Pulls between molecules

Section 1 of 4

Covalent bonds hold the atoms of a molecule together. Molecules also pull on each other, much more weakly. These small pulls between molecules decide whether a substance is a gas, a liquid or a solid.

In a liquid, the molecules touch and tumble past each other, held close by their pulls. Heat makes them move faster. At the boiling point, they move fast enough to break away from each other, and the liquid turns into a gas. Every molecule stays whole: steam from a kettle is made of H₂O molecules, just like the water.

Boiling water: the molecules move apart and stay whole Left, liquid water: nine bent water molecules, each a red oxygen atom with two white hydrogen atoms, packed close together. An arrow labelled boils points right to the gas: four water molecules spread far apart. Every molecule on both sides is still a whole H2O molecule. boils liquid close together gas far apart
When water boils, its molecules move apart from each other. The bonds inside each molecule stay as they were, so the gas is still made of H₂O molecules.

The stronger the pulls between molecules, the more heat it takes to separate them, and the higher the boiling point.

Longer chains hold on harder

Alkanes are non-polar, with no charged ends, yet their molecules still pull on each other. The electrons in a molecule are always moving, and for an instant one side can have a few more than the other. That brief unevenness shifts the electrons of the molecule next door, so the two facing sides get opposite charges for that instant, and opposite charges attract. Each tug is tiny, but they add up along every part of two molecules that touch.

In lesson 4 you saw that longer chains boil at higher temperatures. Now you can see why: a long chain touches its neighbours along more of its length, so more of these small pulls hold it in place.

Boiling points of the first eight alkanes A bar chart of boiling points. Methane boils at −162 °C, ethane at −89 °C, propane at −42 °C, butane at −0.5 °C, pentane at 36 °C, hexane at 69 °C, heptane at 98 °C and octane at 126 °C. Each extra carbon raises the boiling point. A dashed line marks room temperature, 20 °C: methane to butane boil below it and are gases, and pentane to octane are liquids. methane −162 °C ethane −89 °C propane −42 °C butane −0.5 °C pentane 36 °C hexane 69 °C heptane 98 °C octane 126 °C 0 °C room temperature, 20 °C
Boiling points of the first eight alkanes. Every extra carbon raises the boiling point. Methane to butane boil below room temperature, so they are gases; pentane to octane are liquids.

Quick check

Hexane boils at 69 °C and octane at 126 °C. Why does octane boil at a higher temperature?