There are several differences between a physical and chemical change in matter
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
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
Film is processed with the assistance of light.
Heat is another factor that facilitates chemical reactions.