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 H2 O.)
- 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).