What is a pure substance?

A pure substance is defined as a material that consists entirely of one type of particle. This means that it has a uniform and definite composition throughout. Pure substances can be elements or compounds, but they do not contain any mixtures of different substances.

For example, distilled water (H2O) is a pure substance because it consists only of water molecules. Similarly, gold is a pure element because it consists solely of gold atoms. This characteristic allows pure substances to have well-defined physical and chemical properties, such as boiling points, melting points, and density.

In contrast, materials composed of two or more types of particles or those that are mixed with other substances would not meet the criteria for being a pure substance. Additionally, the ability to change states—like transitioning from solid to liquid to gas—applies to both pure substances and mixtures, so it doesn't specifically define a pure substance. Understanding this concept is crucial in various scientific contexts, including sonography, where precise material properties are important for diagnostics and imaging.