Hydrogen peroxide (\(\text{H}_2\text{O}_2\)) is a substance frequently found in homes for cleaning and disinfection. Whether hydrogen peroxide is a mixture or a compound depends entirely on its form. The pure chemical substance is classified one way, while the widely sold household product is classified another. Understanding this distinction requires reviewing the foundational categories used to classify matter.
Defining Fundamental Chemical Substances
All physical matter is categorized into fundamental classes based on composition. The simplest is an element, a pure substance made up of only one type of atom, such as oxygen or hydrogen. Elements cannot be broken down further into simpler substances through ordinary chemical means.
A compound is formed when two or more different elements are chemically bonded together in a fixed, definite ratio. For example, water (\(\text{H}_2\text{O}\)) is a compound because it always consists of two hydrogen atoms joined to one oxygen atom. The chemical combination results in a new substance with properties entirely different from its constituent elements.
A mixture combines two or more substances that are physically intermingled but not chemically bonded. In a mixture, each component retains its individual chemical identity and properties. The ratio of components in any mixture can be varied, which separates it from a compound’s fixed ratio.
Mixtures are further divided into two types: heterogeneous, where the components are visibly separate, and homogeneous. A homogeneous mixture, such as sugar dissolved in water, is often referred to as a solution because the substances are uniformly distributed throughout.
The Chemical Identity of Pure Hydrogen Peroxide
Pure hydrogen peroxide (\(\text{H}_2\text{O}_2\)) is definitively a compound. This classification is based on the specific way its atoms are connected: the molecule consists of two hydrogen atoms and two oxygen atoms held together by strong chemical bonds. This fixed atomic ratio means every molecule of pure hydrogen peroxide is identical, a requirement for a compound. The pure substance has a unique set of physical properties, including a density greater than water and a freezing point of \(-0.43\,^{\circ}\text{C}\). Its components cannot be physically separated without breaking the chemical bonds, confirming its status as a compound rather than a mixture.
Hydrogen Peroxide as a Commercial Solution
The product found in pharmacies and supermarkets is not pure hydrogen peroxide; rather, it is sold as an aqueous solution. This means the hydrogen peroxide compound is dissolved in water, which acts as the solvent.
Because this commercial product consists of two distinct chemical substances (\(\text{H}_2\text{O}_2\) and \(\text{H}_2\text{O}\)) physically combined, it meets the definition of a mixture. Specifically, it is a homogeneous mixture because the hydrogen peroxide is dissolved evenly throughout the water, making the solution appear uniform.
The most common concentration for consumer use is approximately 3% by weight, though other concentrations, such as 6% for hair bleaching, are also available. The proportion of hydrogen peroxide to water can be varied, ranging up to 70% for industrial applications, which confirms its status as a mixture.