Weak forces between molecules, low melting and boiling points, reading the state from data, and why polymers are solids.
What you will learn
- Use melting and boiling points to give the state at a temperature
- Explain low boiling points with intermolecular forces
- Explain why polymers are solid at room temperature
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 locked.
In this lesson
- Strong inside, weak between
- The state from melting and boiling points
- Try it: Read the table
- Polymers and models
11 practice questions, marked as you go; about 22 minutes in all.
Key terms
Covalent bonds, Intermolecular forces, Do not conduct electricity, Solid, Liquid, Gas, Monomers
In this unit: Bonding, structure and the properties of matter
Ionic, covalent and metallic bonding, the structures they make, and how structure explains melting points, conduction and hardness, with nanoparticles for separate Chemistry.