What is chemical change?
When a chemical change takes place one or more new substances are formed. The new substances have different chemical properties from the reacting substances. Chemical bonds are broken and new bonds are formed to create these new substances out of the same atoms. An example is iron reacting with oxygen to form iron(III) oxide, also known as rusting:
`4 Fe + 3 O_2 ->2 Fe_2O_3`
The brownish-orange rust that forms is clearly a different substance than the iron or the oxygen, and you can see from the chemical equation that it has a different formula.
Besides color change, other indications of a chemical reaction are:
- The evolution of heat and/or light as seen during combustion
- The evolution of a gas, as seen in the reaction of vinegar with baking soda
- The formation of a solid precipitate, which is often used to test for the presence of an ion in solution
- Odor, which indicates the formation of a new substance
In contrast, a physical change doesn't result in the formation of a new substance. The substance changes form, for example by vaporizing or melting, but its composition and therefore its chemical formula stay the same.
Chemical changes are changes that result in the formation of a new substance. In other words, the composition of a substance changes when the substance undergoes a chemical change. Chemical reactions are chemical changes. Examples of chemical changes (and reactions) are combustion, decomposition, fermentation, and oxidation. Color changes, the formation of bubbles, and the formation of a precipitate are often indicators that a chemical change has occurred.
Another type of change is a physical change. The composition of a substance does not change during physical change. Physical changes may result in a change of the form, appearance, shape, or size of a substance changes. All phase changes are physical changes. Other examples of physical changes include snapping, smashing, cracking, stretching, cutting, and dissolving.