Final answer:
Physical changes alter the appearance or state of a substance without changing its molecular composition, often reversible, while chemical changes transform one substance into another through a chemical reaction, typically harder to reverse.
Step-by-step explanation:
Understanding Physical and Chemical Changes
Changes in matter are categorized in two ways: physical changes and chemical changes. A physical change alters the appearance or state of a substance without changing its molecular composition. For example, when water in the air turns into snow, it is a physical change; the water changes state from gas (vapor) to solid (snow) but it is still H₂O. Similarly, when a person's hair is cut, it is a physical change as the hair remains chemically the same. Physical changes can often be reversible, such as melting which can be reversed by freezing.
On the other hand, a chemical change results in the transformation of one substance into another. This occurs during a chemical reaction. Take, for instance, the process where bread dough becomes fresh bread in an oven: this is a chemical change because the heat causes a chemical reaction that changes the composition of the dough into something new - bread. Other examples include a fire raging in a fireplace or water being warmed to make a cup of coffee, where the former is a chemical change (combustion reaction) and the latter is a physical change (temperature change).
Evidence of a chemical change includes the evolution of gas, formation of a precipitate, color change, temperature change, or light emission. On the contrary, a physical process might include changes such as dissolution, vaporization, freezing, and condensation that do not produce new substances.