# Explain why high-density polyethylene (HDPE) has a higher melting point and greater tensile strength than low-density polyethylene (LDPE), despite both polymers having the same empirical repeat unit, –CH₂–.

> IB DP Chemistry Higher Level (2023 syllabus) — S2.4 From models to materials · Explain · 4 marks

## Mark scheme (4 marks)

1. LDPE has significant chain branching whereas HDPE chains are largely unbranched/linear
2. Linear HDPE chains can pack more closely/regularly together, increasing crystallinity
3. Closer packing leads to greater London/van der Waals dispersion forces between chains in HDPE
4. Stronger intermolecular forces in HDPE require more energy to overcome, explaining its higher melting point and greater tensile strength compared with LDPE

## Key terms

- [HDPE](https://www.gradenine.co.uk/glossary/hdpe)
- [LDPE](https://www.gradenine.co.uk/glossary/ldpe)
- [tensile strength](https://www.gradenine.co.uk/glossary/tensile-strength)
- [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)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-high-density-polyethylene-hdpe-has-0ca433ad) · Published by Druglandscape Ltd.