Boiling is definitively classified as a physical property. Understanding the difference between physical and chemical properties is fundamental to chemistry, as this distinction clarifies how substances behave during changes in energy or state. This classification is based entirely on whether the substance’s fundamental chemical identity is preserved during the process.
Defining Physical Properties
A physical property is a characteristic of a substance that can be observed or measured without changing its chemical composition. These properties describe the substance itself, independent of its ability to react with other materials. Measuring a physical property may alter the arrangement of the substance’s molecules, but it will never break or form new chemical bonds.
Physical properties are further divided into intensive and extensive properties. Intensive properties, such as density and melting point, do not depend on the amount of material present. Extensive properties, like mass and volume, are characteristics that change with the size of the sample. Other observable physical properties include color, hardness, and electrical conductivity.
Defining Chemical Properties
Chemical properties describe a substance’s potential to undergo a chemical change, which results in the formation of an entirely new substance. These properties cannot be observed simply by looking at the substance; they only become evident when the material is interacting with another substance or sufficient energy to trigger a reaction. When a chemical property is exhibited, the molecular structure of the original material is permanently altered.
A classic example of a chemical property is flammability, which is the ability of a substance to burn or ignite, causing fire or combustion. When wood burns, it changes into ash, carbon dioxide, and water vapor, substances with different chemical formulas than the original wood. Other examples include a metal’s reactivity with acid, its tendency toward oxidation (rusting), and its heat of combustion. Observing any of these properties means the substance has been chemically transformed into something new.
Classifying Boiling and Other Phase Changes
Boiling is a physical change because the substance’s identity remains unchanged. When water boils, the liquid H2O molecules gain enough thermal energy to overcome the intermolecular forces holding them together in the liquid state. The molecules then escape into the atmosphere as gaseous water vapor, or steam.
The chemical formula of the water molecules remains H2O in both the liquid and the gaseous state. No chemical bonds between the hydrogen and oxygen atoms are broken during the transition, meaning no new substance is created. Because the process only changes the physical arrangement and energy of the molecules, it fits the definition of a physical property. The change is also easily reversible; steam can be condensed back into liquid water simply by removing heat.
Boiling is one of several phase changes that are all classified as physical properties. Melting, where a solid turns into a liquid, and freezing, where a liquid turns into a solid, are also physical changes. In each case, the chemical composition of the material is conserved.