Physical and Chemical Changes
There are
several differences between a physical and chemical change in matter or
substances.
A physical
change in a substance doesn't change what the substance is. In a chemical
change where there is a chemical reaction, a new substance is formed and energy
is either given off or absorbed.
For example,
if a piece of paper is cut up into small pieces it still is paper. This would
be a physical change in the shape and size of the paper. If the same piece of
paper is burned, it is broken up into different substances that are not paper.
Physical
changes can be reversed, chemical changes cannot be reversed with the substance
changed back without extraordinary means, if at all. For example, a cup of
water can be frozen when cooled and then can be returned to a liquid form when
heated.
If one decided
to mix sugar into water to make sugar water, this would be a physical change as
the water could be left out to evaporate and the sugar crystals would remain.
However, if one made a recipe for a cake with flour, water, sugar and other
ingredients and baked them together, it would take extraordinary means to
separate the various ingredients out to their original form.
When heat is
given off in a chemical change or reaction, it is called an exothermic
reaction. When heat is absorbed in a chemical change or reaction, it is called
an endothermic reaction. The speed at which chemical reactions take place
depend on the temperature pressure and how concentrated the substances involved
in the chemical reaction are. Sometimes substances called catalysts are used to
speed up or help along a chemical reaction. Light is helpful in the processing
of film.
 |  |
| Film is processed with the assistance of light. | Heat is another factor that facilitates chemical reactions. |
Physical Change in Water
 |  |  |
| Water as steam or water vapor. It is formed when water is heated to the boiling point or it evaporates. | Water as a liquid is formed then water vapor is cooled or ice is heated. | Ice is formed when water is cooled to the freezing point. |
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