Re: Please define both a physical and a chemical change.

I took this definition from "College Chemistry with Qualitative Analysis," 
by Nebergall, Schmidt, and Holtzclaw:

"In addition to chemical properties, every substance also possesses 
definite physical properties-those which do not involve a change in 
composition of the material.  Some familiar physical properties of matter 
are color, hardness, crystalline form, ductility, malleability, physical 
state, melting point, boiling point, density, electrical and thermal 
conductivity, and specific heat."

Let's examine two subtances which have definite changes in physical 
properties:

1.  Water (H2O)  below 0 degrees C it is in it's solid form, ice,  from 0-
100 degrees C it is in its liquid form, water, and above 100 degrees C it 
is in its gaseous form, steam.  The molecular formula never changes 
(H2O), but its physical properties do change.

2.  Carbon (C) One form of carbon is graphite.  This substance is a soft 
grayish-black solid which has a mettalic luster.  It is also very 
malleable at room temperature and does conduct electricity.  The other 
form of carbon is known as a diamond.  You know what a diamond looks 
like.  It is formed below the earth when very pure carbon is subjected to 
high temperature and pressure.  It does not conduct electricity and is the 
hardest compound known.  The molecular formula never changes (C), but its 
physical properties do change.

Try to identify the physical changes in water and carbon that are listed 
in the excerpt from "College Chemistry with Qualitative Analysis," by 
Nebergall, Schmidt, and Holtzclaw.  Then try to come up with a couple more 
examples on your own (Hint: CO2 is a good place to start).  Good Luck!!

Dan Berger adds:
A chemical change is one in which the identity of the substance changes:
you can change carbon from graphite to diamond and back again -- that's
a physical change -- but when you burn it, it becomes carbon dioxide.
That's a chemical change.