Section 2.5—Characteristics
of Acids and Bases
Objectives:
•investigate the characteristics of acids & bases
•distinguish between strong & weak acids & bases
•distinguish between concentration & strength
•explain the pH scale and use of indicators
Characteristics of Acids & Bases
Acids
Bases
•Produce H3O+1 (hydronium
ion) in water
•Produce OH-1 (hydroxide
ion) in water
•Tastes sour
•May sting or burn on contact
•Tastes Bitter
•React with active metals to
form hydrogen gas
•Feels slippery
•Turn blue litmus paper pink
•Turns pink litmus paper blue
Acid Strength
Review of how acids produce ions
water
acid
Hydrogen cation with some anion
Review of how acids produce ions
+1
-
Review of how acids produce ions
+1
Hydronium ion
Anion
Strong versus Weak Acids
-
-
How many hydronium ion – anion
pairs can you find?
3
+
-
How many intact (whole) acid
molecules can you find?
1
Strong acid
Most of the acid molecules have
donated their H+1 to water
Strong versus Weak Acids
How many hydronium ion – anion
pairs can you find?
+
1
How many intact acid molecules can
you find?
3
-
Weak acid
Only a few of the acid
molecules have donated
their H+1 to water
Strong versus Weak Acids
A strong acid is an acid that dissociates
(comes apart) and ionizes (forms
ions)almost completely in water.
A weak acid is an acid that dissociates
and ionizes only slightly in water.
Concentrated versus Dilute Acids
A Note About Solutions
 We saw in previous slides how an acid
compound combines with water to form a
hydronium ion and an anion. When this
occurs, the acid is really an acid solution.
 Solutions are homogeneous mixtures. There
is a even mixing of the 2 parts throughout the
mixture.
 The 2 parts are called the solute – the
substance that is dissolved – and the solvent
– the substance that does the dissolving.
Concentrated versus Dilute Solutions
solute
Dilute
Not as many solute (what’s
being dissolved) particles
solvent
Concentrated
More solute (what’s being
dissolved) particles
Concentrated versus Dilute Acids
A concentrated acid will have a lot of the
acid compound mixed into the water.
A dilute acid will not have a lot of the acid
compound mixed into the water.
Concentration vs. Strength
Combinations of Concentration & Strength
Concentrated
Dilute
Strong
A lot of acid
added & most
dissociates
Not much acid
added, but most of
what’s there
dissociates
Weak
A lot of acid
added, but most
stays together
(does not
dissociate)
Not much acid
added and most of
what is there stays
together
Other notes
All of these same ideas about strength and
concentration apply to bases as well
Many people think all acids are dangerous
and all bases are more safe but . . .
Vinegar is an acid we eat…some acids are safe!
Sodium hydroxide is a very caustic base…not all
bases are less harmful than acids!
The stronger an acid or base is (and the more
concentrated it is), the more dangerous it is for
you!!
pH
pH Scale
Is a scale to measure acidity (concentration
of hydronium ions)
1
7
Highly acidic
neutral
14
Very basic
(not acidic)
Chapter 6 will give more detail about how pH is calculated!
pH
Neutral Substances
These are substances that are not considered to
be an acid or a base. They have a pH around 7.
Acidic Substances
These substances have a pH LESS THAN 7. In
fact, the lower the pH, the greater the
concentration of hydronium ions. The lower
the pH, the more ACIDIC the substance is
considered.
pH
Basic Substances
These substances have a pH GREATER THAN
7. In fact, the higher the pH, the lower the
concentration of hydronium ions. The higher
the pH, the more BASIC the substance is
considered.
Ways to measure pH
 Indicators change color based on pH
Liquid indicators – various indicators change colors at
different pH’s
Universal indicator – a combination of liquid indicators to
produce a “rainbow” changing colors at several pH’s
 Paper Indicators
Paper with a liquid indicator on it (Litmus paper or pH
paper)
 pH meters or pH probes
Electronically determine pH and give a read-out
Common pH indicators – pg. 56
This picture © 1998 David Dice
pH of common substances
SUBSTANCE
PH
Intestinal contents
6.5
0.1M HCl
1
Milk
6.5
Stomach contents
2
Urine
6.6
Vinegar
2.9
Bile
6.9
Soda pop
3
Saliva
7
Grapes
4
Blood
7.4
Beer
4.5
Eggs
7.8
Pumpkin pulp
5
0.1M NH3 (aq)
11.1
Bread
5.5
0.1M NaOH
13
•Which substance is most acidic? least?
•Which substance is most basic? least?
•Is bread acidic or basic? eggs?
•Is any substance is neutral?