Acids, Bases, and Neutrals
Name
______________________________
Date __________
For thousands of
years people have known that vinegar, lemon juice, and many other foods taste
sour. However, it was not until a few hundred years ago that it was discovered
why these things taste sour – because they are all acids.
In the
seventeenth century, the Irish writer and amateur chemist Robert Boyle first
labeled substances as either acids or bases (he called bases alkalies),
according to the following characteristics:
Acids taste sour, are corrosive to metals,
change litmus (a dye extracted from lichens) red, and become less acidic when
mixed with bases.
Bases feel slippery, change litmus blue,
and become less basic when mixed with acids.
While Boyle and others
tried to explain why acids and bases behave the way they do, the first
reasonable definition of acids and bases would not be proposed until 200 years
later.
In the late
1800s, the Swedish scientist Svante Arrhenius proposed that water can dissolve many compounds by
separating them into their individual ions. Arrhenius suggested that acids are
compounds that contain hydrogen and can dissolve in water to release hydrogen
ions into solution. For example, hydrochloric acid (HCl) dissolves in water
as follows:
HCl + H2O →
H+ + Cl-
Bases are substances that dissolve in
water to release hydroxide ions (OH-) into solution. For example, a typical base according to
the Arrhenius definition is sodium hydroxide (NaOH):
NaOH + H2O →
Na+ + OH-
Acids act like they do because all acids
release H+ into solution (and all bases release OH-).
This
idea, that a base can make an acid weaker, and vice versa, is called
neutralization.
Neutralization
Acids release H+ into solution and bases
release OH-. If we were to mix an acid and base together, the H+ ion would
combine with the OH- ion to make the molecule H2O, or plain water. Water is NEUTRAL.
H+ + OH- → H2O
The neutralization reaction of an acid
with a base will always produce water and a salt, as shown below:
Acid Base Water Salt
HCl + NaOH → H2O + NaCl
HBr + KOH → H2O + KBr
We
measure the strength of acids and bases using the pH scale.
pH = -log [H+]
This means that
the LOWER the pH number, the STRONGER the acid. The higher the pH, the stronger
the base (more OH-, less H+).
The pH scale measures how acidic or basic
a substance is. The pH
scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A
pH greater than 7 is basic. Pure water is neutral.
Acids with a low
pH of around 1 are very reactive and can be dangerous. The same is true for
bases of a pH near 13.
There are many
strong acids and bases in nature. Many plants have acids and bases in their
leaves, seeds, or even their sap. Citrus fruits like lemons and oranges have
citric acid in their juice. This is what makes lemons taste so sour.
Our bodies use
acids and bases too. Our stomachs use hydrochloric acid to help digest foods.
This strong acid also kills bacteria and helps to keep us from getting sick.
Our muscles produce lactic acid when we exercise. Also, our pancreas uses a
base called an alkali to help with digestion. These are just a few examples of
how the chemistry of bases and acids help our bodies function.
ACID/BASE Indicators
If you want to know if a substance is an acid,
base or neutral, you need to use an acid/base indicator. An indicator is a substance that
changes color in the presence of H+ or OH-.
Some common indicators:
LITMUS
Red litmus paper turns blue
in the presence of a base. (pH = greater than 7)
Blue litmus paper turns red
In the presence of an acid. (pH = less than 7)
If neither color changes, it’s a neutral. (pH =
7)
Phenol
Red
Phenol red liquid
is red in a neutral, yellow in an acid, and magenta in a base.
Universal pH indicator
This liquid is green in a neutral, red in an acid
and blue in a base.
Purple
Cabbage Juice
Cabbage juice is purple in a neutral, reddish
pink in an acid, and blue to greenish yellow in a base.
