Its constituent elements cannot be separated by physical methods.
Explanation
Matter is chemically classified into elements, compounds, and mixtures. A compound is a pure substance composed of two or more elements chemically combined in a fixed proportion by mass. The classification depends on how the constituents are bonded and how they can be separated.
- It exists in liquid state at room temperature. is incorrect: The physical state (solid, liquid, or gas) does not determine whether a substance is a compound. Elements (e.g., Mercury) and mixtures (e.g., Milk) can also exist in the liquid state.
- It has a fixed boiling point. is incorrect: While it is true that pure compounds have fixed boiling points (unlike most mixtures which boil over a temperature range), this is a physical property resulting from their purity. It is not the fundamental definition of a compound. Furthermore, azeotropic mixtures can also exhibit constant boiling points.
- Its constituent elements cannot be separated by physical methods. is correct: The defining characteristic of a compound is that its constituent elements are held together by strong chemical bonds. Therefore, they cannot be separated by simple physical methods such as filtration, distillation, or magnetic separation. Separating hydrogen and oxygen from water requires chemical methods, such as electrolysis.
- It is a universal solvent. is incorrect: The property of being a "universal solvent" describes water's utility and polarity but does not define its chemical status as a compound.
Key Takeaway:
A substance is classified as a compound if its constituent elements are chemically bonded and can only be separated by chemical or electrochemical reactions, not by physical means.