The question of whether hydrogen is classified as a pure substance or a mixture requires an understanding of how matter is fundamentally organized. To determine hydrogen’s status, one must first explore the basic scientific definitions that distinguish between different forms of matter.
How Scientists Classify Matter
Scientists divide all matter into two broad categories: pure substances and mixtures. A pure substance is defined by a uniform and fixed composition, meaning every sample will be identical in its properties. A mixture is a physical combination of two or more substances that are not chemically bonded together.
Pure substances are further separated into elements and compounds. An element is the simplest form of matter, consisting of only one type of atom, and it cannot be broken down into simpler substances. Gold, which consists only of gold atoms, is a common example.
A compound is formed when two or more different elements are chemically bonded together in a fixed ratio. Water (\(\text{H}_2\text{O}\)) is a compound because it is always made of two hydrogen atoms bonded to one oxygen atom. Components of a mixture retain their original properties and can often be separated by physical methods like evaporation or filtration.
Hydrogen’s Status as an Element
Hydrogen is classified as a chemical element. Its classification is based on the fact that it is composed of only one type of atom, represented by the chemical symbol H. It is the first entry on the Periodic Table, having an atomic number of 1, meaning its atom contains a single proton.
A common point of confusion arises because hydrogen gas typically exists as a diatomic molecule, written as \(\text{H}_2\). In this form, two hydrogen atoms are chemically bonded together. Despite the presence of two atoms, because both atoms are of the same type, the substance is still considered an element.
The definition of an element includes substances that exist as either single atoms, like argon, or as molecules composed of only one kind of atom, like nitrogen or hydrogen. Therefore, a container holding only \(\text{H}_2\) molecules is holding a pure substance. This elemental form of hydrogen is highly combustible and is the most abundant element in the universe.
Practical Applications: Hydrogen as a Mixture
While pure hydrogen gas is an element, in many real-world applications, hydrogen is deliberately handled or encountered as a mixture. Industrial hydrogen, stored in pressurized tanks, is often blended with other gases for safety or to achieve specific performance characteristics. For instance, mixtures of hydrogen and nitrogen are used in metal heat-treating processes and in the production of semiconductors.
Another common industrial blend is argon-hydrogen gas, utilized as a shielding gas in specialized welding applications. The hydrogen content in these industrial mixtures can vary, with the remainder being argon. This combination of gases, which are not chemically bonded, fits the definition of a mixture.
Naturally occurring hydrogen also exists in mixtures, such as the atmosphere, where trace amounts of hydrogen gas are physically intermingled with nitrogen, oxygen, and other gases. In these practical contexts, the hydrogen is not chemically bonded to the other components, confirming its status as part of a mixture.