Effects of Intermolecular Forces
Intermolecular forces control how well molecules stick together. This affects many of the measurable physical properties of substances:
Melting and Boiling Points
Viscosity
Surface Tension
Vapour Pressure
Melting and Boiling Points
- If molecules stick together more, they'll be tougher to break apart
- Stronger intermolecular forces → higher melting and boiling points
Viscosity
- Viscosity is a measure of how well substances flow.
- Stronger intermolecular forces → higher viscosity.
Surface Tension
- Surface tension is a measure of the toughness of the surface of a liquid
- Stronger intermolecular forces → higher surface tension.
Vapour Pressure
- This is a small amount of gas that is found above all liquids. Refer to our lesson about vapour pressure to learn about it.
- Stronger intermolecular forces → Lower vapour pressure.
Answering Questions about Intermolecular Force Strength
These are one of my favourite types of questions. If you are asked to rank molecules in order of melting point, boiling point, viscosity, surface tension or vapour pressure ... what they are actually asking is for you to rank them by strength of intermolecular forces (either increasing or decreasing).
Here is my strategy for this:
Here is my strategy for this:
- Look for molecules with hydrogen bonding. They will have the strongest intermolecular forces.
- Look for molecules with dipoles. These will have the next strongest intermolecular forces.
- Larger molecules will have stronger London dispersion forces. These are the weakest intermolecular forces but will often be the deciding factor in multiple choice questions.
I have created a worksheet showing how I answer these types of questions:
| Sample Questions: Ranking Substances by Melting Point, Boiling Point, Surface Tension, Visoosity, or Vapour Pressure |
