Acidic,
Basic and
Neutral
Solutions
Chapter 9, Lesson
2
Acids
• An acid is a substance that releases a
positively charged hydrogen ion, H+, in
water.
– Acids taste sour and turn blue litmus paper
red.
– The hydrogen ion from an acid combines
with a water molecule in solution to form a
hydronium ion, which has the formula H3O+
– Because acid solutions contain ions, they
conduct electricity.
Hydronium ion
• When an acid dissolves, it releases a
hydrogen ion.
• The H+ ion combines with water to form
H3O+, the positively charged hydronium
ion.
Hydrogen ion
Water molecule
Hydronium ion
What is a hydronium ion?
Uses of Acids
• Acids are important in several body
processes, including breaking down food
in the stomach.
• Acids are also used in making many
products, such as fertilizers, detergent,
and cleaners.
Bases
• A base is a substance that produces
hydroxide ions when dissolved in water.
– The formula for the hydroxide ion is OH– Bases feel slippery on your skin, taste bitter,
and turn red litmus paper blue.
– Most bases are ionic compounds.
Hydroxide ion
• In water, bases separate into a positive ion
and the hydroxide ion (OH–).
• All the properties of bases are a result of
the presence of hydroxide ions that form
when the base is dissolved in water.
Uses of Bases
• Common bases include baking soda and
cleaning agents.
• Bases are also used to produce new
products, such as soap, paper, and
plaster.
Acids
and
Bases
What is pH?
• pH is a numerical scale used to indicate
how acidic or basic a solution is.
• The pH scale runs from 0 to 14.
– Acids are below 7; strong acids are
near 0.
– Bases are above 7; strong bases
near 14.
– Neutral solutions have a pH of 7.
pH and Hydronium ion
Concentration
• The higher the concentration of hydronium
ions, the more acidic a solution is.
• All acid and base solutions contain both
hydronium ions and hydroxide ions.
• In a neutral solution (pH=7) the hydronium and
hydroxide ion concentrations are equal.
• A change in one pH unit represents a tenfold
change in the acidity or alkalinity (how basic)
of a solution.
Neutralization
• When an acid and a base are mixed, a
neutralization reaction occurs.
• A neutralization reaction produces water
and a salt.
• Stomach acid reacts with magnesium
hydroxide in the following equation:
2HCl + Mg(OH)2 → MgCl2 + 2H2O
• Neither of the products is acidic or basic.
How is pH measured?
• An indicator is a compound that changes
form one color to another within a
particular pH range.
– Indicators can be used to determine
approximate pH, simply whether something
is acidic or basic.
How is pH measured?
pH strips
• Litmus is one of the simplest indicator
test papers but also the least accurate.
• pH testing strips are a quick way of
determining the approximate pH of a
solution.
pH Meters
• pH meters are electronic instruments with
an electrode that is sensitive to
hydronium ions present in a solution.
– Accurately measures pH
– Requires an electric source to operate
pH Meters (cont’d)
Lesson Review 9-2
1. An acid with a pH of 3 is contains how many
times as many hydronium ions as an acid with
a pH of 6?
A 1000
B3
C 30
D 100
Lesson Review 9-2
2. When an acid and base are mixed, the
products are ____ and ____.
A water; a base
B water; ammonia
C water; salt
D ammonia; salt
Lesson Review 9-2
3. Red litmus paper is dipped in a solution. The
paper turns blue. The solution must have been
____.
A acidic
B homogeneous
C basic
D polar
Lesson Review 9-2
4. Blue litmus paper turns ____ in acid solutions.
A red
B orange
C green
D blue
Lesson Review 9-2
5. Which of the following is a
hydroxide ion?
A H3O+
B H+
C OH–
D NH4+
Lesson Review 9-2
6. Which is a property of a base?
A sour taste
B produces OH– in water
C produces H+ in water
D reacts with metal to produce hydrogen gas
Lesson Review 9-2
7. What ions are present in the greatest amount
in a solution with a pH of 2.5?
A hydroxide
B hydrogen
C hydronium
D oxygen
Lesson Review 9-2
8. What is the symbol for the hydrogen ion?
A H3O+
B H2O
C H+
D OH–