what is the difference between a physical change and a chemical change?

Answer 1

Chemical change is any change that results in the formation of new chemical substances. At the molecular level, chemical change involves making or breaking of bonds between atoms. These changes are chemical:

iron rusting (iron oxide forms)
gasoline burning (water vapor and carbon dioxide form)
eggs cooking (fluid protein molecules uncoil and crosslink to form a network)
bread rising (yeast converts carbohydrates into carbon dioxide gas)
milk souring (sour-tasting lactic acid is produced)
suntanning (vitamin D and melanin is produced)
Physical change rearranges molecules but doesn't affect their internal structures. Some examples of physical change are:


whipping egg whites (air is forced into the fluid, but no new substance is produced)
magnetizing a compass needle (there is realignment of groups ("domains") of iron atoms, but no real change within the iron atoms themselves).
boiling water (water molecules are forced away from each other when the liquid changes to vapor, but the molecules are still H2O.)
dissolving sugar in water (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.)
dicing potatoes (cutting usually separates molecules without changing them.)
Classification of real processes can be tricky. Complex changes can be broken down into many simpler steps. Some of the steps are chemical and others are physical, so the overall process can't cleanly be placed in either category. For example, boiling coffee involves chemical change (the delicate molecules that give coffee its flavor react with air and become new, bitter-tasting substances) and physical change (the water in the coffee is going from liquid to gaseous form).




There are two possible definitions for Physical and Chemical changes which I use in school:

1. A physical change is reversible, a chemical change is not. For example, the freezing of water would be a physical change because it can be reversed, whereas the burning of wood is a chemical change - you can't 'unburn' it

2. A physical change is a change in which no new substance is formed; a chemical change results in the formation of one or more new substances. Again, consider the previous examples: Freezing water into ice just results in water molecules which are 'stuck' together - it's still H2O. Whereas burning wood results in ash, carbon dioxide, etc, all new substances which weren't there when you started.

Answer 2

In most cases, a chemical change involves breaking or forming chemical bonds, whereas a physical change it just a rearrangement of the molecules that are already there.

Example:

Your teeth change a piece of steak physically by breaking it into smaller pieces.

Your stomach changes a piece of steak chemically by using enzymes to break it into its chemical components, amino acids.

Answer 3

chemical change happens at the molecular level, gasoline burning, steel rusting, concrete setting, most things baking
physical change is more board: change of state, water into ice, change in shape, car being crushed, a mixture, like a salad

by no means the limit to this answer

Answer 4

chemical alters its chemical composition like burning a paper, you are left with the ashes... Phyisical alters the appearence like cutting a peice of paper