In this lesson we will answer the
- What are acids and bases?
- How do acids and bases participate in chemical reactions?
- How are acids and bases measured?
In addition to the online lecture, read chapter 6 in Basic Chemistry for Water and Wastewater Operators
Acids and Bases
Acid-base chemistry is based on one simple reaction - the ionization
H+ + OH-
As we saw in the last lesson, this equation means that water can break
apart into a hydrogen ion and a hydroxide ion. A hydrogen and a hydroxide
ion can also join together to form a water molecule.
In pure water, hydrogen and hydroxide ions are present in a 1:1 ratio
since the only source of these ions is the ionization of water. Acids
and bases are substances which change the balance of hydrogen and hydroxide
ions in water. In this lesson, we will first explore
the chemistry of acids and bases, then we will show how to calculate their
concentration mathematically. In the next lesson, we will find out
how to calculate the concentration of acids and bases in the lab.
acetic acid, which makes it taste sour.
Onions release a gas which turns into sulfuric acid when
it reaches your eyes, making them burn.
What do you think of when you think of acids? You might think
of sour-tasting acidic foods such as lemons. Or you might think of
strong acids, such as battery acid, which can burn your skin or corrode metal.
In this lesson, we will be concerned with the less visible properties of
acids - their chemical properties.
Chemically, acids are substances
which increase the concentration of hydrogen ions when they are placed in
water. In this section we will be concerned only with strong acids.
Weak acids act slightly differently and will be discussed in a later section.
You can recognize strong acids because they are ionic compounds which
contain hydrogen ions. For example, consider the strong acids listed
in the table below. Notice that each compound consists of a hydrogen
ion bonded to some sort of anion.
When a strong acid is placed in water, it will
ionize completely, breaking down into its constituent ions. For example,
hydrochloric acid reacts as shown below:
H+ + Cl-
The ionization of a molecule of hydrochloric acid introduces a hydrogen
ion and a chloride ion to the solution. The chloride ion has no effect
on the acidity of the water, but the hydrogen ion makes the solution more
The diagrams above show what happens when
hydrochloric acid is added to water. The top diagram merely contains
water. Notice that most of the water is present in its un-ionized state,
but that two water molecules have ionized into hydroxide and hydrogen ions.
Despite the presence of hydrogen ions, this solution is neutral (meaning that it is neither acidic
nor basic) because the number of hydroxide ions equals the number of hydrogen
The bottom diagram shows what happens to the water after the addition
of hydrochloric acid. Notice that all of the hydrochloric acid has
ionized, so now there are five hydrogen ions and only two hydroxide ions
in the solution. Since there are more hydrogen ions than hydroxide
ions present, the solution has become acidic.
You probably have less familiarity with bases than with acids, so you
may be surprised to learn how many bases you deal with in your everyday life.
Soap, baking soda, milk of magnesia, and ammonia all contain bases.
These substances exhibit some of the physical properties of bases, such as
feeling slippery, tasting bitter, and dissolving greases. Bases also
irritate the skin and eyes just as acids do.
Chemically, a base is a substance
which decreases the concentration of hydrogen ions when it is placed in water.
Strong bases decrease the hydrogen ion concentration by increasing the hydroxide
Let's consider what happens when we put sodium hydroxide (a strong base)
in water. Like strong acids, strong bases ionize completely, so the
sodium hydroxide breaks down into a sodium ion and a hydroxide ion:
Na+ + OH-
The hydroxide ion released by the sodium hydroxide then reacts with
a hydrogen ion from the water, forming a water molecule:
The reaction between the hydroxide ion and the hydrogen ion removes
the hydrogen ion from the solution, making the solution less acidic and more
basic. The two diagrams below show the net reaction of sodium hydroxide
with water schematically:
Notice that in the top diagram, three water molecules have ionized
to produce hydrogen and hydroxide ions. When three sodium hydroxide
molecules are added (in the bottom diagram) the hydroxide ions from the base's
dissociation combine with the three hydrogen ions in the solution, forming
water. So the net result of the addition of sodium hydroxide to the
water is that the concentration of hydrogen ions becomes lower and the acidity
of the solution is decreased.
In the table below, I have listed some of the strong bases which you
may run into in the lab:
Weak Acids and Bases
So far we have talked about strong acids which contain a hydrogen
ion and strong bases which contain a hydroxide ion. However, some acids
and bases work slightly differently. Technically, an acid is any substance
which donates a hydrogen ion to a solution and a base is any substance which
accepts a hydrogen ion.
The table below lists some weak acids and bases:
of these weak acids and bases ionize just like their strong counterparts,
donating hydrogen or hydroxide ions to the solution. For example, ammonia
will ionize as follows:
NH4+ + OH-
The difference between strong and weak bases (and between strong and
weak acids) is that weak bases do not ionize completely when placed in solution.
So, while every molecule of sodium hydroxide will break down into a sodium
ion and a hydroxide ion when placed in solution, only some of the ammonia
molecules will ionize under the same conditions. The illustration
below gives a schematic representation of what happens under these
The top rectangle is an example of a solution containing sodium
of the molecules have ionized into their constituent ions. The
bottom rectangle is an example of a solution containing ammonia. Most
of the ammonia stayed in its current state, with only a small
percentage breaking apart into ammonium and hydroxide ions.
Some weak bases not only do not ionize fully, they also do not release
a hydroxide ion. For example, ammonia in its gaseous form reacts as
O + NH3
Notice that the ammonia accepted a hydrogen ion from the water, decreasing
the acidity of the solution. So it acted as a base even though it didn't
produce hydroxide by ionization.