Science of Water : Concepts and Applications

Water Chemistry 105
As stated previously, a chemical equation tells what elements and compounds are present before
and after a chemical reaction. Sulfuric acid poured over zinc will cause the release of hydrogen and
the formation of zinc sulfate. This is shown by the following equation:
Zn + H 2 SO 4 → ZnSO 4 + H 2
One atom (also one molecule) of zinc unites with one molecule sulfuric acid giving one
molecule of zinc sulfate and one molecule (two atoms) of hydrogen. Notice that there is
the same number of atoms of each element on each side of the arrow. However, the atoms are
combined differently.
Let us look at another example.
When hydrogen gas is burned in air, the oxygen from the air unites with the hydrogen
and forms water. The water is the product of burning hydrogen. This can be expressed as
an equation.
2H 2 + O 2 → 2H 2 O
This equation indicates that two molecules of hydrogen unite with one molecule of oxygen to
form two molecules of water.
WATER SOLUTIONS
A solution is a condition in which one or more substances are uniformly and evenly mixed or dissolved.
A solution has two components, a solvent and a solute. The solvent is the component that
does the dissolving. The solute is the component that is dissolved. In water solutions, water is the
solvent. Water can dissolve many other substances—given enough time, there are not too many
solids, liquids, and gases that water cannot dissolve. When water dissolves substances, it creates
solutions with many impurities.
Generally, a solution is usually transparent and not cloudy. However, a solution may be colored
when the solute remains uniformly distributed throughout the solution and does not settle with
time.
When molecules dissolve in water, the atoms making up the molecules come apart (dissociate)
in the water. This dissociation in water is called ionization. When the atoms in the molecules come
apart, they do so as charged atoms (both negatively and positively charged) called ions. The positively
charged ions are called cations and the negatively charged ions are called anions.
A good example of the ionization occurs when calcium carbonate ionizes:
CaCO3 Ca CO3
calcium carbonate calcium ion carbonate ion
(cat
2
↔
ion) (anion)
Another good example is the ionization that occurs when table salt (sodium chloride) dissolves
in water:
NaCl ↔ Na Cl
sodium chloride sodium ion chloride ion
(cation) (anion)