# Explain why calcium fluoride (CaF₂) has a much higher melting point than potassium fluoride (KF), even though both compounds contain fluoride ions.

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

## Mark scheme (4 marks)

1. CaF₂ contains Ca²⁺ ions whereas KF contains K⁺ ions, so the cation in CaF₂ has a greater charge.
2. Ca²⁺ is smaller than K⁺ (has a smaller ionic radius), so the interionic distance in CaF₂ is shorter.
3. Greater charge and smaller ionic radius result in stronger electrostatic attraction between oppositely charged ions in CaF₂.
4. More energy is therefore required to overcome the stronger electrostatic attractions in CaF₂, resulting in a higher melting point.

## Key terms

- [melting point](https://www.gradenine.co.uk/glossary/melting-point)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-calcium-fluoride-caf-has-d5d499ed) · Published by Druglandscape Ltd.