What is a compound in chemical terms?

A compound in chemical terms is defined as a substance formed by a union of two or more elements in a definite proportion by weight. This definition accurately captures the essence of what constitutes a compound: it is not merely a combination of elements; instead, it refers to a specific chemical combination where the elements are bonded together in a fixed ratio. For example, water (H₂O) is a compound made up of two hydrogen atoms and one oxygen atom in a precise proportion. This fixed composition is key to identifying compounds as distinct from mixtures.

In contrast, a substance made of only one type of element describes an element rather than a compound. Similarly, a mixture includes various substances without a fixed composition, which differentiates them from the strict structural makeup of compounds. Lastly, a physical blend that cannot be separated suggests a mixture or solution, which again lacks the specific chemical bonding and definite proportions that characterize compounds. Thus, the correct understanding of a compound hinges on its formation through the union of differing elements in defined proportions, making option C the accurate choice.