Acids and Bases
Atoms can gain or lose electrons in order to form ions in a process called ionization (compounds
formed in this way are called ionic compounds). When ionic compounds dissolve in water, their
ions separate from one another in a process called dissociation. One interesting feature of water
and many other covalent compounds is that they too can dissociate into ions.
Dissociation of Water
When water dissociates, one of the hydrogen nuclei leaves its electron behind with the oxygen
atom to become a hydrogen ion, while the oxygen and other hydrogen atoms become a
hydroxide ion. Since the hydrogen ion has no electron to neutralize the positive charge on its
proton, it has a full unit of positive charge and is symbolized as H+. The hydroxide ion retains
the electron left behind and thus has a full unit of negative charge, symbolized by OH-. The
hydrogen ion (proton) does not wander long by itself before it attaches to the oxygen atom of a
second un-ionized water molecule to form a hydronium ion (H3O +).In any sample of water,
very few of the molecules are dissociated at any one time: in fact, only about one in 550 million.
Therefore, water is neutral.
Hydrochloric Acid
Certain molecules, ionic and covalent, dissociate in such a way that they release a hydrogen ion
without releasing a hydroxide ion. These substances are called acids. Since a hydrogen ion is
really just a single proton in most cases, this definition of an acid is a “proton donor.” If very
many protons (hydrogen ions) are “donated” the effect can be very profound, such as burning
your skin or dissolving metal. The acid illustrated is hydrochloric acid. Since nearly all of it is
dissociated in water, it is called a strong acid. Acids that do not dissociate completely are called
weak acids.
Sodium Hydroxide
The opposite of an acid is a base, also known as an alkali. A typical strong base is sodium
hydroxide. Sodium hydroxide dissociates to form a sodium ion and a hydroxide ion. A base is
defined as a “proton acceptor.” A strong base can give your skin a much worse burn than an
acid.
Neutralization
When a base and an acid are mixed, the hydroxide ion and the base combines with the hydrogen
ion from the acid to form water. This process is called neutralization.
What does it mean for a solution to be acidic or basic (alkaline)?
It all has to do with hydrogen ions (H+). In water (H2O), a small number of the molecules
dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxide ions
(OH−). The "lost" hydrogen ions are referred to as hydrogen ions H+. In pure water, there are an
equal number of hydrogen ions and hydroxide ions. The solution is neither acidic nor basic.
An acid is a substance that donates hydrogen ions (proton donor). Because of this, when an acid
is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now
there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic.
A base is a substance that accepts hydrogen ions (proton acceptor). When a base is dissolved in
water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because
the base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than
hydrogen ions. This kind of solution is basic (alkaline).
Acidity and alkalinity are measured with a logarithmic scale called pH. Here is why: a strongly
acidic solution can have one hundred million million (100,000,000,000,000) times more
hydrogen ions than a strongly basic solution! The flip side, of course, is that a strongly basic
solution can have 100,000,000,000,000 times more hydroxide ions than a strongly acidic
solution.
In order to deal with these large numbers more easily, scientists use a logarithmic scale, the pH
scale. Each one-unit change in the pH scale corresponds to a ten-fold change in hydrogen ion
concentration. The pH scale ranges from 0 to 14.
Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7
are alkaline (basic). A buffer solution is one which resists changes in pH when small quantities
of an acid or a base are added to it. Examples: sea water and human blood are both buffers.
Adapted from Biologycorner.com and
http://www.sciencebuddies.org/science-fairprojects/project_ideas/Chem_AcidsBasespHScale.shtml
Classwork: Acids and Bases
Name_____________ Date_________ Per_
Instructions
Read the paper titled “Acids and Bases.”
After reading the information, complete both sides of this worksheet.
Reading Comprehension Questions:
1. What happens when an atom gains or loses an electron?
2. In your own words, explain why water generally has a neutral pH, even though water
molecules dissociate.
3.Why are acids called proton donors?
4. What happens during neutralization?
5. Give an example of a strong base, and a strong acid.
The pH Scale
1) What does the pH scale indicate?
________________________________________________________________________
2) What pH number is water (neutral)? ____
3) What pH number is apple juice (acid)? ___
4) What pH number is bleach (base)? _____
5) Does an acid like apple juice have more or less H+ ions than a base like bleach? ______
6) Does an acid have a higher or a lower pH than a base? _________________________
7) What is a buffer? ____________________ ____________________________________
8) Why do you think it’s important that human blood is a buffer?
__________________________________________________________________________
9) Label the pH scale in the correct place with “basic”, “acidic” and “neutral.”