What distinguishes a physical change from a chemical change?

A physical change alters a substance's form without changing its chemical identity, while a chemical change transforms it into a new substance.

A physical change is a type of change in which the form of matter is altered but one substance is not transformed into another. This often involves changes in the state of matter, such as melting, freezing, condensation, and evaporation. For example, when ice melts into water, it undergoes a physical change. The water is still H2O, just in a different state. Physical changes are usually reversible, meaning you can change the substance back to its original form.

On the other hand, a chemical change, also known as a chemical reaction, involves a process that transforms one or more substances into new substances with different properties. These changes are often irreversible. For instance, when you burn a piece of wood, it turns into ash, smoke, and gases. These are all new substances, and you can't change them back into the original piece of wood.

In a chemical change, the atoms in the reactants rearrange themselves to form one or more new products with different characteristics. This is often accompanied by observable phenomena, such as heat or light production, colour change, gas production, or the formation of a precipitate.

In summary, the key difference between a physical change and a chemical change lies in whether the original substance maintains its chemical identity (physical change) or transforms into a new substance with different properties (chemical change). Understanding this distinction is fundamental to the study of chemistry.