In a chemical reaction, substances are transformed into new substances with different chemical compositions and properties. Bonds between atoms are broken and new bonds are formed, resulting in the creation of entirely new molecules or compounds. This change is often accompanied by a release or absorption of energy, such as heat or light. The original substances cease to exist in their previous forms, and new substances are produced. Chemical reactions are typically not easily reversible.
On the other hand, a physical change involves a change in the physical state or appearance of a substance without altering its chemical composition. This means that the molecules and atoms of the substance remain the same before and after the change. Common examples of physical changes include changes in state (solid to liquid, liquid to gas, etc.), changes in shape or size, and changes in physical properties like density or color. Physical changes are usually reversible, as they do not involve the formation of new substances.
In summary, chemical reactions involve the creation of new substances with different chemical properties, often accompanied by an energy change, while physical changes involve alterations in the physical appearance or state of a substance without changing its chemical identity.