared to
the colour change to
determine the pH
Other pH Indicators
These indicators change
colour wihtin a small
range of pH values.
This is useful if more
precise measurements are
required.
pH Indicators in Nature
A variety of plants contain juices that can act as natural
acid-base indicators.
Learning Checkpoint!
Questions 7-9 on clicker technology