Matter is classified into two primary categories: pure substances and mixtures. Gases, such as helium, require a clear understanding of basic chemical definitions for proper categorization. This classification is based on composition and the nature of the bonds between constituent particles.
Understanding Pure Substances and Mixtures
A pure substance is defined as matter that possesses a constant composition and uniform properties throughout any sample. Its chemical and physical characteristics, such as melting or boiling points, are fixed and predictable. Pure substances cannot be separated into simpler components by physical means.
In contrast, a mixture is formed when two or more substances are physically combined, not chemically bonded. Each component retains its individual chemical identity and properties. Mixtures have a variable composition and do not exhibit fixed physical properties. The components of a mixture can often be separated using physical processes.
Elements and Compounds The Categories of Purity
Pure substances are divided into two sub-categories: elements and compounds. Elements are the simplest substances, composed of only one type of atom. They cannot be broken down into chemically simpler substances.
Compounds are also pure substances, but they consist of two or more different elements chemically bonded in a fixed ratio. For example, water is a compound because it always consists of two hydrogen atoms bonded to one oxygen atom. A compound can only be broken down into its constituent elements through a chemical reaction, not a physical process.
Why Helium Gas is Chemically Pure
Helium (He) gas is classified as an element and, therefore, a pure substance. It is found on the periodic table with the atomic number 2, meaning every helium atom contains two protons. Helium fits the definition of an element because it consists solely of one kind of atom.
As a noble gas, helium exists naturally as single, unbonded atoms. It does not readily form chemical bonds with other atoms. The gas cannot be chemically separated into anything simpler because its atoms are all identical, confirming its status as a pure substance.
Commercial Helium and Trace Impurities
While helium is chemically defined as a pure substance, the commercial product sold in tanks is rarely 100% pure. Various grades exist, often containing trace amounts of other gases like neon, nitrogen, or oxygen. For example, Grade-A helium typically has a nominal purity of 99.995 percent.
These trace contaminants technically make the bulk gas a mixture, but the primary classification remains based on the majority component. For industrial applications, the purity level is strictly graded, with some high-purity grades reaching 99.999% or higher. The presence of minor impurities does not change the fundamental chemical identity of helium as an element.