Which of the following best describes an everyday pure substance?

A pure substance is defined as a material that consists of only one type of particle, meaning it has a uniform and definite composition. When considering the options provided, describing a pure substance as one that has had nothing added to it accurately reflects this definition.

In practical terms, this means that a pure substance can be a single element, like gold or oxygen, or a chemical compound, like water (H₂O) or carbon dioxide (CO₂), as long as it is not mixed with any other substances. This distinguishes it from mixtures, which involve multiple components that can vary in proportion and composition.

The other options highlight characteristics that do not align with the definition of a pure substance. For example, a chemically altered substance may contain multiple new compounds, and a substance with multiple ingredients clearly indicates a mixture rather than purity. The assertion that a pure substance must always be a solid is also misleading, as pure substances can exist as solids, liquids, or gases. Therefore, identifying a pure substance as one that has had nothing added to it captures its essence accurately.