Which statement is true about compounds?

Compounds are substances formed when two or more elements chemically bond together in fixed proportions. The defining characteristic of a compound is that it possesses a unique set of properties that differ from the properties of its constituent elements. One of the key features of compounds is that they can be broken down into simpler substances, which may be either elements or simpler compounds, through chemical reactions. This process often requires energy input or a specific reaction condition, such as heat or catalysts, to initiate the breakdown.

In contrast to this understanding, the other statements present inaccuracies about compounds. For instance, compounds are distinct from mixtures, which can have varying proportions of their components. Moreover, while some compounds may be gaseous at room temperature, they can also exist as solids or liquids, depending on the nature of the compound and the conditions present. Thus, the ability to break down compounds into simpler substances is a fundamental aspect of their classification in chemistry, affirming the correctness of the chosen answer.