Answer to Question #138362 in General Chemistry for Sky

Question

What is the difference between a chemical change and a physical change on the “particulate” level?

Answer 1

In principle chemical changes occur when the electrons in an atom's orbitals change, either being shared, added or subtracted from shells.

Physical changes are changes to the arrangement of atoms or molecules in the material without changing the relationships between the nuclei of the atoms and their electrons.

However what used to be thought as physical changes, such as melting, boiling etc because they are easily reversed, are in fact just as chemical as chemical changes.

For example if one takes some H₂O gas and slowly cool it at constant pressure the molecules start meeting each other more often as they crowd in to a smaller and smaller volume. Eventually they stick loosely together forming a liquid - we call this water;)

Carrying on the cooling results in crystals being formed because of strong hydrogen bonding.

This is still called a physical change because it is easily reversed but it is chemistry, the properties of the molecules and even their average mass ill change.