Matter continually undergoes various transformations. Scientists categorize these transformations based on whether the substance’s fundamental identity changes during the process. Understanding this distinction between physical and chemical changes is key to grasping basic principles of chemistry and physics. Determining the type of change involves examining what is happening at the molecular level.
What Defines a Physical Change
A physical change alters the form, state, size, or appearance of a substance without changing its core chemical composition. During this transformation, the molecules remain intact; only their arrangement or spacing is affected. For example, when an ice cube melts into liquid water, the substance is still H2O, having simply changed from a solid to a liquid state.
Changing the dimensions of an object is another common example. Cutting paper or stretching a rubber band alters the shape and size, but the material’s chemical structure remains the same. Many physical changes are reversible, such as recovering dissolved salt by evaporating the liquid. Even irreversible actions, like smashing a rock, are physical because the rock’s chemical makeup is preserved.
What Defines a Chemical Change
A chemical change, also known as a chemical reaction, occurs when atoms and molecules are rearranged. This results in the formation of one or more entirely new substances with different properties. The process involves the breaking and forming of chemical bonds, fundamentally altering the identity of the material.
Several observable indicators signal that a chemical change has taken place. These signs include an unexpected change in color, the production of an odor, or a change in temperature (release or absorption of heat). Other indicators are the formation of a solid precipitate when two liquids are mixed or the evolution of gas bubbles not caused by boiling. A classic example is the burning of wood, which produces ash, water vapor, and carbon dioxide.
The Answer: Breaking a Pencil
Breaking a pencil is categorized as a physical change. When mechanical force is applied, the wood casing and the graphite core are fractured, resulting in smaller pieces. This action changes the overall size and shape of the object, altering its physical properties.
Crucially, the chemical composition of the materials does not change. The wood remains wood (cellulose and lignin), and the graphite remains graphite (a crystalline form of carbon). There is no rearrangement of atoms, no new substances are created, and none of the indicators of a chemical reaction are observed.
The process involves a physical deformation—a reduction in size and a change in shape—without affecting the molecular structure of the components. The pieces of the broken pencil still possess the same chemical properties as the original pencil. Therefore, the act of breaking the pencil falls under the definition of a physical change because the identity of the substance is conserved.