ACIDS, BASES, and
SALTS
PS-3.8
Acid pH Base Neutralization
reactions Neutral solution
By definition, an acid is a substance
that dissociates ( dissociate means it
breaks apart) to produce Hydrogen
ions in an aqueous ( aqueous means
water) solution.
Look at the diagram shown below. It shows a
formula unit of Hydrogen Chloride dissociating in
water to form an aqueous solution of Hydrochloric
acid.
An Acidic
Solution of HCl(aq)
The solution in the beaker shown above contains free
hydrogen ions, H+. The free Hydrogen ions attach loosely to
the water molecule forming H3O+. H3O+ is referred to as
the Hydronium ion. This solution is an acid because it
contains positively charged Hydrogen ions.
There are many acids. They all have one thing in common, they
donate H+ ions to an aqueous solution.
The pH ( Positive Hydrogen Ion Concentration)
of an acid solution will fall somewhere below 7 on
the pH scale. pH is a way to measure the
Hydrogen ion concentration of the solution.
Solutions that are acidic will have a pH below 7
on the pH scale. Strong acid solutions will have a
pH closer to 0. Weaker acid solutions will have
a pH closer to 7.
A Base is a substance that dissociates to give a
solution Hydroxide ions, OH-. One common Basic
substance is Sodium Hydroxide, NaOH. Sodium
Hydroxide is commonly used as drain cleaner. The
diagram below shows Sodium Hydroxide
dissociating in water to give the solution
Hydroxide ions, OH-.
The solution in this beaker is basic because it contains free
Hydroxide (HO-)ions. The pH of this solution will fall
somewhere above 7 on the pH scale. Solutions that are
basic will have a pH above 7 on the pH scale. Strong base
solutions will have a pH closer to 14. Weaker base solutions
will have a pH closer to 7.
There are many
bases. They all
have one thing in
common, they
contribute OHions to an
aqueous solution.
Shown below is the pH scale. Note that acids are
below 7 and that the bases are above 7 on this
scale.
On the pH scale, a pH of 7 is neutral. A neutral
solution contains equal numbers of H+ and OH- ions.
If H+ and OH- ions are added to the same
solution, they will combine chemically to produce
water. If there are exactly the same number of
H+ and OH- ions in the solution, there will be no
leftover Hydrogen ions or Hydroxide ions. When
there are no excess Hydrogen ions or Hydroxide
ions, the solution is neutral.
•Of course, acids do not just contain Hydrogen and bases do not
just contain Hydroxide ions. There is always something else that
comes along with the ion.
•In a formula unit of HCl there is a Chlorine atom that comes
along with every Hydrogen ion.
•In a formula unit of NaOH, there is a Sodium atom that comes
along with every Hydroxide ion.
•In solution, the Hydrogen ions combine with the Hydroxide ions
to form water, but at the same time the Sodium atom combines
with the oppositely charged Chlorine atom making a formula unit
if Sodium Chloride, NaCl. In general terms:
an acid + a base
a salt + water.
an acid + a base  a salt + water.
•This generalized chemical equation is called a
neutralization reaction. Any acid plus any base
produces a salt plus water.
•The term, "salt" simply means an ionically
bonded, chemical combination, of a metal and a
nonmetal.
•There are lots of salts, this one in this example
just happens to be common table salt, NaCl.
Let's take a close look at what happens in the beaker of water
when an acid is added to a base.
The solution produced by adding equal quantities of acid and
base is neutral. It consists of a salt plus water.
Acids can be distinguished from bases by their physical and
chemical properties.
•Acid solutions are electrolytes. Electrolytes are solutions that
will conduct electricity
•Acid solutions have a tart or sour taste. (This is why Citric acid
and Phosphoric acid are added to candy and soda. It gives them a
little sour kick. Citrus fruits, such as lemons, limes, and Oranges,
contain Citric acid.)
•Acids react with active metals such as zinc and magnesium to
form a salt and Hydrogen gas. We used an acid-metal reaction
to fill the balloon with Hydrogen gas in the Hydrogen Demo.
•For the purposes of Physical Science, the formula of an acid
can be recognized because (except for water) the first
element in the formula hydrogen.
Bases can be distinguished from acids by their
physical properties.
•Basic solutions have a slippery feel. The base Sodium
Hydroxide, NaOH is used in soap making...soap is slippery!
•Basic solutions conduct electricity (are electrolytes)
•Basic solutions have a pH greater than 7.
•For the purposes of Physical Science, the formula of a base
can be recognized because the formula ends in OH. Sodium
Hydroxide, NaOH is a base...note the OH at the end of the
molecule. Calcium Hydroxide, Ca(OH)2 is a base, it also has
hydroxide, OH, at the end of the molecule.
INDICATORS
•Indicators are used to determine the pH of a solution. The
indicator litmus turns red in the presence of an acid. Other
indicators will turn a specific color for each pH value.
Indicators can change color to show if a solution is an acid or
a base, because they change color in response to the
concentration of H+ ions and OH ions in the solution.
•Blue litmus turns red in an acid solution, red litmus paper
turns blue in the presence of a basic solution; other indicators
will turn a specific color for each pH value.