MIXING CHEMICALS227SEPARATING MIXTURES AND COMPOUNDS
CHEMICAL
REACTIONS
When the atoms of two or
more substances rearrange
themselves to form a new
compound, we say that a
chemical reaction has taken
place. Most chemical
reactions are irreversible—
you cannot turn a cake
back into eggs and flour.
Some reactions can be
reversed but may need heat
or pressure to change back.It is much easier to separate a mixture than a
compound. Mixtures can be separated using
physical methods, such as evaporation, filtration,
flotation, or distillation. Separating compounds
may require several steps before you get the substance
you want, including mixing with other chemicals,
heating, and filtering. IRON can be
prevented from
rusting by coating
it with a less
reactive metal,
such as zinc (left).SCIENCE PANNING FOR GOLD
Gold prospectors separate
grains of gold from river
gravel by swirling the gravel
around in a shallow pan.
The heavier gold sinks to
the bottom and can be
picked out.Reversible reaction When iron is exposed to air
or water it starts to react. The metal reacts with
oxygen, which turns the iron into the reddish
brown iron oxides we call rust. However, if you
were to heat the iron oxides in a blast furnace they
would change back into iron and oxygen. ALLOY This is a
solid solution in which
one metal has dissolved
in another. Alloys are
often tougher and more
durable than the
original metals.
CHROMATOGRAPHY
This is used to identify colored
substances in a mixture. A drop
of the mixture is placed onto
chromatography paper and solvent
is then dripped onto it. As the
solvent travels across the paper, the
different substances travel across
the paper at different speeds.
Scientists can then figure out what
each substance is by the distance it
has traveled.Irreversible reaction Burning wood
causes irreversible changes. The
carbon atoms in the wood react with
oxygen in the air to form ash, smoke,
and carbon dioxide. The wood also
loses energy as heat and light. Even if
you put all these things together in
a test tube they would not change
back into wood. Another irreversible
change happens when food starts to
rot. Tiny microorganisms feed on the
food and turn it into new substances.
This process is called decomposition.