Which of the following describes a compound?

A compound is defined as a pure substance that consists of two or more different elements that are chemically bonded together. This definition highlights that the elements in a compound are not just mixed physically, as in a mixture, but rather they participate in chemical reactions to form new substances with unique properties that differ from those of the individual elements.

For example, water (H2O) is a compound made up of hydrogen and oxygen. In water, these elements combine in a fixed ratio and exhibit properties that are distinct from those of hydrogen gas or oxygen gas.

This is in contrast to a mixture, which might involve various substances but does not involve any chemical bonding, thereby allowing the individual substances to retain their own properties. Additionally, a compound typically has a definite chemical structure and composition, whereas mixtures do not.

Understanding the nature of compounds as pure substances formed from elements helps distinguish them from the other options, such as a physical change or an unstable mixture of gases, which do not accurately describe the concept of a compound.