Chemistry · Chapter 10

Compare the Pulls Between Molecules

Every substance has dispersion forces. Only lopsided molecules add a dipole pull, and only an H on N, O or F can make a hydrogen bond. Pick two substances, set a temperature, and see which one is still holding itself together.

The modelliquid holds, gas flies apart

carbon, sulfur or a halogen N, O or F hydrogen

Dashed lines mark an attraction that is winning. The jiggle grows with temperature, because warmer molecules carry more energy.

Boiling point of all sixteengold line = your temperature

Choose two

Substance A

Substance B

Temperature: 25 °C

The numbers

A–
B–
Forces in A–
Forces in B–

What you are seeing

State at this temperature
Pick two substances
A substance is a liquid below its boiling point and a gas above it. Stronger pulls between molecules push that boiling point higher.

Ranking rule. Compare molecules of similar size first. Hydrogen bonding beats a plain dipole pull, which beats dispersion alone. When one molecule is far bigger, its dispersion forces can win anyway.