Brainlag PlusWeak forces between molecules, low melting and boiling points, reading the state from data, and why polymers are solids.
Strong inside, weak between
A substance made of small molecules has two kinds of force:
- Covalent bonds inside each molecule: strong.
- Intermolecular forces between molecules: weak.
Melting or boiling only pulls the molecules apart, so only the weak intermolecular forces are overcome. Little energy is needed, so the melting and boiling points are low and many of these substances are gases or liquids at room temperature.
Bigger molecules have stronger intermolecular forces, so boiling points rise with size: methane (CH4) boils at −162 °C, octane (C8H18) at 126 °C.
The molecules have no overall charge and there are no free electrons, so these substances do not conduct electricity.
Boiling water does not break the O and H atoms apart. If it did, you would get hydrogen and oxygen, not steam.
The rest of this lesson and its 11 practice questions are in Plus.
Screen 1 of 4, free to read.
- 11 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards