--- title: "Forces Between Molecules" description: "Melting and boiling a simple molecular substance breaks the forces between molecules, not the bonds inside them. Those forces come in a few strengths, and hydrogen bonding is the strong end of the ran" canonical: https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB source: https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Forces Between Molecules Melting and boiling a simple molecular substance breaks the forces between molecules, not the bonds inside them. Those forces come in a few strengths, and hydrogen bonding is the strong end of the range. Subject: Science · Area: Matter & Materials · Ages 15 to 16 Page: https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB ## Ready when they can - Distinguish the energy needed to separate molecules from the energy needed to break a covalent bond - Rank substances by boiling point using molecular size, polarity and hydrogen bonding - Explain why water boils far above the temperature its molecular mass alone would suggest ## Lesson: What boiling really breaks You already know some bonds are polar. Now keep two kinds of pull apart. Covalent bonds hold atoms together inside one molecule. Intermolecular forces stick whole molecules to each other. Melting and boiling only separate molecules, while the bonds inside stay intact. Splitting liquid HCl into gas takes about 17 kilojoules per mole, but breaking its inner bonds takes about 430, near 25 times more. Boiling is a physical change, not a chemical one. These between-molecule pulls come in three strengths. Dispersion forces are weakest and every molecule has them. They grow as molecules get bigger: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Dipole-dipole attractions sit in the middle and act between polar molecules. Hydrogen bonding is strongest. It happens when hydrogen joined to nitrogen, oxygen, or fluorine is pulled toward a lone pair on a neighbor. **Example.** Look at water. It is tiny, so by size alone you would expect it to boil far below 0. Instead it boils at 100, because hydrogen bonds grip its molecules tightly and much heat is needed to pull them apart. You can see the grip in a brimful glass: the water holds a dome above the rim. That dome is cohesion, water molecules holding each other through hydrogen bonds. Each one links several neighbors at once. **Tip.** To rank boiling points, check three things in order. First look for hydrogen bonding, which beats the rest. Then check polarity, since dipole-dipole beats plain dispersion. Then compare size, because bigger molecules carry stronger dispersion. Candle wax melts far above tiny methane on size alone. **Recap.** Boiling separates molecules, and stronger pulls between them mean higher boiling points. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [How Materials Change State](https://lightmysky.com/learn/science/how-materials-change-state-mt_CyV7crZ8hl) - [Electronegativity and the Polar Bond](https://lightmysky.com/learn/science/electronegativity-and-the-polar-bond-mt_TwSlh2VvUT) - [Reading Structure from Properties](https://lightmysky.com/learn/science/reading-structure-from-properties-mt_yCA66GjWRw) ## Opens up - [Why Some Things Dissolve and Others Do Not](https://lightmysky.com/learn/science/why-some-things-dissolve-and-others-do-not-mt__njIrlqqqL)