Physical properties are the defining features of matter used by scientists to identify, classify, and study the world around us. They represent the inherent qualities of a material that can be determined without altering its chemical composition or identity. These measurable and observable qualities differentiate one material from another.
Defining Physical Properties
A physical property is a characteristic of matter that can be observed or measured without changing the substance’s chemical composition or identity. The act of measuring the property does not result in a new substance being formed. The measurement process might involve a physical change, such as a change in state, but the material’s fundamental chemical makeup remains the same.
The melting point of ice is a common example, which is the temperature at which solid water turns into liquid water. Although the state changes, it is still water (H2O) in both forms, meaning its chemical identity is preserved. Other readily observable physical properties include color, odor, and the state of matter.
Properties like boiling point, hardness, and solubility are measured with specific tools and conditions. Determining the boiling point involves heating a liquid until it vaporizes, but the vapor is chemically identical to the liquid. When salt dissolves in water, its solubility is measured, confirming no new substance was created.
Types of Physical Properties
Physical properties are categorized into two types based on their relationship to the quantity of matter present: intensive and extensive properties. This classification helps scientists determine which properties are useful for identifying an unknown substance.
Intensive properties do not depend on the amount of matter in the sample. These properties are intrinsic to the material itself and remain constant regardless of the sample size. Examples include density (mass per unit volume), temperature, color, and boiling point. A small drop of water and a gallon of water both have a boiling point of 100°C, demonstrating their independence from quantity.
Extensive properties, conversely, are entirely dependent on the amount of matter present. These properties change proportionally as the size of the sample changes. Common examples are mass, volume, and length. Doubling the amount of a substance doubles its mass and volume, confirming their dependence on quantity.
Physical Properties Versus Chemical Properties
The clearest way to understand physical properties is by contrasting them with chemical properties. A physical property describes a substance’s characteristics without reference to its potential to change into something new. A chemical property describes a substance’s potential to undergo a chemical reaction, requiring the substance to transform into one or more new materials with different chemical identities.
Chemical properties include flammability and reactivity with acids or bases. Flammability is a chemical property because observing it requires combustion, irreversibly turning wood into ash, carbon dioxide, and water vapor. The ability of iron to rust, known as its susceptibility to corrosion, is another chemical property because it involves iron reacting with oxygen to form iron oxide, a completely different material.
The distinction lies in the outcome of the observation or measurement. When water boils, it changes from liquid to gas but remains H2O, which is a physical change. The tarnishing of silver is a chemical change, resulting in a new compound on the surface. Chemical properties are only apparent as the substance is in the process of being changed into a different substance.