Acids and Bases
Chapter 25
Acids
• Contain at least one hydrogen atom that can be
removed when the acid is dissolved in water
• Example: HCl (hydrochloric acid)
• You can recognize an acid by the chemical
formula (starts with a H ion)
• Some acids can burn and are dangerous to
handle
• Most acids in foods are safe to eat
– Pickles, lemons
Properties of Acids
• An acid is a substance that produces
hydrogen ions in a water solution
• Ability to produce H+ ion is what gives
acids their characteristic properties
• When acid dissolves in water, H+ ions
interact with water molecules to form
H3O+ ions, which are called hydronium
ions
Properties of Acids
• Several common properties:
– All acids taste sour
• Never use taste to determine the presence of
an acid
– Acids are corrosive
• “eat away” materials they come into contact
with
• Can use an indicator to determine presence of an
acid
• An indicator is an organic compound that changes
color in an acid and base (Litmus paper turns red)
Common Acids
• Many foods contain acids
• Examples: citric acid in fruits, lactic acid
in yogurt, and acetic acid in anything
pickled
• Your stomach uses hydrochloric acid
(HCl) to help digest food
• Sulfuric, phosphoric, nitric and
hydrochloric acid are all vital to industry
Bases
• Bases feel slippery, like soapy water
(soap is a base)
• Important in many cleaning materials
• Bases can be defined two ways:
–Any substance that forms hydroxide
ions (OH-) in a water solution
–Any substance that accepts H+ from
acids
Properties of Bases
•
•
•
•
•
Opposites of acids
Many are crystalline solids
Feel slippery
Bitter taste
Strong bases are corrosive, and can be just as
dangerous as acids
• React with indicators to produce a change in
color (Litmus turns blue)
Common Bases
• Cleaning products in the home
• Shampoos, hand soap, laundry soap,
oven cleaner, drain cleaner, etc.
Solutions of Acids and Bases
• When acids are added to water, the H+ ion
leaves the acid and joins the water to form a
hydronium ion (H3O+)
• Example:
– HCl + H2O → H3O+ + Cl-
• When bases are added to water, the OH- group
completely dissociates (leaves) from the ion and
the two become individual ions in solution
• Example:
– NaOH (aq)→ Na+ + OH-
Strength of Acids
and Bases
Chapter 22
Strong and Weak Acids and
Bases
• Acids and bases can be classified as
strong or weak
• Strong acids ionize (separate)
completely in water
• Weak acids only partially ionize
• A strong base is going to completely
separate in solution
• A weak base does not completely
separate in solution
pH of a Solution
• pH of a solution is a measure of the
concentration of H+ ions in the solution
• *Be careful not to confuse the terms
“weak base/acid” and “strong base/acid”
for pH
• The pH measures how acidic or basic
the solution is
• To indicate pH a scale ranging from 0-14
has been created
pH of a Solution
pH of a Solution
• Solution with a pH of lower than 7 are
acidic
• The lower the value is the more acidic
the solution is
• Solution with a pH greater than 7 are
basic
• A solution with a pH of exactly 7 is
neutral (ex: pure water)
pH of a Solution
• There are many ways to determine pH
of a solution
• A universal indicator undergoes a color
change in the presence of an acid or
base
• The color of the pH paper is matched
with colors in a chart to find the pH
• A pH meter can be used; operated by
immersing a probe into the solution;
convenient for testing in the field
pH of a Solution
pH of a Solution
• The pH of blood must remain between 7.07.8
• Enzymes (act as catalysts) cannot work
outside of this pH range
• Blood contains compounds called buffers,
which allow you to eat foods outside of
this pH range, but do not affect your
body’s overall pH
• Buffers are solutions containing ions that
react with additional acids or bases to
minimize the effect on pH
pH of a Solution
Salts
Chapter 22
Neutralization
• Ads for antacid tablets (Tums, Pepcid AC) describe
how effectively these products neutralize
stomach acid
• What does this mean?
• Normally your stomach is acidic (HCl), too much
acid can cause discomfort
• Antacids contain bases that lower acid
concentrations
• Bases and acids “equal” each other out
Neutralization
• Neutralization is a chemical reaction between an
acid and a base that takes place in a water
solution
• A salt is formed in a neutralization reaction
• A salt is a compound formed when the negative
ions from an acid combine with the positive ions
from a base
• Example: HCl + NaOH → H2O + NaCl
• Acid-base reaction: acid + base = water + salt
Salts
• Salts are essential for many animals
• Humans need salt, because they lose salt
during perspiration (sweating)
• Many salts will form when acids react with
metals in a single-displacement reaction
• The metal displaces the hydrogen from the
acid, and a salt is formed along with a
hydrogen molecule
Titrations
• Sometimes we will need to know the
concentration of an acidic or basic solution
• To determine the concentration, we can use a
process called titration
• Titration is a process in which a solution of
known concentration is used to determine the
concentration of another solution
(concentration unknown)
Titration
• The solution in which the concentration is
known is called the standard solution
• The standard solution is added slowly to the
solution of unknown concentration to which
an acid/base indicator has been added
• If the unknown solution is a base, an acidic
standard solution is used
• If the unknown solution is an acid, a basic
standard solution is used
Titration
• You begin by adding a few drops of indicator to
the solution of unknown concentration
• Then, slowly add the standard solution one drop
at a time
• Once you observe a color change in the unknown
solution, you have reached the endpoint
• You can determine the concentration of the
unknown solution by looking at how much of the
standard solution it took before you reached the
endpoint
Titration
• Many natural substances are acid-base
indicators
• Red cabbage varies from a deep red color at
pH 1 to lavender at pH 7 to yellow-green at pH
10
• Grape juice
• Roses
• Radishes