The correct statement regarding the melting and boiling points of pure substances is that the melting and boiling points can vary depending on the substance.
The melting point of a substance is the temperature at which it changes from a solid to a liquid state. On the other hand, the boiling point is the temperature at which a substance changes from a liquid to a gas state.
Both melting and boiling points are unique for each substance. The melting and boiling points are influenced by the strength of the forces of attraction between the molecules or atoms that make up the substance.
Substances with strong intermolecular forces will have higher melting and boiling points, while substances with weak intermolecular forces will have lower melting and boiling points. For example, metals tend to have high melting and boiling points because the metallic bonds between the metal atoms are strong.
Ionic compounds also have high melting and boiling points because of the strong electrostatic attraction between the positively and negatively charged ions. In contrast, molecular substances generally have lower melting and boiling points because the forces of attraction between their molecules are weaker.
This is why substances like water (H2O) have lower melting and boiling points compared to metals or ionic compounds. So, to summarize, the melting and boiling points of pure substances are not always the same and can vary depending on the substance.
The strength of the intermolecular forces determines the melting and boiling points, with substances having stronger forces generally having higher melting and boiling points.