Ordering ethane, propane and butane from lowest to highest boiling point gives:

1. All three are nonpolar hydrocarbons, so only dispersion forces are in play. 2. Dispersion strength grows with molecular size and surface area. 3. The ordering from lowest to highest boiling point is therefore ethane, then propane, then butane. 4. Each extra carbon adds to the contact area between molecules. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 10 'Liquids and Solids', sections 10.1-10.6_