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In chemistry, there are two classifications of reactions
physical reactions and chemical reactions.
Physical reactions involve a change in form or state of a material,
but no change in chemical composition.
Take for example water, represented by the formula H2O.
It can be cooled to a point that it freezes to form a solid ice,
or it can be heated to a point that it boils to form
a gas water vapor.
Regardless of its state solid, liquid, or gas
its chemical composition remains H2O.
Freezing and boiling are physical reactions because
the form may have changed, but the chemical composition did not.
A chemical reaction produces new substances with characteristics and
compositions which are
different than those of the starting materials.
An example of a chemical reaction is burning.
Fire comes from a chemical reaction between
oxygen in the atmosphere and some sort of fuel, such as wood.
First, something such as a match, focused light, friction, or
lightening heats the wood to a very high temperature.
When wood reaches 300 degrees Fahrenheit,
the heat decomposes some of the material that makes up the wood.
Some of the decomposed material is released as smoke,
and some as char.