The classification of matter provides a fundamental framework for understanding the composition of the physical world. All substances can be broadly categorized based on whether their composition is fixed and uniform, or variable and mixed. Chemistry distinguishes between pure substances, which have an unvarying chemical makeup, and mixtures, which are physical combinations of two or more distinct substances.
Defining Pure Substances: Elements and Compounds
Pure substances have a uniform and definite composition that remains constant regardless of the sample size or source. This category is divided into elements and compounds.
Elements represent the simplest form of matter that cannot be broken down into simpler substances by ordinary chemical methods. Each element is uniquely defined by the number of protons in its atoms, known as the atomic number, and is listed on the periodic table. Examples include iron (Fe), oxygen (O), and gold (Au).
Compounds are pure substances formed when two or more different elements are chemically bonded together. This combination occurs in a fixed, specific ratio, meaning the formula for a compound like water (\(\text{H}_2\text{O}\)) is always the same. A compound’s properties are entirely distinct from the properties of the individual elements from which it is formed. For instance, combining sodium and chlorine creates sodium chloride (table salt).
Defining Mixtures: Homogeneous and Heterogeneous
Mixtures are formed when two or more substances are combined physically, rather than chemically bonded. Unlike compounds, the components retain their individual chemical properties and can be combined in variable ratios. Mixtures are separated based on the uniformity of their composition throughout the sample.
A homogeneous mixture, often called a solution, has a composition that is uniform throughout, meaning the components are evenly distributed and indistinguishable, even under magnification. A sample of saltwater, for example, is homogeneous because the dissolved salt particles are spread evenly, giving it a uniform appearance. Homogeneous mixtures are characterized by having only a single phase.
A heterogeneous mixture, in contrast, has a composition that is not uniform throughout. Its components remain visibly separate or are unevenly distributed. A mixture of sand and water is a classic example, as the components exist in two or more distinct phases.
Determining the Classification of Calcium
Calcium (Ca) is definitively classified as an element. It is a metallic substance that appears on the periodic table with the atomic number 20. This classification means that calcium is composed exclusively of atoms that all contain 20 protons and cannot be broken down into simpler substances. When isolated in its pure, elemental state, calcium is a silvery-white, soft metal.
Elemental calcium is not a compound because a compound must be formed from two or more different elements chemically bonded together. Calcium only consists of a single type of atom. Furthermore, calcium is not a mixture, as a mixture involves the physical combination of two or more distinct substances. Pure calcium metal does not fit the definition of either a homogeneous or heterogeneous mixture.
While the substance Calcium itself is an element, it is rarely found in its pure metallic form in nature. Calcium is a highly reactive metal, and it readily combines with other elements to form various compounds. The calcium people encounter most often is found in compounds, such as calcium carbonate (\(\text{CaCO}_3\)) in limestone and chalk, or calcium phosphate in bones and teeth. These naturally occurring calcium-containing materials are correctly classified as compounds, but this does not change the fundamental classification of Calcium (Ca) as a chemical element.