A solid substance with high melting and boiling points is likely to be a/an

Step 1: Analyze the question The question asks which type of substance is a solid with both a high melting point and a high boiling point. This depends on the type of bonding and the strength of the forces holding the particles together.

Step 2: Explain the concept

• Electrovalent (ionic) compounds are made of positive and negative ions arranged in a giant crystal lattice. The electrostatic attraction between the oppositely charged ions is very strong, so a lot of energy is needed to break them apart. This gives them high melting and boiling points. Examples: NaCl, MgO, CaCl₂.
• Covalent compounds (simple molecular) have strong bonds within molecules but only weak intermolecular forces between molecules. They usually have low melting and boiling points, and are often gases, liquids, or soft solids. (Giant covalent structures like diamond and silicon(IV) oxide are exceptions, but "covalent compound" in general is not the best answer.)
• Dative covalent compounds (e.g. NH₄⁺ compounds, H₃O⁺) are a type of covalent bonding, so the same reasoning applies. Overall they do not by default have high melting points.
• Non-metals are mostly gases or low-melting solids (like sulphur and iodine), with a few exceptions such as carbon (diamond/graphite).

Step 3: Work through the options

• A: Usually low melting/boiling points, so not the best choice.
• B: Same reasoning as A.
• C: Strong ionic lattice, so high melting and boiling points. ✔
• D: Most non-metals have low melting and boiling points.

Step 4: Answer The substance is most likely an electrovalent compound, which is option C.

Exam tip: Remember the link "Ionic = Intense attraction = high melting point". When you see "solid, high melting and boiling point, (and conducts when molten or dissolved)", think ionic/electrovalent first.