Explain why calcium fluoride (CaF₂) is a solid at room temperature with a high melting point, whilst hydrogen fluoride (HF) is a gas at room temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
CaF₂ is an ionic compound that forms a giant lattice structure. The electrostatic attraction between the Ca²⁺ cations and the F⁻ anions is very strong, so a large amount of energy is required to break these attractions. Consequently CaF₂ has a high melting point and remains a solid at room temperature.
HF is a covalent molecule. While the H–F bond itself is strong, the forces between HF molecules are only weak van der Waals forces (and very weak hydrogen bonds). These intermolecular forces are easily overcome, giving HF a low melting/boiling point so it is a gas at room temperature.
HF is a covalent molecule. While the H–F bond itself is strong, the forces between HF molecules are only weak van der Waals forces (and very weak hydrogen bonds). These intermolecular forces are easily overcome, giving HF a low melting/boiling point so it is a gas at room temperature.
Examiner tips
- Use the word ‘ionic lattice’ for CaF₂ and ‘molecular/covalent’ for HF. Mention electrostatic attractions for CaF₂ and weak intermolecular forces for HF. Explain how the energy required to break these forces determines the state at room temperature.
Common mistakes
- Confusing ionic with covalent for CaF₂. Forgetting to mention the type of forces (electrostatic vs van der Waals). Using vague terms like ‘strong’ or ‘weak’ without linking them to melting point.
Mark scheme (4 marks)
- CaF₂ consists of an ionic lattice / giant ionic structure held together by strong electrostatic attractions between oppositely charged ions (Ca²⁺ and F⁻)
- A large amount of energy is required to overcome these electrostatic attractions, resulting in a high melting point, so CaF₂ is solid at room temperature
- HF is a simple molecular / covalent substance; the H–F covalent bond is strong but the intermolecular forces (van der Waals / hydrogen bonds) between HF molecules are weak
- Little energy is needed to overcome the weak intermolecular forces between HF molecules, so the melting/boiling point is low and HF is a gas at room temperature
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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