Breaking a mirror is a physical change. This event alters the object’s appearance and structure but does not change the fundamental chemical identity of the materials that make up the mirror. Matter transforms in two primary ways: physical changes, which affect form, and chemical changes, which affect composition.
Defining Changes in Form and State
A physical change occurs when a substance modifies its physical properties without transforming into a new substance. These changes often involve a shift in size, shape, or state of matter, such as from a solid to a liquid or gas. When an ice cube melts, the water molecules (H2O) are rearranged from a rigid solid structure to a flowing liquid form, yet they remain water molecules.
The underlying chemical composition remains constant throughout the process. Tearing paper or cutting wood are simple examples of physical changes. The resulting smaller pieces are still chemically the same material as the original, only their form has been altered. These changes are often easily reversible, and no new chemical bonds are created or broken.
How Chemical Composition Stays the Same
In contrast to a physical change, a chemical change results in the formation of one or more entirely new substances with distinct properties. A chemical reaction is required for this transformation, involving the breaking and reforming of molecular bonds between atoms. This process fundamentally changes the substance’s molecular composition.
Indications of a chemical change include the production of a gas, a noticeable change in temperature, the emission of light, or a permanent color shift. For instance, when iron rusts, metallic iron atoms react with oxygen to form iron oxide, a completely new compound with different characteristics. The resulting iron oxide cannot simply be turned back into iron by a physical action.
Applying the Concepts to a Broken Mirror
When a mirror shatters, the process is purely one of physical deformation, not chemical transformation. A standard mirror consists of a glass substrate, primarily silicon dioxide, and a reflective coating, typically metallic silver or aluminum. Breaking the mirror merely separates these original components into smaller fragments.
The resulting pieces are still chemically glass and silver or aluminum. The molecular bonds within the glass and the metal coating remain intact, and no reaction has occurred to convert the silicon dioxide into a new compound. The change is limited to the physical properties of size and shape, confirming the event as a physical change.