The material world is composed of substances, and scientists use specific characteristics, known as properties, to describe, classify, and distinguish one substance from another. A substance’s properties are the fundamental traits that allow for its identification and prediction of its behavior in various conditions. Understanding these defining traits is foundational to the study of chemistry and physics, as they reveal how matter is structured and how it interacts. These observable and measurable characteristics make it possible to track matter as it undergoes changes.
Defining Characteristics: Physical Properties
Physical properties are those characteristics of a substance that can be measured or observed without causing a change in its chemical composition or identity. Observing a physical property does not transform the substance into a new one, meaning the original material remains intact. Properties like color can be noted simply by looking at the material, while others require measurement during a physical change, such as a phase transition.
The melting point of a solid is a distinct physical property, representing the temperature at which the substance changes from a solid to a liquid state. Similarly, the boiling point is the temperature at which a liquid converts to a gas. In both processes, the substance’s identity is preserved despite the change in physical form.
Density is calculated as the mass per unit of volume, and it is a unique identifier for a pure substance under specified conditions. Hardness, which is a substance’s resistance to scratching or indentation, is also a physical property. Other physical traits include odor, electrical conductivity, and solubility (the ability to dissolve in a solvent).
Defining Characteristics: Chemical Properties
Chemical properties describe a substance’s potential to undergo a chemical change, which results in the formation of a completely new substance. These properties are only observable during or after a chemical reaction, as the original material must be transformed to demonstrate the characteristic. A chemical property predicts how a substance will interact with other substances or energy sources.
Flammability is a common chemical property, defined as the ability of a substance to ignite and burn, producing new compounds like carbon dioxide and water vapor. Observing this property inherently destroys the initial substance by converting it into different chemical entities. Reactivity is a broader term that describes how easily a substance combines with other materials.
Corrosion potential describes the propensity of a material, like iron, to react with oxygen in the air to form iron oxide, commonly known as rust. This process, called oxidation, permanently changes the metal’s surface composition and appearance.
Classifying Properties by Quantity: Intensive vs. Extensive
Properties can also be categorized based on their relationship to the amount of substance present, dividing them into intensive and extensive categories. This classification applies to both physical and chemical characteristics. Intensive properties are those whose value does not depend on the quantity of the sample being measured.
Temperature is an intensive property because a cup of water at 50 degrees Celsius has the same temperature as a gallon of water at 50 degrees Celsius. Melting point and boiling point are also intensive, as the temperature at which a phase change occurs is fixed regardless of the size of the sample. These properties are useful for identifying a pure substance because they are inherent to the material’s nature.
In contrast, extensive properties are dependent on the amount of matter in the sample, meaning their value changes proportionally with the size of the substance. Mass, which is the amount of matter in an object, and volume, which is the amount of space an object occupies, are the two most frequently cited extensive properties. If two identical samples are combined, the total mass and total volume will double.
The ratio of two extensive properties yields an intensive property, which is demonstrated by density. Since density is calculated by dividing mass by volume, the resulting value is independent of the sample size.