Matter is defined as anything that has mass and occupies space. To understand and categorize the vast array of substances, scientists rely on characteristic attributes known as properties. These properties act as fundamental identifiers, allowing for the distinction between one type of matter and another. Chemical properties describe a substance’s potential to transform into an entirely new substance. This transformation can only be observed when the matter undergoes a fundamental change in its composition.
Defining Properties of Chemical Identity
A chemical property is an attribute that describes a substance’s capability to participate in a chemical reaction. Observing this property necessarily requires the substance’s chemical identity to be altered. This alteration is referred to as a chemical change, where the original material is converted into one or more products with a new molecular structure and chemical composition.
For a chemical property to be demonstrated, the bonds holding the original atoms together must be broken, and new bonds must form. This process results in a permanent and irreversible change in the substance’s identity. For example, cutting a piece of wood only changes its size, but observing its ability to burn completely changes the wood into ash, smoke, and gases. The resulting products—carbon dioxide, water vapor, and carbon residue—are chemically distinct from the original wood.
Determining a chemical property means transforming the original sample. Chemical properties are always expressed in terms of a reaction or an interaction with another substance. The property is not a static measurement but a statement about the substance’s inherent tendency to undergo a specific compositional change.
Recognizing Common Chemical Properties
One chemical property is flammability, the ability of a substance to ignite and sustain combustion. Combustion is an exothermic, high-temperature oxidation reaction involving a fuel combining rapidly with an oxidant, usually atmospheric oxygen. This reaction requires an initial input of energy, known as the activation energy, to overcome molecular stability. Once initiated, the heat released makes the reaction self-sustaining, primarily producing oxidized products like carbon dioxide and water vapor.
Another property is reactivity, which describes the tendency of a substance to undergo a chemical reaction with specific agents. For instance, alkali metals like sodium demonstrate intense, explosive reactions with water. This interaction involves a rapid transfer of electrons, oxidizing the metal while generating flammable hydrogen gas and a highly alkaline hydroxide solution. Conversely, metals like gold are considered unreactive because they resist oxidation and acid exposure, maintaining their elemental state.
Toxicity is a chemical property defined by the degree to which a substance can cause damage to a living organism. This effect is rooted in specific chemical interactions at the cellular and molecular level. Toxicants exert their influence by binding to target molecules, such as enzymes, proteins, or DNA. This binding disrupts normal cellular function, potentially leading to cellular dysfunction, necrosis, or the initiation of uncontrolled processes like cancer.
The Critical Difference from Physical Properties
To appreciate chemical properties, it is useful to contrast them with physical properties. Physical properties are characteristics that can be observed or measured without altering the chemical identity or molecular structure of the substance. Examples include density, melting point, boiling point, and color. Observing these attributes does not require the substance to undergo any compositional change.
Density is a physical property defined as the ratio of a substance’s mass to its volume. It is an intensive property, meaning it does not depend on the amount of material present. Similarly, the melting point is the specific temperature at which a substance transitions from a solid to a liquid state. This change is merely a phase transition, where the molecules themselves remain chemically identical.
The distinction is clear when comparing observation to action. Observing the metallic luster of iron and measuring its density are physical properties because the iron remains iron. However, observing the iron’s propensity to rust when exposed to moist air demonstrates a chemical property, as the iron metal is irreversibly converted into iron oxide. The ability to measure a property without changing the substance’s core identity is the defining feature of physical properties.