Matter is the fundamental material of the universe, defined as anything that has mass and takes up space. Because matter exists in a variety of forms, scientists organize it into categories to understand its properties and behavior. Classification is broadly done in two ways: by looking at the physical arrangement and motion of its particles, and by examining its chemical composition.
Classification Based on Physical State
The most immediate way to classify matter is by its physical state, which is determined by the energy and movement of its constituent particles. The three states most commonly encountered on Earth are solid, liquid, and gas. These states are largely defined by whether the substance maintains a fixed shape and volume.
In a solid, particles are packed tightly together in fixed positions, allowing only slight vibration. This arrangement results in a definite shape and a definite volume that resists compression. Liquids have a definite volume but no fixed shape, taking on the form of their container because their particles are close but can flow and slide past one another.
A gas has neither a definite shape nor a definite volume; its particles are widely separated and move rapidly and freely, expanding to fill any container they occupy. The state of matter can change based on factors like temperature and pressure, such as when water transitions from solid ice to liquid water to steam. A fourth state, plasma, is a high-energy, ionized gas where electrons are stripped from atoms. Though rare on Earth, plasma is the most abundant state of ordinary matter in the universe, making up the sun and other stars.
The Fundamental Division: Pure Substances and Mixtures
Beyond physical state, matter is fundamentally categorized by its composition into pure substances and mixtures. A pure substance has a uniform and definite composition, meaning every sample of that substance has constant properties. These substances cannot be separated into simpler components by physical methods like filtration or evaporation.
A mixture, in contrast, is a physical combination of two or more pure substances. The components retain their individual properties and are not chemically bonded together. Mixtures can almost always be separated back into their original substances using physical techniques.
Compositional Categories of Pure Substances
Pure substances are further divided into two compositional categories: elements and compounds. Elements are the simplest form of matter and cannot be broken down into other substances by ordinary chemical means. Each element is composed of only one type of atom, which is defined by the unique number of protons in its nucleus.
The elements are organized on the Periodic Table. Examples include Gold (Au), Iron (Fe), and Oxygen (O), each represented by a specific chemical symbol. Compounds are pure substances formed when two or more different elements are chemically bonded together in a fixed ratio.
This fixed proportion makes a compound distinct, such as water (H2O), which always contains two hydrogen atoms for every one oxygen atom. Unlike mixtures, the elements in a compound lose their original properties; for example, hydrogen and oxygen combine to form liquid water. Separating the components of a compound requires a chemical reaction, not a simple physical process.
Understanding Different Types of Mixtures
Mixtures are classified based on the uniformity of their composition. The two primary types are homogeneous and heterogeneous mixtures. The distinction hinges on whether the mixture appears to be uniform throughout.
Homogeneous mixtures, often called solutions, have a composition that is uniform at a microscopic level. The substances are so thoroughly mixed that only one phase of matter is visible, and individual particles are not visually distinguishable. Examples include saltwater, air (a uniform blend of gases), and alloys such as brass.
Heterogeneous mixtures have a non-uniform composition. These mixtures often have visibly distinct parts or regions, allowing for easy identification of the different substances. A mixture of sand and water is a classic example, as the sand particles settle and remain separate from the water. Other examples include salad dressing and granite, where different components can be seen in clumps or distinct phases.