Distinguishing elements, compounds, and mixtures

Steps:

• Define key terms: Elements contain one type of atom; compounds contain two or more; mixtures combine substances variably.
• Evaluate A: Compounds include both ionic and covalent types, not all ionic.
• Evaluate B: Only ionic compounds conduct when molten; covalent ones do not.
• Evaluate D: Mixtures have variable proportions, unlike fixed-ratio compounds.

Why C is correct:

• By definition, an element is a pure substance consisting of only one type of atom, as per the periodic table and atomic theory.

Why the others are wrong:

• A: Covalent compounds, like water, are not ionic.
• B: Molecular compounds, like sugar, do not conduct electricity when molten.
• D: Mixtures, like air, have varying proportions of components.

Final answer: C