Final answer:
Chemical changes occur when substances form new chemical substances, with observable signs like color changes or new substances being formed. An example of a chemical change is a nail rusting, which results in the formation of iron oxide. On the other hand, boiling water is a physical change as it does not change the water's chemical structure.
Step-by-step explanation:
In the context of the provided examples, chemical changes are the result of substances interacting to form new chemical substances, often evidenced by observable phenomena like color changes, temperature changes, or the formation of new substances. For example:
- A nail rusting is a chemical change because the metal reacts with oxygen to form iron oxide (rust), which is a new substance with different properties from the original nail.
- Boiling water is a physical change because it involves the change from liquid to gas without altering the molecular structure of the water (H2O).
- The souring of milk is a chemical change as it involves the milk sugar lactose being converted into lactic acid by bacteria, which changes the milk's characteristics.
- The burning of gasoline is a chemical change because it reacts with oxygen to form carbon dioxide and water, releasing energy in the form of heat and light.
Physical changes typically involve a substance changing its state or form without changing its chemical identity, such as melting, freezing, or evaporating.