When exposed to oxygen in the air, white phosphorus, \(\mathrm{P}_{4}\), catches fire and produces the compound \(\mathrm{P}_{4} \mathrm{O}_{10}\). Is this a description of a physical property or a chemical property of phosphorus? Explain.

Understanding the distinctions between physical and chemical properties is crucial in chemistry. Physical properties are characteristics of a substance that can be observed without altering its chemical composition. Examples of physical properties include texture, color, smell, freezing point, boiling point, and density.

Chemical properties, on the other hand, describe how a substance interacts with other substances, leading to a change in its chemical structure. These properties include reactivity with acids, reactivity with oxygen (combustion), and the ability to undergo fermentation or oxidation. For example, iron's tendency to rust when exposed to moist air is a chemical property because it results in the formation of iron oxide, a new substance with different chemical properties than elemental iron.

In the context of the textbook exercise, classifying the behavior of phosphorus as either a physical or chemical property is straightforward when recalling these definitions. Phosphorus catching fire and reacting with oxygen to form phosphorus pentoxide (\(\mathrm{P}_{4} \mathrm{O}_{10}\)) is a clear example of a chemical reaction because it results in a new product, hence showcasing a chemical property.

Combustion reactions are high-energy chemical reactions that typically involve oxygen and release heat and light. Often seen in everyday life as burning or fire, combustion reactions are always exothermic, meaning they give off energy. The general formula for a combustion reaction is fuel (usually a hydrocarbon) plus oxygen yields carbon dioxide and water. However, this formula can vary depending on the reactants involved.

In a combustion reaction, the substance that combusts is usually known as the fuel, while the source of oxygen can be pure oxygen gas or a component of the air.

Example of a Combustion Reaction

A simple and well-known example of a combustion reaction is the burning of methane gas (\(\mathrm{CH}_{4}\)), where methane reacts with oxygen to produce carbon dioxide, water, and energy: \(\mathrm{CH}_{4} + 2\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2} + 2\mathrm{H}_{2}\mathrm{O} + \text{energy}\).

Regarding white phosphorus, the reaction with oxygen does not produce carbon dioxide or water but instead yields \(\mathrm{P}_{4}\mathrm{O}_{10}\), which is still an example of a combustion reaction. This showcases the energy-releasing chemical change white phosphorus undergoes in the presence of oxygen.

Chemical change, also known as a chemical reaction, refers to the process where one or more substances are converted into different substances with distinct properties. Indications of a chemical change may include color change, temperature change, formation of a precipitate, emission of gas, or light and heat production. Chemical changes are fundamental to the study of chemistry because they describe the transformation of matter.

For a chemical change to occur, bonds between atoms in the reactants must be broken and new bonds must form to produce the products. This rearrangement of atoms results in substances with new chemical properties. Notably, chemical changes are usually irreversible under simple conditions, unlike physical changes which are often reversible.

Identifying Chemical Change

A reliable method to identify a chemical change is to look for signs of a new substance being formed. In the case provided in the textbook exercise, the reaction of white phosphorus with oxygen to form phosphorus pentoxide is a definitive sign of a chemical change. As opposed to merely melting or dissolving, which would be physical changes, the phosphorus atoms are actually reorganizing to create a new chemical structure, one with its own set of properties, unlike the original phosphorus or oxygen.