# Does a freezing point of a substance always increase as its boiling point increases?

I am studying for an introductory to physical sciences test and came upon a chart describing substances found in petroleum and the distillation process.

Each freezing point increases as each boiling point increases. Each of these properties also increases with the rise in density.

Is this just true for substances in petroleum (methane, ethane, propane, butane, pentane...) or is this consistent throughout all substances?

Update:

(Pure substances)

• Nope. See: methanol vs ethanol. – SendersReagent Apr 17 '16 at 23:13
• I don't think this is true even for all substances in petroleum. Wait, doesn't propane have lower melting point than ethane? – Ivan Neretin Apr 18 '16 at 6:41
• Since there are zillions of chemicals you should in general be skeptical when "always" is used for some trend. There always seem to be exceptions. – MaxW Jun 22 '16 at 2:09

Yes and no.

Yes in the sense that boiling point and freezing point usually correlate. If there is a line of homologous compounds, than this properties rise in general, as both are increased with increase in number of dispersion interactions involved. However, it is likely to find several compounds with odd behavior, especially around beginning of the line.

As a counterexample. Toluene has higher boiling point than benzene, but much lower freezing point. This is because toluene has less symmetrical form and consequently has harder time packing into strictly ordered lattice.

Another counterexample. Gallium melts around 30 celsium, but boils well above 2000 celsium. However, Tallium melts around 300 celsium, but boils around 1700.

However, for similar compounds the values usually correlate in the series. Say, for alkanes both values steadily grow with increase of number of carbons... give or take occasional peaks due to details in crystal packing.

However, if you consider compounds of different families, it is not predictable without looking into structures of the compounds.

An increase in the boiling point does not always follow an increase in the freezing point - these two properties, while seemingly correlated by the fact that they are governed by intermolecular forces, are still independent of another.

Take the case of Carbon Tetrachloride, $\ce{CCl4}$ vs Ethanol, Phenol vs Aniline

$\ce{CCl4}$ boils at 76.8 degrees and freezes at –22.8 degrees Celsius.
Ethanol boils at 78.4 degrees but freezes at –114.6 degrees Celsius.
Phenol boils at 181.75 degrees and freezes at 43 degrees Celsius
Aniline boils at 184.3 degrees but freezes at - 6.0 degrees Celsius

There's really no hard-and-fast rule, but you can make an informed guess that freezing point is affected more significantly by intermolecular forces than boiling is, because in liquid state the molecules are fairly loosely bonded whereas in a solid they would be stacked and packed to optimize the intermolecular forces.

Among single substances there is water. Increase the pressure above 4.56 torr and freezing point goes down until you get to ice (III) (>200 atmospheres), boiling point goes up.

This is true for all solutions and mixtures. Its what is called Colligative properties. Freezing/melting point and Boiling of a homogeneously (evenly) mixed solution depends on the number of dissolved particles in the solution. How much these values change also depends specifically on the mixture.

If you think about it makes sense. Freezing point decreases because its harder for the similar species in the mixture to form nice regular crystal without other junk getting in the way. Similar boiling point increase is the result of solutes lowering the the vapor pressure of a solvent.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/colligative.php

• I think the question is about pure compounds. – Ivan Neretin Apr 18 '16 at 6:42
• This question asks "Is this just true for substances in petroleum (methane, ethane, propane, butane, pentane...) or is this consistent throughout all substances?" Ie. Pure or not – Scient Apr 20 '16 at 19:37
• The question is a bit confusing as to whether it is for pure substances (the methane, ethane, ... listed), or for their solutions. After re-reading it I fall firmly in the pure substance interpretation. – Jon Custer Apr 20 '16 at 20:07