Unit 1: Back to the Basics

Section 7: Water, Solutions, and Electrolytes

* Importance of Water in Chemical Reactions
* Nonpolar Substances
* Electricity and Solutions
* Composiont of Solutions

Importance of Water in Chemical Reactions

The most important substance on earth is water. Water has so many different
uses that water becomes very valuable in the chemical community. For
instance, for most living things to survive, water is necessary; for
cooking, water is necessary, and for keeping mechanical engines cool, water
is necessary. Well, in chemistry, water is also very important because it is
necessary for many reactions to take place. An example of this is seen in
salt. Table salt (NaCl) when put into water dissolves into its ions, Na+ and
Cl-. Due to the salt dissolving, the Na+ ions and the Cl- ions can react
with other substances by chemical reactions.

In the example above, NaCl, is dissolved in water. In this example, water is
called the solvent. The solvent can be any substance that allows a substance
to be dissolved in it. Also in this reaction, NaCl, is called the solute, or
more basically put, the substance that gets dissolved. Although water is not
the only substance that can be used as a solvent, it is the main substance
used, so the scientific community came up with a term that is used to
describe a situation when water is used as a solvent. The term is aqueous
solution. Using the example above, the NaCl is an aqueous solution

The Water Molecule has many important properties that allow for substances
to be dissolved in them. Water acts much like a magnet, because one side of
the water molecule is negatively charged, and the other side is positively
charged. The hydrogen side of water is the positively charged side, and the
oxygen side of water is the negatively charged side. This characteristic of
being polar is when a substance has a positively charged side and a
negatively charged side. Water is a polar molecule.

Due to the polar nature of water, NaCl can be broken up into its ions. For
example, NaCl breaks up into Na+ and Cl-, which means that the hydrogen side
of water (+) attracts the Cl- ions, and the Na+ ions are attracted to the
oxygen side of water. This is how solid substances are dissolved into water,
and this is why water is so valuable to the chemistry world.

Nonpolar Substances

Not all substances are polar. Some are nonpolar. Benzene is a common
nonpolar solvent. Because of nonpolar and polar substances there is a rule
that students and chemists use a like, to describe whether substances should
dissolve. The rule is likes dissolve likes. This means that if a chemist is
trying to dissolve a polar solute, a polar solvent must be used, and
likewise, if a chemist is trying to dissolve a nonpolar solute, a nonpolar
solvent must be used. This works about ninety-five percent of the time, but
like all rules there are always exceptions.

Electricity and Solutions

Another useful characteristic of solutions is the ability to conduct
electricity. To determine if a solution has the ability to conduct
electricity, an electrical conductivity apparatus is used. An electrical
conductivity apparatus is basically a battery and light bulb setup which
lights up when electricity is conducted through the solution.

Some solutions conduct electricity better than others do, and to allow
scientists to understand how well substances conduct electricity, they came
up with a scaling system. The highest level on this system is when solutions
conduct electricity very well. This is called a strong electrolyte. The
middle level, or when solutions conduct electricity, but not very well, is
called a weak electrolyte. The lowest level, or when a solution does not
conduct electricity at all is called a non-electrolyte. These levels are
purely subjective and are open to interpretation by each scientist. To
determine the level of conductivity that a solution has, a scientist uses a
electrical conductivity apparatus and uses the brightness of the light bulb
to determine the level of which the substance should be. (The brighter the
light bulb is, the more conductive the solution is, and the stronger the
electrolyte is.)

As stated above, the most conductive solutions are called strong
electrolytes. There are three common ways in which strong electrolytes can
be created. The first way to create a strong electrolyte is to dissolve
salts in a solvent by breaking the salt up into its ions. (An example of
this is the NaCl example above.) The second way involves acids. Arrhenius
discovered that acids give off H+ ions and allow for a good flow of
electricity through a solution. The third class was also proposed by
Arrhenius. Arrhenius discovered that bases give off OH- ions and OH- ions
also allow for a good flow of electricity through the solution.

Weak Electrolytes have two major classes of which they can be made from. The
first class is called weak acids. A weak acid is much like a strong acid
because it produces H+ ions, but the difference is that a weak acid only
dissociates partially. In the terms of the common man, only partially
dissociating means that only a small percentage of the acid gives off their
H+ ions. For example, if HC2H3O2 were put in water, maybe 25% of the HC2H3O2
break up to H+ and C2H3O2- and the other 75% stays together. The second
class of weak electrolytes are weak bases. Weak bases are exactly like weak
acids except they release OH- ions instead of H+ ions.

Non-Electrolytes are substances that will dissolve, but do not produce any
ions. For example, sugar will dissolve in water, but will not produce ions.
Thus there is nothing to transfer electricity through the solution. That is
why non-electrolytes do not conduct electricity.

Composition of Solutions

In Chemistry, many reactions take place because two solutions are mixed
together, but to do any mathematical calculations to determine what the
final solution will be composed of or how much of the reactants there were,
the Chemist needs to know a few things. The first thing that the Chemist
needs to know is what type of reaction is taking place. This allows the
Chemist to correctly determine what the products will be. The second piece
of information that is necessary is the amount of chemicals that are
reacting. There are many ways to express how much of something that is
reacting, but the most common way is through Molarity.

Molarity is defined as:

Moles of Solute / L of solution

The units on molarity is mol/L or more simply written M.