Physical properties are often explained by structure.
For example, butane and 2-methylpropane both have 4 carbon atoms and 10 hydrogen atoms. Butane boils at -0.5 degrees Celsius, while 2-methylpropane boils at a lower temperature of -11.6 degrees.
The 2-methylpropane molecules can be viewed as spherically shaped, like a ball, while the butane molecules look more like a hot dog.
The butane molecules can pack together in such a way that more of their surface area contacts. Since both molecules have similar attractive forces, the attraction is more effective for butane than for 2-methylpropane.
Consequently, it takes more energy (ie. a higher boiling point) for the butane molecules to leave the liquid state and enter the gaseous state.
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