Which term best describes a compound?

The term that best describes a compound is when two or more elements are joined together to produce an entirely different substance. A compound is formed through a chemical reaction, resulting in substances that have properties different from the elements that make it up. For example, when hydrogen and oxygen combine to form water, the resulting compound possesses distinct characteristics that are not present in the individual elements.

In this context, a compound is a specific type of pure substance; it consists of a definite ratio of its constituent elements, held together by chemical bonds. This distinguishes compounds from other forms of matter, such as mixtures, where the individual components retain their properties and can often be separated by physical means. Thus, the defining feature of a compound is the transformation that occurs when elements bond chemically, creating a new substance.

The other options do not accurately describe a compound. The mention of a single element, uniform composition, or a mixture of gases does not capture the essence of a compound as defined by the combination of elements into a new substance.