Matter is anything that has mass and takes up space, categorized into pure substances and mixtures. A pure substance is a form of matter that is chemically uniform and distinct from other forms. This classification focuses on the material’s composition and inherent characteristics.
The Core Definition of a Pure Substance
A pure substance possesses a constant composition and uniform properties throughout any sample. Every particle is identical, leading to a definite structure. For example, water is always H2O, and pure gold consists only of gold atoms.
The fixed composition of a pure substance distinguishes it from materials like saltwater, where component ratios vary. This definite composition ensures a single, characteristic set of physical and chemical properties. A pure substance cannot be broken down into simpler components by physical means, such as filtering or boiling.
Classifying Pure Substances
Pure substances are organized into two categories based on their atomic makeup: elements and compounds. Elements are the simplest forms of matter because they cannot be broken down by ordinary chemical methods. Gold, oxygen, and aluminum are examples of elements, each consisting of only one type of atom.
Compounds are pure substances formed when two or more different elements are chemically bonded in a fixed ratio. Water (H2O) is a compound, always containing two hydrogen atoms for every one oxygen atom. Unlike elements, compounds can be separated into their constituent elements, but only by using chemical reactions, such as passing an electric current through water.
The properties of a compound are entirely different from the elements that formed it. For instance, sodium is a highly reactive metal and chlorine is a toxic gas. When they combine chemically, they form the stable, edible compound sodium chloride, or table salt. This chemical combination results in a new substance with its own unique set of properties.
Understanding Mixtures
A mixture is the physical combination of two or more pure substances, where each substance retains its distinct identity and properties. Unlike compounds, mixture components can be combined in variable proportions. The ability to separate components using physical processes, such as evaporation or filtration, is a key difference.
Mixtures are classified based on how uniformly the components are distributed. A homogeneous mixture, often called a solution, has a uniform composition and appears the same throughout. Examples include saltwater, air, and alloys like bronze, where components are blended at the molecular level.
A heterogeneous mixture is characterized by a non-uniform composition. Different samples will not be identical, and the components are typically visible as separate parts. Common examples include sand mixed with water or a salad, where the individual ingredients are easily seen.
Identifying Substances Through Properties
Pure substances are identified by a set of specific, constant properties independent of sample size. These physical and chemical properties serve as a unique scientific fingerprint. Physical properties can be observed or measured without changing the substance’s chemical composition.
Physical properties include the temperature at which a substance melts or boils, its density, color, and electrical conductivity. For instance, pure water always boils at 100°C at standard pressure, a fixed characteristic used for identification. These properties are often intensive, meaning they remain the same regardless of the amount of substance present.
Chemical properties describe a substance’s potential to undergo a change in composition and transform into a new substance. These properties are only observed during or after a chemical reaction has taken place. Flammability (the ability to burn) and reactivity with acids are common examples. By comparing this fixed set of properties, scientists can definitively identify any unknown pure substance.