Matter is defined by its characteristics, which scientists classify into properties that describe how a substance exists and interacts. This classification is fundamental in chemistry and physics because it allows for the precise identification of materials and the prediction of how they will behave under different conditions. These classifications are broadly divided into two categories based on whether observing the property changes the substance’s fundamental identity.
What Defines a Physical Property?
A physical property is a characteristic of a substance that can be observed or measured without changing the substance’s chemical composition. These properties can often be detected using our senses, such as sight, touch, or smell. For example, describing a material’s color, density, hardness, or melting point does not alter the substance itself.
Even when a physical change occurs, like water freezing into ice or boiling into steam, the substance remains chemically the same, which is still \(\text{H}_2\text{O}\). The change only affects the physical state or form, not the molecular structure. Other examples include malleability and electrical conductivity.
What Defines a Chemical Property?
A chemical property describes a substance’s potential to undergo a chemical change or reaction that results in the formation of an entirely new substance. Observing a chemical property requires the substance to actually change its identity. These properties relate directly to the substance’s atomic structure and how it forms or breaks chemical bonds.
Examples of chemical properties include flammability, which is the potential to burn, and toxicity. These characteristics indicate how a substance will react when exposed to other materials or conditions. The observation of a chemical property, such as a material rusting, is evidence of a completed chemical reaction.
Reacting with Air: A Chemical Property
The ability of a substance to react with air is classified as a chemical property. This is because the interaction involves a fundamental transformation of the substance’s chemical composition. Air contains several reactive components, most significantly oxygen (\(\text{O}_2\)), but also water vapor (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)).
When a substance reacts with air, it is typically undergoing a process called oxidation, where it combines with oxygen to form a new compound. A common example is iron metal reacting with oxygen in the presence of water to form iron oxide, commonly known as rust. Rust is chemically distinct from the original iron, signaling a change in identity.
Similarly, wood burning in air is a rapid chemical reaction called combustion. The wood is transformed into ash, carbon dioxide, and water vapor. This formation of different molecules confirms that the ability to react with air is a measure of a substance’s chemical reactivity.