--- title: "Forces Between Molecules" description: "Melting and boiling a simple molecular substance change attractions between molecules while leaving their covalent bonds intact. Boiling points depend on dispersion, molecular polarity and hydrogen bo" canonical: https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB source: https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB.md retrieved: 2026-11-12 --- > **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 change attractions between molecules while leaving their covalent bonds intact. Boiling points depend on dispersion, molecular polarity and hydrogen bonding together. 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 - Predict how electron-cloud size affects boiling points when dispersion is the only relevant attraction, and recognise why force names alone do not give a universal ranking - Explain why water boils far above the temperature its molecular mass alone would suggest ## Lesson: What boiling really breaks You have met polar bonds. Covalent bonds join atoms within a molecule. Intermolecular forces attract separate molecules. Melting rearranges molecules; boiling separates them into a gas. Their covalent bonds stay intact. Vaporising one mole of liquid HCl takes about 17 kJ. Splitting the H-Cl bonds in one mole of gaseous HCl takes about 430 kJ, roughly 25 times as much energy. *(drawing: Energy in kJ per mole. Separating HCl molecules takes much less energy than splitting them into atoms.)* Every molecule has dispersion forces, caused by temporary uneven electron distributions. Larger, more easily distorted electron clouds usually give stronger dispersion attractions. Shape matters too, because it affects contact between molecules. At room temperature and atmospheric pressure, F2 and Cl2 are gases, Br2 is liquid and I2 is solid. Their increasing electron-cloud sizes help explain this trend. Polar molecules also have dipole-dipole attractions between opposite partial charges. In the hydrogen bonding studied here, H bonded to N, O or F attracts a lone pair on N, O or F in a nearby molecule. This is a particularly strong dipole-dipole attraction. For small molecules of similar size and shape, hydrogen bonding usually gives the strongest attractions of these three types. The comparison is not universal. **Example.** Water boils at 100°C at standard atmospheric pressure. This is unusually high for such a small molecule because water molecules form hydrogen bonds with several neighbours. Attraction between molecules of the same substance is called cohesion. Cohesion produces surface tension, which helps a brimful glass hold a small dome of water. The covalent O-H bonds do not stretch into a skin. Compare boiling points at the same pressure. Identify all the attractions, then compare electron-cloud size and molecular shape. For similar-sized molecules, polarity and hydrogen bonding often raise boiling points. Among similarly shaped nonpolar molecules, a larger, more easily distorted electron cloud usually raises the boiling point. A large nonpolar molecule can have stronger total attractions than a much smaller molecule that hydrogen-bonds. **Recap.** Boiling separates intact molecules. Compare their combined attractions, including dispersion, polarity and hydrogen bonding, at the same pressure. *Written for this dataset using OpenStax Chemistry 2e and Khan Academy as factual references. Their prose was not adapted.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Electronegativity and the Polar Bond](https://lightmysky.com/learn/science/electronegativity-and-the-polar-bond-mt_TwSlh2VvUT) ## Optional connections - [How Materials Change State](https://lightmysky.com/learn/science/how-materials-change-state-mt_CyV7crZ8hl) - [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)