Matter is broadly categorized based on its composition: as a pure substance or as a mixture. Pure substances have a fixed, uniform chemical makeup, while mixtures are physical blends of two or more substances that can vary in proportion. Determining calcium’s classification requires understanding these chemical definitions. This analysis will clarify calcium’s identity as a chemical element and explain how this classification answers the question of its purity.
Understanding Chemical Purity
A pure substance is defined as any form of matter that possesses a constant and uniform composition throughout. Every sample of that substance, under the same conditions, will display identical physical and chemical properties, such as a fixed melting point or boiling point. Mixtures are composed of different particles blended physically, allowing their properties, like density or melting range, to vary depending on the proportions of the components.
Pure substances are divided into two subcategories: elements and compounds. Elements consist of only one type of atom and cannot be broken down chemically into simpler substances, such as oxygen, gold, and calcium. Compounds are formed when atoms of two or more different elements are chemically bonded together in a fixed, definite ratio. Water (\(\text{H}_2\text{O}\)) is a classic example, always containing two hydrogen atoms for every one oxygen atom, giving it a fixed chemical structure.
Calcium as an Elemental Substance
Elemental calcium, represented by the chemical symbol \(\text{Ca}\), is a pure substance. It is classified as an element because it is composed solely of calcium atoms. As an element, pure calcium is found in the fourth period and Group 2 of the periodic table, identifying it as an alkaline earth metal.
The properties of elemental calcium, such as its density of \(1.55 \text{ g}/\text{cm}^3\) and its melting point of \(842^\circ\text{C}\), remain constant for any sample of the pure metal. This uniformity confirms that it meets the definition of a pure substance. Because it is an element, the silvery-white metal cannot be chemically decomposed into simpler substances.
Distinguishing Calcium Forms in Context
While elemental calcium (\(\text{Ca}\)) is a pure substance, the calcium people encounter daily is almost always in the form of a compound. For instance, calcium in bone tissue is primarily a complex compound called calcium hydroxyapatite. Calcium in dietary supplements often exists as calcium carbonate (\(\text{CaCO}_3\)) or calcium citrate.
These common calcium compounds are also pure substances, but they are the compound type, not the element type. Calcium carbonate, for example, is pure because it consistently maintains a fixed ratio of one calcium atom, one carbon atom, and three oxygen atoms. Unlike the element, however, the compound can be chemically broken down into its constituent elements.