Which of the following statements is true regarding the melting and boiling points of pure substances?

Analysis

The question asks us to identify the correct relationship between the melting and boiling points of pure substances. We need to evaluate the physical properties of matter and how these transition temperatures are defined for different substances.

Concept Explanation

• Melting Point: The specific temperature at which a solid substance changes into a liquid state.
• Boiling Point: The specific temperature at which a liquid substance changes into a gaseous state.
• Pure Substances: These have fixed, characteristic physical properties. However, these properties are unique to each specific substance. For example, the melting point of water ($0^\circ\text{C}$) is different from the melting point of iron ($1538^\circ\text{C}$).
• Variation: Because different substances have different intermolecular forces (like hydrogen bonding, van der Waals forces, or metallic bonding), their melting and boiling points vary significantly from one substance to another.

Step-by-Step Solution

1. Evaluate Option A & B: These suggest a universal rule (e.g., "always higher"). Since melting points and boiling points depend on the nature of the substance, there is no universal rule that one must be higher than the other across all matter.
2. Evaluate Option C: This is incorrect because if melting and boiling points were the same for all substances, all matter would exist in the same state at room temperature, which we know is false.
3. Evaluate Option D: This statement acknowledges that these points are specific to the identity of the substance. Since different substances have different molecular structures and bond strengths, their melting and boiling points vary. This is a scientifically accurate observation.

Conclusion

The melting and boiling points are characteristic physical properties that differ from one substance to another. Therefore, they vary depending on the substance being examined.

Exam Tip: When you see "always" or "never" in multiple-choice questions, be very careful. In chemistry, there are almost always exceptions to general rules, so statements using absolute terms are frequently incorrect.