All matter possesses observable characteristics that allow scientists to distinguish one substance from another. These distinct characteristics are classified into two categories: physical properties and chemical properties. The difference between these categories lies in whether measuring or observing the characteristic requires changing the substance’s inherent molecular identity. A common point of confusion in this classification is where to place the melting point, which is the specific temperature at which a substance transitions from a solid to a liquid state.
Understanding Physical Properties and Changes
Physical properties are characteristics that can be observed or measured without altering the chemical composition of the substance. Examples of these measurable traits include color, density, hardness, and electrical conductivity. When a substance undergoes a physical change, its state or appearance is modified, but its molecular structure remains intact. These changes are often reversible, such as when liquid water freezes into ice or when ice melts back into liquid water. The process affects only the arrangement and motion of the molecules, not the chemical bonds holding the molecules themselves together.
A physical change, like a phase change, involves changes in the forces between molecules, known as intermolecular forces. The chemical formula of the substance is identical, meaning no new material is created. Other common examples of physical changes include dissolving salt in water, grinding a solid into a powder, or shaping a piece of metal. Since the chemical identity is preserved during the measurement of these properties, they serve primarily to describe the substance’s appearance and condition.
Understanding Chemical Properties and Changes
Chemical properties describe a substance’s ability to undergo a transformation that results in the formation of entirely new substances. These properties can only be observed during a chemical change, where the substance’s molecular structure is permanently altered. A chemical change, also known as a chemical reaction, involves the breaking and reforming of chemical bonds within the molecules. The final products of a chemical change possess chemical compositions and properties that are distinct from the original starting materials.
A substance’s reactivity, its potential to combine with other materials, is a primary example of a chemical property. Flammability, the ability to burn or ignite, is another common chemical property, as combustion converts the original material into ash, carbon dioxide, and water vapor. The tendency of iron to combine with oxygen and water to form rust, a process called corrosion, is a chemical change that defines a chemical property of iron. These changes are generally considered irreversible, confirming that a chemical transformation has occurred.
Melting Point The Definitive Classification
The melting point is classified as a physical property. This classification is confirmed by observing the molecular events that occur during the phase transition from solid to liquid. When a solid is heated to its melting point, the absorbed energy increases the kinetic energy and vibration of its constituent molecules. The motion of the molecules overcomes the intermolecular attractive forces that hold them in the rigid, ordered structure of the solid crystalline lattice.
This change in state does not involve the reorganization of atoms within the individual molecules. For example, when ice melts at \(0^\circ\text{C}\), the water molecule remains \(\text{H}_2\text{O}\). No new chemical substance is created, which is the requirement for a chemical change. The melting point is a temperature value that describes a physical condition—a change in molecular arrangement—rather than a chemical potential or reactivity. It is also an intensive physical property, meaning it does not depend on the amount of substance present.