# 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.

> IB DP Chemistry Standard Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks

## Mark scheme (4 marks)

1. CaF₂ consists of an ionic lattice / giant ionic structure held together by strong electrostatic attractions between oppositely charged ions (Ca²⁺ and F⁻)
2. 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
3. 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
4. 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

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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