3. Chemical Change
Chemical changes occur when a substance
combines with another to form a new substance,
called synthesis. or, alternatively, decomposes
into two or more different substances. These
processes are called chemical reactions and, in
general, are not reversible except by further
chemical reactions.
Some reactions produce heat and are called
exothermic reactions and others may require heat
to enable the reaction to occur, which are
called endothermic reactions.
4. Examples of chemical changes
iron rusting (iron oxide forms)
gasoline burning (water vapor and carbon
dioxide form)
eggs cooking (fluid protein molecules uncoil
and crosslink to form a network)
bread rising (yeast converts carbohydrates
into carbon dioxide gas)
milk souring (sour-tasting lactic acid is
produced)
suntanning (vitamin D and melanin is
produced)
5. Rusting
Rusting is the special name given to the
corrosion of iron. As iron, in the form of
steel, is the most commonly used metal in the
world, the corrosion of iron is important.
6. Speeding up Rusting
Rusting can be sped up by the use of sulphur
dioxide which causes acid rain.
The quantity of air and water affects the
speed.
7. Cathodic Protection
Iron has to lose electrons in order to
rust.
The negative terminal of the battery is
pushing electrons onto this nail and this
prevents this nail from losing any
electrons. This nail cannot rust.
Electrons flowing to the nail stop rusting.
8. Galvanising
Galvanising is when galvanised iron in made
by dipping iron into molten zinc which coats
the iron with zinc. It is used to protect
dustbins, car exhausts and special nails.
9. Sacrificial Protection
Sacrificial protection is when a more
reactive metal sacrifices itself to protect
the less reactive metal in the
electrochemical series.
10. Ways to Protect Iron
painting e.g. the Forth Rail Bridge.
greasing or oiling - protects moving parts of
machinery.
coating with plastic - dish drainers have a
metal core and a plastic coating.
coating with other metals such as tin, zinc,
silver, gold.
11. Rusting
Corrosion is an example of oxidation because it
involves a loss of electrons.
Fe ----------> Fe2+
+ 2e-
The rusting process continues when iron(II) ions lose
another electron to form iron(III) ions.
Fe2+
----------> Fe3+
+ e-
Oxygen and water accept the electrons lost by the
iron.
2H2
O + O2
+ 4e-
----------> 4OH-
